Showing papers in "Journal of the American Chemical Society in 1996"

Development and Testing of the OPLS All-Atom Force Field on Conformational Energetics and Properties of Organic Liquids

Abstract: The parametrization and testing of the OPLS all-atom force field for organic molecules and peptides are described. Parameters for both torsional and nonbonded energetics have been derived, while the bond stretching and angle bending parameters have been adopted mostly from the AMBER all-atom force field. The torsional parameters were determined by fitting to rotational energy profiles obtained from ab initio molecular orbital calculations at the RHF/6-31G*//RHF/6-31G* level for more than 50 organic molecules and ions. The quality of the fits was high with average errors for conformational energies of less than 0.2 kcal/mol. The force-field results for molecular structures are also demonstrated to closely match the ab initio predictions. The nonbonded parameters were developed in conjunction with Monte Carlo statistical mechanics simulations by computing thermodynamic and structural properties for 34 pure organic liquids including alkanes, alkenes, alcohols, ethers, acetals, thiols, sulfides, disulfides, a...

Nucleus-Independent Chemical Shifts: A Simple and Efficient Aromaticity Probe

Abstract: The ability to sustain a diatropic ring current is the defining characteristic of aromatic species.1-7 Cyclic electron delocalization results in enhanced stability, bond length equalization, and special magnetic as well as chemical and physical properties.1 In contrast, antiaromatic compounds sustain paratropic ring currents3 despite their localized, destabilized structures.1-7 We have demonstrated the direct, quantitative relationships among energetic, geometrical, and magnetic criteria of aromaticity in a wide-ranging set of aromatic/antiaromatic fivemembered rings.5a While the diamagnetic susceptibility exaltation (Λ) is uniquely associated with aromaticity, it is highly dependent on the ring size (area2) and requires suitable calibration standards.6 Aromatic stabilization energies (ASEs) of strained and more complicated systems are difficult to evaluate. CC bond length variations in polybenzenoid hydrocarbons can be just as large as those in linear conjugated polyenes.2 The abnormal proton chemical shifts of aromatic molecules are the most commonly employed indicators of ring current effects.1 However, the ca. 2-4 ppm displacements of external protons to lower magnetic fields are relatively modest (e.g., δH ) 7.3 for benzene vs 5.6 for dC-H in cyclohexene). In contrast, the upfield chemical shifts of protons located inside aromatic rings are more unusual. The six inner hydrogens of [18]annulene, for example, resonate at -3.0 ppm vs δ ) 9.28 for the outer protons. This relationship is inverted dramatically in the antiaromatic [18]annulene dianion, C18H18, where δ ) 20.8 and 29.5 (in) vs. -1.1 (out).8 Similar demonstrations of paratropic ring currents in antiaromatic compounds are well documented.3,8,9 Chemical shifts of encapsulated 3He atoms are now employed as experimental and computed measures of aromaticity in fullerenes and fullerene derivatives.10 While the rings of most aromatic systems are too small to accommodate atoms internally, the chemical shifts of hydrogens in bridging positions have long been used as aromaticity and antiaromaticity probes.9 δLi+ can be employed similarly, with the advantage that Li+ complexes with individual rings in polycyclic systems can be computed.4,11 We now propose the use of absolute magnetic shieldings, computed at ring centers (nonweighted mean of the heavy atom coordinates) with available quantum mechanics programs,12 as a new aromaticity/antiaromaticity criterion. To correspond to the familiar NMR chemical shift convention, the signs of the computed values are reversed: Negative “nucleus-independent chemical shifts” (NICSs) denote aromaticity; positive NICSs, antiaromaticity (see Table 1 for selected results). Figure 1, a plot of NICSs vs the ASEs for our set of five-membered ring heterocycles,5a