Showing papers in "ChemPhysChem in 2004"

Activation of Integrin Function by Nanopatterned Adhesive Interfaces

Abstract: To study the function behind the molecular arrangement of single integrins in cell adhesion, we designed a hexagonally close-packed rigid template of cell-adhesive gold nanodots coated with cyclic RGDfK peptide by using block-copolymer micelle nanolithography. The diameter of the adhesive dots is or = 73 nm between the adhesive dots results in limited cell attachment and spreading, and dramatically reduces the formation of focal adhesion and actin stress fibers. We attribute these cellular responses to restricted integrin clustering rather than insufficient number of ligand molecules in the cell-matrix interface since "micro-nanopatterned" substrates consisting of alternating fields with dense and no nanodots do support cell adhesion. We propose that the range between 58-73 nm is a universal length scale for integrin clustering and activation, since these properties are shared by a variety of cultured cells.

Non-Haloaluminate Room-Temperature Ionic Liquids in Electrochemistry—A Review

Abstract: Some twenty-five years after they first came to prominence as alternative electrochemical solvents, room temperature ionic liquids (RTILs) are currently being employed across an increasingly wide range of chemical fields. This review examines the current state of ionic liquid-based electrochemistry, with particular focus on the work of the last decade. Being composed entirely of ions and possessing wide electrochemical windows (often in excess of 5 volts), it is not difficult to see why these compounds are seen by electrochemists as attractive potential solvents. Accordingly, an examination of the pertinent properties of ionic liquids is presented, followed by an assessment of their application to date across the various electrochemical disciplines, concluding with an outlook viewing current problems and directions.

Biomolecule‐Functionalized Carbon Nanotubes: Applications in Nanobioelectronics

Abstract: Carbon nanotubes (CNTs) revealing metallic or semiconductive properties depending on the folding modes of the nanotube walls represent a novel class of nanowires. Different methods to separate semiconductive CNTs from conductive CNTs have been developed, and synthetic strategies to chemically modify the side walls or tube ends by molecular or biomolecular components have been reported. Tailoring hybrid systems consisting of CNTs and biomolecules (proteins and DNA) has rapidly expanded and attracted substantial research effort. The integration of biomaterials with CNTs enables the use of the hybrid systems as active field-effect transistors or biosensor devices (enzyme electrodes, immunosensors, or DNA sensors). Also, the integration of CNTs with biomolecules has allowed the generation of complex nanostructures and nanocircuitry of controlled properties and functions. The rapid progress in this interdisciplinary field of CNT-based nanobioelectronics and nanobiotechnology is reviewed by summarizing the present scientific accomplishments, and addressing the future goals and perspectives of the area.

How To Make Ohmic Contacts to Organic Semiconductors

Abstract: The process of charge injection plays an important role in organic semiconductor devices. We review various experimental techniques that allow injection to be separated from other competing processes, and quantify the injection efficiency of a contact. We discuss the dependence of the injection efficiency on parameters such as the energy barrier at the interface, the carrier mobility of the organic semiconductor, its carrier density (doping level), the presence of mobile ions, and the sample geometry. Based on these findings, we outline guidelines for forming ohmic contacts and present examples of contact engineering in organic semiconductor devices.

Tetrachloro-substituted perylene bisimide dyes as promising n-type organic semiconductors: Studies on structural, electrochemical and charge transport properties

Abstract: Twisted p-systems: The highly twisted 1,6,7,12-tetrachloro-substituted perylene bisimides possess an improved electron affinity. The nonplanar nature of these molecules facilitates a slipped brickstone-type rather than a columnar stacking of the p-systems, with a potentially useful two dimensional contact feature. These compounds show isotropic charge carrier mobilities as high as up to 0.14 cm2?V-1 s-1 (see graphic).

Issues in the synthesis of crystalline molecular sieves: towards the crystallization of low framework-density structures.

Abstract: Fundamental and practical interest in crystalline, microporous, molecular sieves is largely a direct consequence of the fact that their bulk properties can be manipulated through variations in atomic structure. This correspondence between the macroscale and the atomic scale is due to the uniformity of these crystalline materials. Control of the atomic structure therefore is of extreme importance, and is the thesis of this Review. Synthesis mechanisms and the parameters that can direct the crystal assembly pathway and the ultimate product formed are discussed and rationalized.

