Showing papers in "Journal of the American Chemical Society in 1993"

Synthesis and characterization of nearly monodisperse CdE (E = sulfur, selenium, tellurium) semiconductor nanocrystallites

Abstract: A simple route to the production of high-quality CdE (E=S, Se, Te) semiconductor nanocrystallites is presented. Crystallites from ∼12 A to ∼115 A in diameter with consistent crystal structure, surface derivatization, and a high degree of monodispersity are prepared in a single reaction. The synthesis is based on the pyrolysis of organometallic reagents by injection into a hot coordinating solvent. This provides temporally discrete nucleation and permits controlled growth of macroscopic quantities of nanocrystallites. Size selective precipitation of crystallites from Portions of the growth solution isolates samples with narrow size distributions (<5% rms in diameter). High sample quality results in sharp absorption features and strong «band-edge» emission which is tunable with particle size and choice of material

Conversion of light to electricity by cis-X2bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) charge-transfer sensitizers (X = Cl-, Br-, I-, CN-, and SCN-) on nanocrystalline titanium dioxide electrodes

Abstract: cis-X 2 Bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) complexes (X=Cl - , Br, I - , CN - , and SCN - ) were prepared and characterized with respct to their absorption, luminescence, and redox behavior. They act as efficient charge-transfer sensitizers for nanocrystalline TiO 2 films (thickness 8-12 μm) of very high internal surface area (roughness factor ca. 1000), prepared by sintering of 15-30-nm colloidal titania particles on a conducting glass support. The performance of cis-di(thiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) (1) was found to be outstanding and is unmatched by any other known sensitizer

High-spin molecules: [Mn12O12(O2CR)16(H2O)4]

Abstract: The syntheses and electrochemical and magnetochemical properties of [Mn 12 O 12 (O 2 CPh) 16 (H 2 O) 4 ] (3), its solvate 3.PhCOOH-CH 2 Cl 2 , and [Mn 12 O 12 (O 2 CMe) 16 (H 2 O) 4 ].MeCOOH-3H 2 O (4) are reported. Complex 3 can be prepared either by reaction of Mn(OAc) 2 .4H 4 O, benzoic acid, and NBu n 4 MnO 4 in pyridine or by reaction of PhCOOH with complex 4 slurried in CH 2 Cl 2 .

Adsorption of proteins onto surfaces containing end-attached oligo(ethylene oxide): a model system using self-assembled monolayers

Abstract: This paper reports a study of the adsorption of four proteins-fibrinogen, lysozyme, pyruvate kinase, and RNAse A-to self-assembled monolayers (SAMs) on gold. The SAMs examined were derived from thiols of the structure HS(CH 2 ) 10 R, where R was CH 3 , CH 2 OH, and oligo(ethylene oxide). Monolayers that contained a sufficiently large mole fraction of alkanethiolate groups terminated in oligo(ethylene oxide) chains resisted the kinetically irreversible, nonspecific adsorption of all four proteins. Longer chains of oligo(ethylene oxide) were resistant at lower mole fractions in the monolayer. Resistance to the adsorption of proteins increased with the length of the oligo(ethylene oxide) chain: the smallest mole fraction of chains that prevented adsorption was proportional to n -0.4 , where n represents the number of ethylene oxide units per chain

Application of RESP charges to calculate conformational energies, hydrogen bond energies, and free energies of solvation

Abstract: We apply a new restrained electrostatic potential fit charge model (two-stage RESP) to conformational analysis and the calculation of intermolecular interactions Specifically, we study conformational energies in butane, methyl ethyl thioether, three simple alcohols, three simple amines, and 1,2-ethanediol as a function of charge model (two-stage RESP us standard ESP) and 1-4 electrostatic scale factor We demonstrate that the two-stage RESP model with a 1-4 electrostatic scale factor of ∼1/12 is a very good model, as evaluated by comparison with high-level ab initio calculations For methanol and N-methylacetamide interactions with TIP3P water, the two-stage RESP model leads to hydrogen bonds only slightly weaker than found with the standard ESP changes

Addition of azomethine ylides to C60: synthesis, characterization, and functionalization of fullerene pyrrolidines

Abstract: We report herein a new and very general fullerene functionalization, based on the 1,3-dipolar cycloaddition of azomethine ylides to C[sub 60]. Azomethine ylides, planar species of general formula (R[sup 1]R[sup 2])-C=N[sup +](R[sup 3]) - C[sup [minus]](R[sup 4]R[sup 5]), represent one of the most reactive and versatile classes of 1,3-dipoles. They can be generated from a wide variety of easily accessible starting materials and react readily with a range of dipolarophiles. The products of cycloaddition, substituted pyrrolidines, are well suited for further functionalization. A very easy way of generating azomethine ylides is the [open quotes]decarboxylation route.[close quotes] Another approach to azomethine ylides, the thermal ring-opening of aziridines, gave the same profitable results. The present new fullerene functionalization offers high potential in the materials chemistry field. Of the several uses that can be envisioned, two representative examples are reported. 17 refs., 1 fig.