provides calibration. The equally good correlations with magnetic susceptibility exaltations and with structural variations establish NICS as an effective aromaticity criterion. Unlike Λ,6 NICS values for [n]annulenes (Table 1) show only a modest dependence on ring size. The 10 π electron systems give significantly higher values than those with 6 π electrons. The antiaromatic 4n π electron compounds, cyclobutadiene (27.6), pentalene (18.1), heptalene (22.7), and planar D4h cyclooctatetraene (30.1), all show highly positive NICSs. Like the Li+-complex probe,4 the NICS evaluates the aromaticity and antiaromaticity contributions of individual rings in polycyclic systems. Scheme 1 (HF/6-31+G*, data from Table 1) shows NICSs for selected examples. The benzenoid aromatic NICSs provide evidence both for localized and “perimeter” models. The naphthalene (1) NICS (-9.9) resembles that of benzene (-9.7), as do the NICSs for the outer rings of phenanthrene (2) (-10.2) and triphenylene (3); the aromaticity of the central rings of the latter two are reduced. The NICS of the central ring of anthracene (4) (-13.3) exceeds the benzene value in contrast to the outer ring NICS (-8.2). Remarkably, the NICS (-7.0) for the seven-membered ring of azulene (5) is very close to that of the tropylium ion (-7.6 ppm), whereas the azulene five-membered ring NICS (-19.7) is even larger in magnitude than that of the cyclopentadienyl anion (-14.3). The four-membered rings in benzocyclobutadiene (6) (NICS ) 22.5) and in biphenylene (7) (19.0) are antiaromatic, but less so than cyclobutadiene itself (27.6). The six-membered rings in these polycycles are still aromatic, but their NICSs (-2.5 (1) (a) Minkin, V. I.; Glukhovtsev, M. N.; Simkin, B. Y. Aromaticity and Antiaromaticity; Wiley: New York, 1994. (b) Garratt, P. J. Aromaticity; Wiley: New York, 1986. (c) Eluidge, J. A.; Jackman, L. M. J. Chem. Soc. 1961, 859. (2) Schleyer, P. v. R.; Jiao, H. Pure Appl. Chem. 1996, 28, 209. (3) Pople, J. A.; Untch, K. G. J. Am. Chem. Soc. 1966, 88, 4811. (4) Jiao, H; Schleyer, P. v. R. AIP Conference Proceedings 330, E.C.C.C.1, Computational Chemistry; Bernardi, F., Rivail, J.-L., Eds.; American Institute of Physics: Woodbury, New York, 1995; p 107. (5) (a) Schleyer, P. v. R.; Freeman, P.; Jiao, H.; Goldfuss, B. Angew. Chem., Int. Ed. Engl. 1995, 34, 337. (b) Jiao, H.; Schleyer, P. v. R. Unpublished IGLO results. (c) Kutzelnigg, W.; Fleischer, U.; Schindler, M. In NMR: Basic Princ. Prog.; Springer: Berlin, 1990; Vol. 23, p 165. (6) Dauben, H. J., Jr.; Wilson, J. D.; Laity, J. L. In Non-Benzenoid Aromatics; Synder, J., Ed.; Academic Press, 1971; Vol. 2, and references cited. The partitioning of ring current or ring current susceptabilitites among various rings in polycyclic syestems were considered earlier, e.g., by Aihara (Aihara, J. J. Am. Chem. Soc. 1985, 207, 298 and refs cited) and by Mallion (Haigh, C. W.; Mallion, J. Chem. Phys. 1982, 76, 1982). (7) Fleischer, U.; Kutzelnigg, W.; Lazzeretti, P.; Mühlenkamp, V. J. Am. Chem. Soc. 1994, 116, 5298. (8) Sondheimer, F. Acc. Chem. Res. 1972, 5, 81. (9) (a) Hunandi, R. J. J. Am. Chem. Soc. 1983, 105, 6889. (b) Pascal, R. A., Jr.; Winans, C. G.; Van Engen, D. J. Am. Chem. Soc. 1989, 111, 3007. (10) (a) Bühl, M.; Thiel, W.; Jiao, H.; Schleyer, P. v. R.; Saunders, M.; Anet, F. A. L. J. Am. Chem. Soc. 1994, 116, 7429 and references cited. (b) Bühl, M.; van Wüllen, C. Chem. Phys. Lett. 1995, 247, 63. The authors have shown that the negative absolute shielding in the center of C60 is nearly the same as δ3He, computed at the same level. (11) Paquette, L. A.; Bauer, W.; Sivik, M. R.; Bühl, M.; Feigel, M.; Schleyer, P. v. R. J. Am. Chem. Soc. 1990, 112, 8776. (12) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T. A.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94, ReVision B.2; Gaussian Inc., Pittsburgh, PA, 1995. Figure 1. Plot of NICSs (ppm) vs the aromatic stabilization energies (ASEs, kcal/mol)5a for a set of five-membered ring heterocycles, C4H4X (X ) as shown) (cc ) 0.966). 6317 J. Am. Chem. Soc. 1996, 118, 6317