Synthesis of Silver and Gold Nanoparticles by a Novel Electrochemical Method

Abstract: Spherical silver and gold nanoparticles with narrow size distributions were conveniently synthesized in aqueous solution by a novel electrochemical method. The technological keys to the electrochemical synthesis of monodispersed metallic nanoparticles lie in the choice of an ideal stabilizer for the metallic nanoclusters and the use of a rotating platinum cathode. Poly(N-vinylpyrrolidone) (PVP) was chosen as the stabilizer for the silver and gold clusters. PVP not only protects metallic particles from agglomeration, but also promotes metal nucleation, which tends to produce small metal particles. Using a rotating platinum cathode effectively solves the technological difficulty of rapidly transferring the (electrochemically synthesized) metallic nanoparticles from the cathode vicinity to the bulk solution, avoiding the occurrence of flocculates in the vicinity of the cathode, and ensuring the monodispersity of the particles. The particle size and particle size distribution of the silver and gold nanoparticles were improved by adding sodium dodecyl benzene sulfonate (SDBS) to the electrolyte. The electrochemically synthesized nanoparticles were characterized by TEM and UV/Vis spectroscopy.

The Metastability of an Electrochemically Controlled Nanoscale Machine on Gold Surfaces

Abstract: The advent of supramolecular chemistry has provided chemists with the wherewithal to construct molecule-level machines 3] in an efficient manner using the protocol of template-direction. Synthetically accessible, linear motor molecules come in the guise of bistable [2]rotaxanes in which the ring component can be induced to move relative to the dumbbell-shaped one by altering the redox characteristics of the molecules. Such precisely controllable nanoscale molecular machines and switches have attracted a lot of attention 3] because of their potential to meet the expectations of a visionary and to act as some of the smallest components for the engineering of nanoelectromechanical systems (NEMs) and the fabrication of nanoelectronic devices. Although the redox-switching properties of numerous bistable [2]rotaxanes have been demonstrated in solution, the lack of coherence of the switches in this phase makes it difficult to harness the potential envisaged by Feynman. It is essential that we establish how to self-assemble these tiny switches in an orderly manner at surfaces and to investigate their switching properties in conjunction with their introduction into solid-state devices that have been shown to function as two-dimensional molecular electronic circuits. The fabrication of such devices required the design and synthesis of bistable [2]rotaxanes that are amphiphilic, so that they can be transferred 14±16] as molecular monolayers using the Langmuir ± Blodgett (LB) technique into a device setting. A molecular switch tunnel junction (MSTJ) has been fabricated by sandwiching such self-organized LB monolayers between a bottom Si electrode and a top Ti/Al electrode in a crossbar device architecture. The switch-on (high conductance) and switch-off (low conductance) states of each junction can be addressed respectively upon applying a 2 V or a 2 V bias. The proposed electromechanical switching mechanism (Figure 1) suggests that the ground state, where the cyclobis(paraquat-p-phenylene) (CBPQT , blue) ring initially encircles the tetrathiafulvalene (TTF, green) unit, represents the switch-off state. When a 2 V bias is applied, the CBPQT ring moves mechanically to the 1,5-dioxynaphthalene (DNP, red) ring system as a result of oxidation of the TTF unit to its radical cation. Although, when the bias is removed, neutrality is restored to the TTF unit, the CBPQT ring continues to reside on the DNP ring system, forming the metastable state. The observation of a switch-on state can be attributed to this slow-decaying metastable state that can be erased by applying a 2 V bias for a fleeting moment during the switching cycle. Since the mechanical motion associated with this decay is an activated process, these devices exhibit a hysteretic current ± voltage response. Herein, we describe how we have utilized a custom-designed variable temperature (VT) electrochemical apparatus to investigate the redox-switching behavior of an Au surface-confined linear motor-molecule, that is, a disulfide-tethered bistable [2]rotaxane SSR ¥ 4PF6, together with the corresponding dumbbell-shaped control compound SSD. In both cases, the appended disulfide function is used to immobilize the redox-active [2]rotaxane and dumbbell control onto gold surfaces as selfassembled monolayers (SAMs). The [2]rotaxane SSR ¥ 4PF6 was obtained (Figure 2) by a template-directed protocol wherein a CBPQT ring was clipped around the TTF unit of the dumbbellshaped precursor SSD. Here, we report i) the results of a semiquantitative electrochemical investigation carried out on the surface-confined SSR and the control (SSD) at room temperature in MeCN, leading to the identification of a [28] W. L. Jorgensen, J. Tirado-Rives, J. Am. Chem. Soc. 1988, 110, 1657. [29] Gaussian 98 (Revision A.11.3), M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen, M. W. Wong, J. L. Andres, M. Head-Gordon, E. S. Replogle, J. A. Pople, Gaussian, Inc. , Pittsburgh, PA, 2002. [30] C. Breneman, K. Wiberg, J. Comput. Chem. 1990, 11, 361. [31] S. J. Weiner, P. A. Kollman, D. A. Case, U. C. Singh, C. Ghio, G. Alagona, S. Profeta, P. Weiner, J. Am. Chem. Soc. 1984, 106, 765. [32] D. Fincham, D. Heyes, Adv. Chem. Phys. 1985, 63, 493. [33] M. Parrinello, A. Rahman, J. App. Phys. 1981, 52, 7182. [34] T. Darden, D. York, L. Pedersen, J. Chem. Phys. 1993, 98, 10089. [35] C. Zannoni, in The Molecular Physics of Liquid Crystals (Eds. : G. R. Luckhurst, G. W. Gray), Academic Press, London 1979, pp. 51 ± 83. [36] I. Haller, Prog. Solid State Chem. 1975, 10, 103. [37] K. Toyne, in Thermotropic Liquid Crystals (Ed. : G. W. Gray), Wiley, London, 28 ±63. [38] M. Hird, in Physical Properties of Liquid Crystals, Vol. 1: Nematics, (Eds. : D. A. Dunmur, A. Fukuda, G. R. Luckhurst), EMIS, IEE, London 2000, pp. 3 ± 16. [39] A. Ferrarini, P. L. Nordio, J. Chem. Soc. Perkin Trans. 1998, 2, 456. [40] M. E. Tuckerman, B. Berne, G. Martyna, J. Chem. Phys. 1992, 97, 1990.