Synthesis and Applications of RuCl2(CHR‘)(PR3)2: The Influence of the Alkylidene Moiety on Metathesis Activity

Abstract: The reactions of RuCl2(PPh3)3 with a number of diazoalkanes were surveyed, and alkylidene transfer to give RuCl2(CHR)(PPh3)2 (R = Me (1), Et (2)) and RuCl2(CH-p-C6H4X)(PPh3)2 (X = H (3), NMe2 (4), OMe (5), Me (6), F (7), Cl (8), NO2 (9)) was observed for alkyl diazoalkanes RCHN2 and various para-substituted aryl diazoalkanes p-C6H4XCHN2. Kinetic studies on the living ring-opening metathesis polymerization (ROMP) of norbornene using complexes 3−9 as catalysts have shown that initiation is in all cases faster than propagation (ki/kp = 9 for 3) and that the electronic effect of X on the metathesis activity of 3−9 is relatively small. Phosphine exchange in 3−9 with tricyclohexylphosphine leads to RuCl2(CH-p-C6H4X)(PCy3)2 10−16, which are efficient catalysts for ROMP of cyclooctene (PDI = 1.51−1.63) and 1,5-cyclooctadiene (PDI = 1.56−1.67). The crystal structure of RuCl2(CH-p-C6H4Cl)(PCy3)2 (15) indicated a distorted square-pyramidal geometry, in which the two phosphines are trans to each other, and the alkyli...

Electrochemical and photoelectrochemical investigation of single-crystal anatase

Abstract: Single crystals of TiO2 anatase containing 0.22% of Al and traces of V, Zr, Nb, and La were grown by chemical transport reactions employing TeCl4 as the transporting agent. Electrodes having the (101) face exposed doped by reduction with hydrogen were employed. The electrochemical and photoelectrochemical behavior of a single crystal of anatase were scrutinized for the first time. Properties were compared to those of single-crystal rutile having the (001) face exposed. Impedance analysis established that the flatband potential of anatase (101) is shifted negatively by 0.2 V with regards to that of rutile (001). Interfacial capacitance measurements under forward bias indicate smaller density of surface states on anatase. Photoelectrochemical oxidation of water occurs on both rutile and anatase with incident photon-to-current conversion efficiencies close to unity at λ = 300 nm. From the comparison of Ufb and Eg, it follows that anatase (101) and rutile (001) electrodes differ mainly in the position of the ...

Multiple Morphologies and Characteristics of “Crew-Cut” Micelle-like Aggregates of Polystyrene-b-poly(acrylic acid) Diblock Copolymers in Aqueous Solutions

Abstract: Crew-cut micelle-like aggregates of various morphologies prepared from polystyrene-b-poly(acrylic acid), PS-b-PAA, diblock copolymers under near-equilibrium conditions, were studied by transmission electron microscopy (TEM). The insoluble block (PS) contents in the copolymers ranged from 80 to 98 wt %. In spherical micelles, the micelle cores, formed by aggregation of the PS blocks, were generally monodisperse. A comparison between star and crew-cut micelles showed that the latter are distinguished by a low density of corona chains on the core surface and a low degree of stretching of the PS blocks in the cores. As the PAA content in block copolymer decreased, the morphology of the aggregates changed progressively from spheres to cylinders, to bilayers (both vesicles and lamellae), and eventually to compound micelles consisting of an assembly of inverted micelles surrounded by a hydrophilic surface. The compound micelles are believed to be a new morphology for block copolymers. The addition of homopolysty...

Influence of Peptide Composition, Gas-Phase Basicity, and Chemical Modification on Fragmentation Efficiency: Evidence for the Mobile Proton Model

Abstract: Relative energetics of fragmentation of protonated peptides are investigated by using electrospray ionization/surface-induced dissociation (ESI/SID) tandem mass spectrometry. ESI/SID fragmentation efficiency curves (percent fragmentation versus laboratory collision energy) are presented for 20 oligopeptides and are a measure of how easily a peptide fragments. The relative positions of the ESI/SID fragmentation efficiency curves depend on several parameters which include peptide composition (e.g., presence/absence of a basic amino acid residue) and peptide size. The ESI/SID fragmentation efficiency curves, in combination with quantum chemical calculations, provide a unique approach to substantiate and refine the mobile proton model for peptide fragmentation. Selected peptides are also investigated to further test and confirm the mobile proton model; these include doubly-protonated peptides and chemically-modified peptides (i.e., acetylated and fixed-charge derivatized peptides). Doubly-protonated peptides ...

Specific Intermolecular Interaction of Carbon Dioxide with Polymers

Abstract: Fourier transform IR spectroscopy has been used to investigate the interaction of carbon dioxide with polymers. IR transmission and attenuated total reflectance spectra were obtained for CO2 impregnated into polymer films. It has been shown that the polymers possessing electron-donating functional groups (e.g., carbonyl groups) exhibit specific interactions with CO2, most probably of Lewis acid−base nature. An unusual aspect is the use of the bending mode (ν2) of CO2 to probe polymer−CO2 interactions. The evidence of the interaction is the observation of the splitting of the band corresponding to the CO2 ν2 mode. This splitting indicates that the double degeneracy of the ν2 mode is removed due to the interaction of electron lone pairs of the carbonyl oxygen with the carbon atom of the CO2 molecule. This splitting has not been observed for polymers lacking electron-donating functional groups (e.g., poly(ethylene)). In contrast, the ν3 mode shows little if any sensitivity to this interaction, which is in ac...

Construction of Porous Solids from Hydrogen-Bonded Metal Complexes of 1,3,5-Benzenetricarboxylic Acid

Abstract: The reaction of M(II) acetate hydrate (M = Co, Ni, and Zn) with 1,3,5-benzenetricarboxylic (BTC) acid yields a material formulated as M3(BTC)2·12H2O. These compounds are isostructural as revealed by their XRPD patterns and a single crystal structure analysis performed on the cobalt containing solid [monoclinic, space group C2, a = 17.482 (6) A, b = 12.963 (5) A, c = 6.559 (2) A, β = 112.04°, V = 1377.8 (8) A, Z = 4]. This solid is composed of zigzag chains of tetra-aqua cobalt(II) benzenetricarboxylate that are hydrogen-bonded to yield a tightly held 3-D network. Upon liberating 11 water ligands per formula unit a porous solid results, M3(BTC)2·H2O, which was found to reversibly and repeatedly bind water without destruction of the framework. The proposed 1-D channels of the monohydrate have a pore diameter of 4 × 5 A, which is typical of those observed in zeolites and molecular sieves. The successful inclusion of ammonia into the porous solid was demonstrated. Larger molecules and others without a reactiv...

Macromolecule-like Aspects for a Colloidal Suspension of an Exfoliated Titanate. Pairwise Association of Nanosheets and Dynamic Reassembling Process Initiated from It

Abstract: A layered protonic titanate of lepidocrocite-type, HxTi2-x/4□x/4O4·H2O (x ∼ 0.7; □, vacancy), has been exfoliated on the action of an aqueous solution of tetrabutylammonium (hereafter TBA) hydroxide, which resulted in a stable colloidal suspension. A colloidal aggregate centrifuged from the suspension was examined by an in situ X-ray diffraction technique under conditions where drying speed was controlled. The diffraction immediately after separation from the liquid phase was principally amorphous except for a series of sharp reflections detected in a small angle scattering region. On the basis of the line profile analysis, the latter diffraction feature was accounted for by the fundamental intersheet interference of a spacing >10 nm, which demonstrates the existence of a novel associated pair of the titanate nanosheets accommodating a large volume of water cluster between them. These XRD data provide persuasive evidence for delamination into single layers. Upon drying, the amorphous halo disappeared and ...