Surfactant-Directed Polypyrrole/CNT Nanocables: Synthesis, Characterization, and Enhanced Electrical Properties

Abstract: We describe here a new approach to the synthesis of size-controllable polypyrrole/carbon nanotube (CNT) nanocables by in situ chemical oxidative polymerization directed by the cationic surfactant cetyltrimethylammonium bromide (CTAB) or the nonionic surfactant polyethylene glycol mono-p-nonylphenyl ether (Opi-10) When carbon nanotubes are dispersed in a solution containing a certain concentration of CTAB or Opi-10, the surfactant molecules are adsorbed and arranged regularly on the CNT surfaces On addition of pyrrole, some of the monomer is adsorbed at the surface of CNTs and/or wedged between the arranged CTAB or Opi-10 molecules When ammonium persulfate (APS) is added, pyrrole is polymerized in situ at the surfaces of the CNTs (core layer) and ultimately forms the outer shell of the nanocables Such polypyrrole/CNT nanocables show enhanced electrical properties; a negative temperature coefficient of resistance at 77-300 K and a negative magnetoresistance at 10-200 K were observed

Molecule‐to‐Metal Bonds: Electrografting Polymers on Conducting Surfaces

Abstract: Electrografting is a powerful and versatile technique for modifying and decorating conducting surfaces with organic matter Mainly based on the electro-induced polymerization of dissolved electro-active monomers on metallic or semiconducting surfaces, it finds applications in various fields including biocompatibility, protection against corrosion, lubrication, soldering, functionalization, adhesion, and template chemistry Starting from experimental observations, this Review highlights the mechanism of the formation of covalent metal-carbon bonds by electro-induced processes, together with major applications such as derivatization of conducting surfaces with biomolecules that can be used in biosensing, lubrication of low-level electrical contacts, reversible trapping of ionic waste on reactive electrografted surfaces as an alternative to ion-exchange resins, and localized modification of conducting surfaces, a one-step process providing submicrometer grafted areas and which is used in microelectronics

Supported membranes with well-defined polymer tethers: Incorporation of cell receptors

Abstract: We report the design of supported lipid membranes attached to the surface by tailored lipopolymer tethers. A series of well-defined lipopolymers were synthesized by means of living cationic polymerization of 2-methyl-2-oxazolines. The polymers were equipped with a silane coupling group on the proximal, and lipid anchors on the distal chain ends. The length of the intermediate hydrophilic polymer tether was varied (n = 14, 18, 33) to change the distance between the membrane and the substrate. Supported membranes have been prepared in two-steps. First, a suitable lipopolymer/lipid mixture was deposited by Langmuir-Blodgett transfer, and annealed to establish the covalent coupling to the surface. On the dry lipopolymer/lipid monolayer, the upper leaflet was deposited by vesicle fusion. Optimization of both preparation steps resulted in the formation of stable and defect-free membranes. Impacts of the spacer length and the lipopolymer fraction upon the lateral diffusivity of the lipids were systematically compared by fluorescence recovery after photobleaching (FRAP). First experiments on the incorporation of a large transmembrane cell receptor (integrin alpha IIb beta 2) into the polymer-tethered membrane suggested that the length of the polymer tether plays a crucial role in distribution of the proteins on the surface.