The Nature and Geometry of Intermolecular Interactions between Halogens and Oxygen or Nitrogen

Abstract: The nature of intermolecular interactions between carbon-bonded halogens (C−X, X = F, Cl, Br, or I) and electronegative atoms (El = N, O and S) has been analysed, focusing on the role of specific attractive forces and the anisotropic repulsive wall around halogen atoms. Searches of the Cambridge Structural Database show that electronegative atoms in various hybridization states clearly prefer to form contacts to Cl, Br, and I (but not F) in the direction of the extended C−X bond axis, at interatomic distances less than the sum of the van der Waals radii. Ab initio intermolecular perturbation theory calculations show that the attractive nature of the X···El interaction is mainly due to electrostatic effects, but polarization, charge-transfer, and dispersion contributions all play an important role. The magnitude of the interaction for the chloro-cyanoacetylene dimer is about 10 kJ/mol, demonstrating the potential importance of these kinds of nonbonded interactions. The directionality of the interaction is ...

T-shaped molecular building units in the porous structure of ag(4,4'-bpy).no3

Abstract: and magnetic 4 properties. One of the simplest strategies employed in the production of such 3-D networks is schematically illustrated in Figure 1a,b, where open-framework diamondlike networks have been achieved by the addition copolymerization of either rod-like ligands 5a-c or metal clusters with tetrahedral metal ions. 2c,5d,e In this report, we show how the tendency of silver(I) for linear coordination may be exploited 5f,g

Energy transfer in dendritic macromolecules: Molecular size effects and the role of an energy gradient

Abstract: Perylene-terminated monodendrons 1−7 and phenyl-terminated reference monodendrons 8−14 have been synthesized, and the intramolecular energy transfer has been studied using steady-state as well as time-resolved fluorescence spectroscopy. In the series 2−7, the light-harvesting ability of these compounds increases with increasing generation due to the increase in molar extinction coefficient. However, the efficiency of the energy transfer decreases with increasing generation in this series. With increasing generation, the photoluminescence intensity from the perylene core still increases and the expected level-off in the photoluminescence intensity has not been reached in this series of compounds. Dendrimer 1 is unique in that the energy transfer in this molecule occurs at a very fast rate. The rate constant for energy transfer in 1 is at least 2 orders of magnitude larger than in 2−7. In contrast to monodendrons 2−7, 1 possesses a variable monomer type at each generation that creates an energy funnel. The ...

Structure of the [Nife] Hydrogenase Active Site: Evidence for Biologically Uncommon Fe Ligands

Abstract: Crystallographic data on the [NiFe] hydrogenase from Desulfovibrio gigas are presented that provide new information on the structure and mode of action of its dihydrogen activating metal center. Recently we found this center to contain, besides Ni, a second metal ion which was tentatively assigned to Fe (Volbeda, A.; Charon, M. H.; Piras, C.; Hatchikian, E. C.; Frey, M.; Fontecilla-Camps, J. C. Nature 1995, 373, 580−587). This assignment is now unambiguously confirmed by a crystallographic analysis using 3 A resolution X-ray data collected at wavelengths close to either side of the Fe absorption edge. Moreover, we report the structure of another crystal form of the as-purified D. gigas hydrogenase refined at 2.54 A resolution, showing that the active site Fe binds three diatomic ligands. The electron density map shows an additional small peak at a position bridging the two active site metal ions, which may be assigned to some form of oxygen. This bridging oxygen species is proposed to be the signature of ...