Molecular origins of the remarkable chiral sensitivity of second-order nonlinear optics.

Abstract: Recent observations of remarkably large chiroptical effects in second-harmonic generation (SHG) and sum-frequency generation (SFG) measurements suggest exciting possibilities for the development of new chiral-specific spectroscopies and novel chiral materials for nonlinear optics. Several fundamental studies designed to elucidate the molecular and macromolecular origins of the chiral responses are reviewed to provide a framework for development of this emerging field. In general, the chiral activity in SHG and SFG has the potential to arise from complex interactions between hosts of different competing effects. Fortunately, relatively simple electric dipole-allowed mechanisms routinely dominate the nonlinear optical chiral activities of most practical systemsexpressions can often be generated to link the . This substantial reduction in complexity allows for the development of simple models connecting the macroscopic nonlinear optical response to intuitive molecular and supramolecular properties.

Electronic structure and chemical reactivity of carbon nanotubes: a chemist's view.

Abstract: A qualitative description of the electronic structure of single-wall carbon nanotubes from a chemical perspective is presented using real-space orbital representations and traditional concepts of aromaticity, orbital symmetry and frontier orbitals. This unusual view of carbon nanotubes allows us to merge the solid-state physics description of band structures with the molecular orbitals framework of reaction mechanisms used in organic chemistry and to predict intriguing chemical selectivity based on electronic structure.

Noniterative Biexponential Fluorescence Lifetime Imaging in the Investigation of Cellular Metabolism by Means of NAD(P)H Autofluorescence

Abstract: The cofactors NADH and NADPH, hereafter NAD(P)H [NAD(P)= nicotinamide adenine dinucleotide (phosphate)], belong to the principal endogenous indicators of energetic cellular metabolism. Since the metabolic activity of cells is given by the ratio between the concentrations of free and protein-bound NAD(P)H, the development of autofluorescence techniques which accurately measure the modifications to this ratio is particularly significant. Hitherto the methods applied in the monitoring of cellular metabolism have provided either imprecise results, due to interference of the NAD(P)H signal by perturbing factors, or they have required a complicated internal calibration. We employ biexponential fluorescence lifetime imaging (FLIM) in order to discriminate between the free and protein-bound NAD(P)H without any previous calibration. Thus, we have obtained directly, and for the first time, a high-resolution map of cellular metabolism, that is, an image of the contribution of the protein-bound NAD(P)H to the cumulative NAD(P)H fluorescence signal. Moreover, we demonstrate that protein-NAD(P)H complexes characterised by different fluorescence lifetimes are not uniformly distributed all over the cell, as assumed until now, but are concentrated in certain cellular regions. The different fluorescence lifetimes indicate either different protein-NAD(P)H complexes or different bond strengths between NAD(P)H and the protein in these complexes. Since an important aspect in biological applications is to monitor the dynamics of the relevant processes (such as cellular metabolism), rapid dynamical techniques, for example, rapid biexponential fluorescence lifetime imaging, are needed. Furthermore, it is necessary to reduce the evaluation effort as much as possible. Most of the evaluation techniques in multiexponential FLIM are time-expensive iterative methods. The few exceptions are connected with a loss of information, for example, global analysis; or a loss in accuracy, for example, the rapid evaluation technique (RLD). We implement for the first time in FLIM a noniterative, nonrestrictive method originally developed by Prony for approximations of multiexponential decays. The accuracy of this method is verified in biexponential FLIM experiments in time-domain on mixtures of two chromophores both in homogenous and in heterogeneous media. The resulting fluorescence lifetimes agree (within error margins) with the lifetimes of the pure substances determined in monoexponential FLIM experiments. The rapidity of our evaluation method as compared to iterative pixel-by-pixel methods is evidenced by a reduction of the evaluation time by more than one order of magnitude. Furthermore, the applicability of this method for the biosciences is demonstrated in the investigation of cellular metabolism by means of NAD(P)H endogenous fluorescence.

Femtosecond time-resolved hydrogen-atom elimination from photoexcited pyrrole molecules.

Abstract: In recent years, the understanding of the photochemical processes in biologically relevant molecules in the gas phase has gained broad interest. In particular, aromatic biomolecules with an OH (hydroxy) or an NH (azine) group such as phenol, indole, pyrrole, or adenine were studied in detail. These molecules are important building blocks of naturally abundant compounds, such as proteins, porphyrins, or nucleic acids. As ab initio electronic-structure calculations and spectroscopic investigations suggested (see, e.g. , ref. [1]), the key role of the photophysics in such molecules is played by an excited ps singlet state, which is characterized by a repulsive potential curve with respect to the O H or N H stretching coordinate. Generally, the dark ps state is not excited directly but is populated through neighboring pp states. The coupling between the initially excited pp state and the ps state, as well as the shape of the conical intersection of the ps state with the electronic ground state upon elongation of the X H bond (X=O,N), determines the dynamics and energetics of the photochemical processes in these molecules. The existence and properties of the repulsive ps state were demonstrated recently for different biomolecular systems, for example, for indole—the chromophore of the amino acid tryptophan—and its derivatives, on the basis of spectroscopic measurements. The H-atom transfer reactions in phenol(NH3)n [3,4] or indole(NH3)n clusters, [5, 6] which were studied experimentally and theoretically, have also been interpreted on the basis of the ps state. The transfer of the H atom (from the OH group in phenol or the NH group in indole) to the NH3 molecules of the cluster proceeds on the ps potential surface. The detection of neutral (NH3)n 1NH4 radicals proves that a Htransfer reaction, followed by fragmentation, occurs. However, until very recently, it was not clear whether the repulsive state could be confirmed only indirectly (for example, by the detection of the reaction products) or whether the Hatom elimination could also be observed directly. The experiments reported by the group of F. Temps demonstrated, for the first time, the H-atom detachment from pyrrole in the excited ps state using photofragment velocity map imaging. Pyrrole is the photoreactive center of indole and represents one of the simplest heterocyclic aromatic ring molecules with a p-electron system. The lowest electronic state (at around 240 nm) has been assigned to the 1A2(3s) Rydberg state, [8] which has, according to the calculations performed by Sobolewski and Domcke, a ps character due to a Rydberg-to-valence transformation. The antibonding s orbital is located on the N H bond, which leads to a repulsive potential with respect to the N H coordinate. As the transition from the ground state to the 1A2(ps ) state is electric-dipole-forbidden, its excitation around 240 nm is only possible by vibronic coupling with the nearby pp states (2A1 or 1 B2) induced by out-of-plane vibrations. This behavior represents a Herzberg–Teller coupling similar to that observed, for example, in the ffiB2u !X1A1g system of benzene. At larger N H distances the ps state of pyrrole crosses the electronic ground state by a symmetry-allowed conical intersection. This leads to an ultrafast internal conversion to the ground state and should be the reason for the missing fluorescence of pyrrole in this wavelength region. In the one-color experiments of Wei et al. at 243.1 nm the photodissociation of the pyrrole molecule is initiated by a onephoton excitation, whereas the produced H atoms are detected by (2+1) resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The authors found a dominant contribution ( 76%) of fast H atoms with a narrow kinetic-energy distribution, which was assigned to a rapid, direct N H bond dissociation, in agreement with the repulsive character of the excited ps state. This observation was also confirmed by the corresponding anisotropy parameter indicating the vibronic coupling between pp and ps. The second contribution is characterized by H atoms that exhibit a broad kinetic-energy distribution with a maximum at smaller velocities. This contribution is attributed to the fragmentation of the molecule in the electronic ground state after an internal conversion. Replacing the H atom of the NH group in pyrrole by a methyl group leads to the disappearance of the fast component; these results confirm that the fast component is, in fact, due to the direct dissociation of pyrrole in the ps state along the N H bond. Quite similar conclusions were drawn in a previous study performed using photofragment translational spectroscopy, where three dissociation channels were observed at 248 nm. Each channel involved a H-atom elimination. The dissociation from an excited electronic state ( 47%) and the N H bond cleavage after an internal conversion to the ground state ( 42%) are the main contributions. The third channel ( 11%) was attributed to a C H bond cleavage in the excited electronic state. In the present work we report on the results of a two-color pump–probe experiment with femtosecond laser pulses. The pyrrole molecules were excited at 250 nm and the products were probed by (2+1) REMPI at 243.1 nm. We were indeed able to detect H atoms using a femtosecond laser, despite the inefficient excitation of a narrow atomic resonance with the spectrally broad laser pulse. Thus, we were able to observe an ultrafast H-atom elimination on the excited ps state of a biomolecule directly for the first time. The analysis of the time-dependent H signal reveals two time constants (t1 0.1 ps, t2 [a] H. Lippert, Dr. H.-H. Ritze, Prof. Dr. I. V. Hertel, Prof. Dr. W. Radloff Max Born Institute, Max-Born-Strase 2A 12489 Berlin (Germany) Fax: (+49)30-6392-1259 E-mail : radloff@mbi-berlin.de [b] Prof. Dr. I. V. Hertel Freie Universit6t Berlin, Physics Department Arnimallee 14, 14195 Berlin (Germany)

Can nematic transitions be predicted by atomistic simulations? A computational study of the odd-even effect.