Tuning the Excited-State Properties of Platinum(II) Diimine Dithiolate Complexes

Abstract: Two series of Pt(diimine)(dithiolate) complexes have been prepared in order to investigate the effects of molecular design on the excited-state properties of this chromophore. The first series comprises Pt(dbbpy)(dithiolate) complexes where dbbpy = 4,4‘-di-tert-butyl-2,2‘-bipyridine and the dithiolates are 1-(tert-butylcarboxy)-1-cyanoethylene-2,2-dithiolate (tbcda), 1-diethylphosphonate-1-cyanoethylene-2,2-dithiolate (cpdt), 6,7-dimethyl-quinoxaline-2,3-dithiolate (dmqdt), maleonitriledithiolate (mnt), and toluene-3,4-dithiolate (tdt). The second series comprises Pt(diimine)(tdt) complexes where the diimines are 3,4,7,8-tetramethyl-1,10-phenanthroline (tmphen), 4,4‘-di-tert-butyl-2,2‘-bipyridine (dbbpy), 4,4‘-dimethyl-2,2‘-bipyridine (dmbpy), 2,2‘-bipyridine (bpy), 1,10-phenanthroline (phen), 5-chloro-1,10-phenanthroline (Cl-phen), 4,4‘-dichloro-2,2‘-bipyridine (Cl2bpy), and 4,4‘-bis(ethoxycarbonyl)-2,2‘-bipyridine (EC-bpy). All of the compounds display solvatochromic absorption bands and solution lumine...

Polymer Light-Emitting Electrochemical Cells: In Situ Formation of a Light-Emitting p-n Junction.

Abstract: Solid-state polymer light-emitting electrochemical cells have been fabricated using thin films of blends of poly(1,4-phenylenevinylene) and poly(ethylene oxide) complexed with lithium trifluoromethanesulfonate. The cells contain three layers: the polymer film (as the emissive layer) and indium-tin oxide and aluminum films as the two contact electrodes. When externally biased, the conjugated polymers are p-doped and n-doped on opposite sides of the polymer layer, and a light-emitting p-n junction is formed in between. The admixed polymer electrolyte provides the counterions and the ionic conductivity necessary for doping. The p-n junction is dynamic and reversible, with an internal built-in potential close to the band gap of the redox-active conjugated polymer (2.4 eV for PPV). Green light emitted from the p-n junction was observed with a turn-on voltage of about 2.4 V. The devices reached 8 cd/m(2) at 3 V and 100 cd/m(2) at 4 V, with an external quantum efficiency of 0.3-0.4% photons/electron. The response speed of these cells was around 1 s, depending on the diffusion of ions. Once the light-emitting junction had been formed, the subsequent operation had fast response (microsecond scale or faster) and was no longer diffusion-controlled.

Structural and Dynamic Parameters Obtained from 17O NMR, EPR, and NMRD Studies of Monomeric and Dimeric Gd3+ Complexes of Interest in Magnetic Resonance Imaging: An Integrated and Theoretically Self-Consistent Approach1

Abstract: We present the results of new and previously published 17O NMR, EPR, and NMRD studies of aqueous solutions of the Gd3+ octaaqua ion and the commercial MRI contrast agents [Gd(DTPA)(H2O)]2- (MAGNEVIST, Schering AG, DTPA = 1,1,4,7,7-pentakis(carboxymethyl)-1,4,7-triazaheptane), [Gd(DTPA-BMA)(H2O)] (OMNISCAN, Sanofi Nycomed, DTPA-BMA = 1,7-bis[(N-methylcarbamoyl)methyl]-1,4,7-tris(carboxymethyl)-1,4,7-triazaheptane), and [Gd(DOTA)(H2O)]- (DOTAREM, Guerbet, DOTA = 1,4,7,10-tetrakis(carboxymethyl)-1,4,7,10-tetraazacyclododecane). High-field EPR measurements at different concentrations give evidence of an intermolecular dipole−dipole electronic relaxation mechanism that has not previously been described for Gd3+ complexes. For the first time, the experimental data from the three techniques for each complex have been treated using a self-consistent theoretical model in a simultaneous multiple parameter least-squares fitting procedure. The lower quality of the fits compared to separate fits of the data for each o...