Abstract: Computer simulations at the atomistic level are often called realistic as they offer, in principle, the possibility of reproducing in full the properties of a molecular system. Unfortunately, the number of atomic centers for molecules forming liquid-crystalline phases is normally so large, and correspondingly the number of molecules considered so low, that the proof of true realism, for example, the approximate reproduction of transition temperatures and of the relevant observables (such as order parameters together with their temperature dependence) has to our knowledge never been given to date. This is understandable in view of the complexity of liquid crystal molecules and probably depends on at least three issues: interaction potentials, sample size, and equilibration time. The force fields available have not been developed to reproduce liquid crystal properties, and only recently have some recommendations on the most appropriate choice amongst the many force fields available been made. Sample sizes have varied from a number of particles N 50, to 64 N 75, to 100 N 125, 11±15] and to N 200 molecules. In a few recent cases, larger samples with N 10 18] particles have been simulated but without really studying the full temperature dependence and identifying transitions. At atomistic resolution, the observation time window in molecular dynamics (MD) calculations has also been quite short, ranging from less than 600 ps 3, 5±7, 10, 11, 14±18] to 1 or 2 ns. 9, 13, 15, 16] Recently, a few simulations lasting up to 8 and 12 ns have appeared, 12] but these have involved the study of one, or very few, state points: again not sufficient to assess the ability of the methodology to predict transitions. The rather short runs are probably also connected to the contradictory preliminary findings on size dependence: thus in a recent study comparing N 118 and N 994 simulations, no size dependence was found: in contrast to what was found on going from N 125 to 1000. These strong limitations have been paralleled by the development of molecular-level models, where molecules are considered as simple objects, either just hard or endowed with attraction and repulsion, as in the Gay ±Berne models. These approximate models have proven extremely useful in studying the general properties of liquid crystals (LC), such as phase transitions and correlations, and in discussing the existence of new and yet undiscovered phases such as the biaxial and the ferroelectric nematic, suggesting design hints to the synthetic chemist. However, there is a class of problems and observations that are very important for the understanding of anisotropic fluids, and completely depends on an atomistic level description. The foremost example is probably the prediction of the nematic ± isotropic (NI) transition temperature TNI , a crucial element in the design of viable LC display materials that have to exist and operate in a certain temperature range, that is now tackled empirically. 23] The task is far from trivial since even small changes of structure can dramatically alter TNI . A classical demonstration is the so called TModd±even∫ effect, which corresponds to large alternation in properties and in mesophase transition temperatures, in particular for homologous series containing an n-alkyl chain, as n varies from even to odd. Examples of this effect have been known, in particular from the work of Gray and co-workers 25] and in Table 1 we report

Probing the vibrations of shared, OH+O-bound protons in the gas phase.

Abstract: Hydrogen bonding plays a crucial role in a wide range of important chemical and biological phenomena, from the solvent properties of water to the structures of proteins and DNA. Hydrogen bonds are typically categorized by strength according to the separation between the heavy atoms that share the proton (rAB). Long distances (rAB>2.8 ä) correspond to weak hydrogen bonds, where the proton is localized in one side of a double well potential. In the intermediate regime (2.8 ä> rAB> 2.5 ä), the barrier between the wells is decreased so that it is comparable with the zero-point energy, and at the shortest distances (rAB<2.5 ä), the proton occupies a symmetric position between the heteroatoms in a single well potential. Shorter, stronger hydrogen bonds falling in the latter two categories are involved in enzymatic catalysis and are also important for proton transfer mechanisms, such as in biological TMproton pumps∫ and for proton exchange in acidic and basic aqueous solutions. Infrared spectroscopy has long been used as a tool for characterization of strong hydrogen bonds in condensed phases. Until recently, it was not possible to obtain the corresponding gas-phase data by similar methods (that is, direct absorption), due to the inherent difficulties in getting sufficiently high ion densities. The advent of widely tunable, high fluence sources in the mid-infrared, such as free electron lasers (FELs) and optical parametric oscillators (OPOs), has allowed gas-phase ion spectroscopy of strongly bound ions via infrared multiple photon dissociation (IRMPD). In a recent IRMPD study, Asmis and co-workers obtained the first spectra for the protonated water dimer (H5O2 ), the quintessential example of a strong hydrogen bond, in the region corresponding to direct excitation of the vibrational modes of the bound proton between 600 and 1900 cm . It was expected that the vibrational potential of the symmetric OHO moiety in H5O2 + would be quite flat and anharmonic, making spectral predictions based on harmonic calculations unreliable. However, even more sophisticated computational techniques failed to obtain agreement with the observed spectra, leaving the assignment of the experimental spectrum somewhat in doubt, and making it clear that more experimental work is called for. In this work we report IRMPD spectra directly probing strong, OHO hydrogen bonds in other systems: the protonated dimers of dimethyl and diethyl ether [denoted (Me2O)2H + and (Et2O)2H ] , and protonated diglyme [1,1’-oxybis(2-methoxyethane)] . These spectra share an intriguing similarity with the H5O2 + spectrum, suggesting that this type of OHO TMproton bridge∫ may have a spectral signature in this region of the infrared. It should be noted that two of these systems [(Et2O)2H + [19] and protonated diglyme] have been investigated previously using IRMPD, but those studies were limited to the region around 10 mm accessible to CO2 lasers. Shown in Figure 1a±c are the IRMPD spectra of the protonated methyl and ethyl ether dimers as well as of protonated diglyme. All of the spectra are qualitatively similar, although