Theoretical Studies of GG-Specific Photocleavage of DNA via Electron Transfer: Significant Lowering of Ionization Potential and 5‘-Localization of HOMO of Stacked GG Bases in B-Form DNA

Abstract: Ab initio molecular orbital calculations of stacked DNA bases were performed at the 3-21G(*) and 6-31G* levels to elucidate the origin of the 5‘-GG-3‘ sequence specificity for the photocleavage of DNA in the presence of electron-accepting photosensitizers. Ionization potentials (IP) were estimated as Koopman's theorem values for 16 sets of two stacked nucleobases and seven sets of stacked nucleobase pair systems in a B-form geometry. It was found that the GG/CC system is the lowest among the 10 possible stacked nucleobase pairs and that approximately 70% of the HOMO is localized on the 5‘-G of 5‘-GG-3‘. These calculations indicate that the 5‘-G of 5‘-GG-3‘ is the most electron donating site in B DNA and suggest that one-electron transfer from DNA to an electron acceptor occurs most effectively at 5‘-GG-3‘ sites which are fully consistent with the experimental data. In order to know the fate of the cation radical, the vertical IPs were estimated for seven stacked nucleobase pairs. It was found that the GG/...

Structural and Functional Aspects of RGD-Containing Cyclic Pentapeptides as Highly Potent and Selective Integrin αVβ3 Antagonists

Abstract: The αVβ3 integrin is implicated in human tumor metastasis and in angiogenesis. The design of low-molecular-mass αVβ3 antagonists by “spatial screening” led to the highly active peptides c(RGDFV) and c(RGDFV). Here the influence of the amino acids in positions 4 and 5 flanking the RGD-sequence on the inhibition of vitronectin and fibrinogen binding to the isolated αVβ3 and αIIbβ3 receptors was investigated. The influence of the side chain and the backbone conformation on activity and selectivity was studied. The compounds were divided into conformational classes. For each class at least one representative peptide was subjected to detailed structure determination in solution. The peptides of classes 1, 2, and 3 show a βII‘/γ-turn arrangement with the d-amino acid in the i + 1 position of the βII‘-turn. By contrast, the peptides of class 4 reveal a modified βII‘/γ-turn pattern with glycine in the i + 1 position of the βII‘-turn and the d-amino acid in the i + 1 position of the γ-turn. Class 1 is divided into...

Self-Assembling Peptide Nanotubes

Abstract: The general design criteria and synthesis of four new peptide-based solid-state tubular array structures are described. Peptide nanotubes, which are extended tubular β-sheet-like structures, are constructed by the self-assembly of flat, ring-shaped peptide subunits made up of alternating d- and l-amino acid residues. Peptide self-assembly is directed by the formation of an extensive network of intersubunit hydrogen bonds. In the crystal structures, nanotubes are stabilized by intertubular hydrophobic packing interactions. Peptide nanotubes exhibit good mechanical and thermal stabilities in water and are stable for long periods of times in most common organic solvents including DMF and DMSO. The remarkable stability of peptide nanotubes can be attributed to the highly cooperative nature of the noncovalent interactions throughout the crystal lattice. Nanotube structures were characterized by cryoelectron microscopy, electron diffraction, Fourier-transform infrared spectroscopy, and crystal structure modelin...

Sonochemical Synthesis of Iron Colloids

Abstract: We present here a new method for the preparation of stable ferromagnetic colloids of iron using high-intensity ultrasound to sonochemically decompose volatile organometallic compounds. These colloids have narrow size distributions centered at a few nanometers and are found to be superparamagnetic. In conclusion, a simple synthetic method has been discovered to produce nanosized iron colloid using high-intensity ultrasound. Nanometer iron particles dispersed in polyvinylpyrrolidone (PVP) matrix or stabilized by adsorption of oleic acid have been synthesized by sonochemical decomposition of Fe(CO){sub 5}. Transmission electron micrographs show that the iron particles have a relatively narrow range in size from 3 to 8 nm for polyvinylpyrrolidone, while oleic acid gives an even more uniform distribution at 8 nm. magnetic measurements revealed that these nanometer iron particles are superparamagnetic with a saturation magnetization of 101 emu/g (Fe) at 290 K. This work is easily extended to colloids of other metals and to alloys of two or more metals, simply by using multiple volatile precursors. 29 refs., 4 figs.