Accurate pKa Determination for a Heterogeneous Group of Organic Molecules

Abstract: Single-molecule studies that allow to compute pK a values, proton affinities (gas-phase acidity/basicity) and the electrostatic energy of solvation have been performed for a heterogeneous set of 26 organic compounds. Quantum mechanical density functional theory (DFT) using the Becke-half&half and B3LYP functionals on optimized molecular geometries have been carried out to investigate the energetics of gas-phase protonation. The electrostatic contribution to the solvation energies of protonated and deprotonated compounds were calculated by solving the Poisson equation using atomic charges generated by fitting the electrostatic potential derived from the molecular wave functions in vacuum. The combination of gas-phase and electrostatic solvation energies by means of the thermodynamic cycle enabled us to compute pK a values for the 26 compounds, which cover six distinct chemical groups (carboxylic acids, benzoic acids, phenols, imides, pyridines and imidazoles). The computational procedure for determining pK a values is accurate and transferable with a rootmean-square deviation of 0.53 and 0.57 pK a units and a maximum error of 1.0 pK a and 1.3 pK a units for Becke-half&half and B3 LYP DFT functionals, respectively.

Chemical derivatisation of multiwalled carbon nanotubes using diazonium salts.

Abstract: A chem. activated method of covalently derivatizing carbon powder, via the chem. redn. of aryl diazonium salts with hypophosphorous acid, to include the covalent derivatization of multiwalled carbon nanotubes (MWCNTs) is demonstrated. The specific mol. environments of 1-anthraquinonyl moieties attached to MWCNTs (see picture) produce interesting effects. [on SciFinder(R)]

Principles of spin-echo modulation by J-couplings in magic-angle-spinning solid-state NMR.

Abstract: In magic-angle-spinning solid-state NMR, the homonuclear J-couplings between pairs of spin-1/2 nuclei may be determined by studying the modulation of the spin echo induced by a pi-pulse, as a function of the echo duration. We present the theory of J-induced spin-echo modulation in magic-angle-spinning solids, and derive a set of modulation regimes which apply under different experimental conditions. In most cases, the dominant spin-echo modulation frequency is exactly equal to the J-coupling. Somewhat surprisingly, the chemical shift anisotropies and dipole-dipole couplings tend to stabilise--rather than abscure--the J-modulation. The theoretical conclusions are supported by numerical simulations and experimental results obtained for three representative samples containing 13C spin pairs.

Intramolecular charge transfer in pyrromethene laser dyes: photophysical behaviour of PM650.

Abstract: Absorption and fluorescence (steady-state and time-correlated) techniques are used to study the photophysical characteristics of the pyrromethene 650 (PM650) dye. The presence of the cyano group at the 8 position considerably shifts the absorption and fluorescence bands to lower energies with respect to other related pyrromethene dyes; this is attributed to the strong electron-acceptor character of the cyano group, as is theoretically confirmed by quantum mechanical methods. The fluorescence properties of PM650 are intensively solvent-dependent. The fluorescence band is shifted to lower energies in polar/protic solutions, and the evolution of the corresponding wavelength with the solvent is analysed by a multicomponent linear regression. The fluorescence quantum yield and the lifetime strongly decrease in polar/protic solvents, which can be ascribed to an extra nonradiative deactivation, via an intramolecular charge-transfer state (ICT state), favoured in polar media.

Solvate-supported proton transport in zeolites.

Abstract: Solvate-supported proton transport in zeolite H-ZSM-5 was studied by means of complex impedance spectroscopy. The zeolite shows enhanced proton mobility in the presence of NH3 and H2O that depends on the concentration of the solvate molecule, temperature (298-773 K), and the SiO2/Al2O3 ratio of the zeolite (30-1000). In general, proton conductivity in H-ZSM-5 is most effectively supported in the presence of NH3 and H2O at high concentrations, low temperatures, and low SiO2/Al2O3 ratios (< or = 80). For the aluminum-rich samples desorption measurements reflect different transport mechanisms that depend on the respective temperature range. Up to about 393 K a Grotthus-like proton transport mechanism is assumed, whereas at higher temperatures (393-473 K) vehiclelike transport seems to dominate. The activation energies for NH4+ and H3O+ vehicle conductivity depend on the SiO2/Al2O3 ratio, and the values are in the range of 49-59 and 39-49 kJ mol-1, respectively, and thus significantly lower than those for "pure" proton conduction in solvate-free samples.

Biofunctionalized polymer surfaces exhibiting minimal interaction towards immobilized proteins.

Abstract: designs are often based on hybrid (biotic–abiotic) nanoscale interface technologies, in which biomolecules are immobilized on solid substrates. [2, 3] The delicate structure of proteins requires coating of these surfaces with carefully designed films that interact only weakly with the protein, except through a strong tether for protein attachment, and thus preserve the functionally competent, properly folded conformation. The surface must not only resist interaction with the hydrophilic surface of the native protein structure, rather, for stability over long periods of times and/or under destabilizing conditions (extreme temperatures, cosolvents), the surface must also be inert towards hydrophobic protein moieties that become transiently or permanently exposed upon unfolding. Here we present a versatile surface preparation that matches exactly these requirements. It was specifically developed for single-molecule studies of protein folding, the intriguing process by which the linear polypeptide chain assumes its specific, functionally competent three-dimensional architecture. [4–6] Protein folding is an intrinsically heterogeneous process because a huge number of folding pathways on the free energy surface connect the myriad of unfolded conformations with the much smaller set of conformations belonging to the native state. [7] F rster resonance energy transfer (FRET) between a donor and acceptor fluorophore has been used extensively in equilibrium and time-resolved investigations of protein folding on large protein ensembles. [8–10] With the advent of single-molecule fluorescence spectroscopy, a technique has become available that enables us to examine protein folding pathways of individual protein molecules in real time, and to detect intermediate states and trajectories leading to misfolded structures, which have been implicated in diseases such as the spongiform encephalopathies (BSE, CJD, Alzheimer’s disease). [11] The strong distance dependence of FRET can be exploited to observe the reconfiguration of a single polypeptide chain in real time. [12] To this end, a donor–acceptor pair of fluorescent dye molecules is attached to specific locations along the polypeptide chain so that the two dyes are in close proximity in the folded structure and further apart in the unfolded chain. Such experiments, performed on proteins diffusing freely in solution, [13, 14] have already yielded insightful results. Free diffusion, however, limits the observation time to the transit time of the protein through the sensitive volume of the microscope, which is typically less than 1 ms. Longer observation time can be realized by fixing the molecules in space to within a volume smaller than the extent of the point-spread function of the microscope. Immobilization of proteins inside surface-bound lipid vesicles is an elegant approach, [15] but this does not allow easy solvent exchange. More versatile is the direct tethering of the protein to the surface. This strategy has been pursued to observe conformational dynamics of a small peptide on an amino-silylated glass surface. [16] In close proximity to a nanostructured surface, a biomolecule experiences an environment that is distinctly different from that in solution. Surface-protein interactions can easily be in the range of the overall stabilization energy of the native fold and thus can profoundly change the energy landscape of the protein. Therefore, experiments with surface-immobilized proteins reveal properties of the protein-surface assembly and not of the protein alone—unless a suitable surface treatment ensures minimal mutual interactions. We have developed nanostructured surfaces with excellent properties for protein functionalization even under destabilizing conditions. These polymer coatings on glass or quartz substrates are based on poly(ethylene glycol) (PEG), hydrophilic and uncharged polymers known to resist unspecific protein adsorption. [17, 18] While both star-shaped and linear PEGs have this advantageous property, [19] linear, non-crosslinked PEG

Mechanical Properties of Microtubules Explored Using the Finite Elements Method

Abstract: Reference EPFL-ARTICLE-150102doi:10.1002/cphc.200300799View record in Web of Science Record created on 2010-07-21, modified on 2016-08-08