Quantitative J correlation: a new approach for measuring homonuclear three-bond J(HNH.alpha.) coupling constants in 15N-enriched proteins

Abstract: A new approach is described for the measurement of homonuclear H N -H α J couplings in 15 N-enriched proteins. The method relies on measurement of the diagonal-peak to cross-peak intensity ratio in a 3D 15 N-separated quantitative J-correlation spectrum. The experiment is demonstrated for the protein staphylococcal nuclease, uniformly enriched with 15 N, ligated with thymidine 3'-5'-bisphosphate and Ca 2+ . Amide to C α H correlations are observed for all but three of the amide protons that have regular intensities in a 2D 1 H- 15 N correlation spectrum, and J couplings could be measured for 96 residues with sufficiently resolved 1 H- 15 N correlations, including 6 glycines

Gemini surfactants: a new class of self-assembling molecules

Abstract: «Gemini surfactant» is a name assigned to a family of synthetic amphiphiles possessing, in sequence, a long hydrocarbon chain, an ionic group, a spacer, a second ionic group, and another hydrocarbon tail. Intramolocular chain/chain association was inhibited through the use of rigid spacers, thereby averting self-assembly into conventional micellar structures. Aggregation of the geminis was investigated by (a) surface tension, (b) film-balance methods, (c) dynamic light scattering, (d) 1 H and 23 Na NMR, and (e) spectral changes in an adsorbed dye

Structure and binding of alkanethiolates on gold and silver surfaces: implications for self-assembled monolayers

Abstract: We performed ab initio geometry optimization of HS and CH 3 S on cluster models of Au(111), Au(100), Ag(111), and Ag(100) surfaces, at the RECP Hartree-Fock+electron correlation (MBPT2) level. From these we determined classical force field parameters, thus opening the possibility for realistic molecular dynamics (MD) simulations of self-assembled alkanethiolate monolayers on gold and silver surfaces. We find that there are two chemisorption modes, very close in energy, for thiolates on Au(111) surfaces. In the first, the surface-S-C bond angle is ∼180 o (sp hybridization), while in the second it is ∼104 o (spα 3 hybridization). This suggests a possible mechanism for the annealing of alkanethiolate monolayers

Inhibition of the HIV-1 protease by fullerene derivatives: model building studies and experimental verification

Abstract: The ability of C[sub 60] fullerene ([open quotes]Bucky Ball[close quotes]) derivatives to interact with the active site of HIV-1 protease (HIVP) has been examined through model building and simple physical chemical analysis. The model complexes generated via the program DOCK3 suggest that C[sub 60] derivatives will fit snugly in the active site, thereby removing 298 A[sup 2] of primarily nonpolar surface from solvent exposure and driving ligand/protein association. The prediction that these compounds should bind to the active site and thereby act as inhibitors has been borne out by the experimental evidence. Kinetic analysis of HIVP in the presence of a water-soluble C[sub 60] derivative, bis(phenethylamino-succinate) C[sub 60], suggests a competitive mode of inhibition. This is consistent with and supports the predicted binding mode. Diamino C[sub 60] has been proposed as a [open quotes]second-generation[close quotes] C[sub 60] derivative that will be able to form salt bridges with the catalytic aspartic acids in addition to Van der Waals contacts with the nonpolar HIVP surface, thereby improving the binding relative to the tested compound. 15 refs., 6 figs., 1 tab.

The Importance of Not Saturating H2o in Protein NMR : application to Sensitivity Enhancement and Noe Measurements

Abstract: It has long been recognized that presaturation of the water resonance is undesirable in many types of protein and nucleic acid NMR experiments, as it obliterates the resonances of rapidly exchanging protons and, via spin diffusion or direct NOE with water, also attenuates the entire 'H spectrum.' A commonly used procedure which avoids the necessity of presaturation uses inhomogeneity of either the radiofrequency fieldz.3 or the static magnetic field4,5 to "scramble" the H2O magnetization during the NMR pulse sequence. Unfortunately, due to the very long TI of H20 protons (4-5 s) compared to protein protons (1.4 s), the water remains in a semisaturated state as the delay time between scans is typically much shorter than the HzO T I . The present Communication describes an approach which, analogous to the original Redfield method,'* leaves the water unperturbed along the Bo axis during most of the NMR pulse sequence, thereby avoiding 'H20 saturation. The approach benefits many of the heteronuclear multidimensional NMR experiments and is demonstrated for 'H-I5N HSQC correlatioq6 for quantitative measurement of the 15N-('HJ NOE, and for the measurement of NOES between HzO and 'Cattached proton~.~#8 Figure 1A shows the pulse scheme for a nonsaturated IH-I5N HSQC correlation. Immediately after the I H N magnetization is transformed into H,N, magnetization (time a) , the H2O magnetization is rotated from they to the -z axis by the subsequent low-power 'H 90' pulse. The pulsed field gradient, GI, ensures that no radiation damping occurs, i.e., the HzO magnetization remains along -z until, t1/2 later, the IH 180' pulse returns it to +z. The final lH 90°, pulse (time b) is preceded by a shaped 90°-, pulse on H20, and, similarly, the final IH 180' pulse is surrounded by low-power 90°, pulses and by pulsed field gradients (G3).9 This leaves the HzO magnetization in a well-defined state along the z axis, providing excellent water suppression. Results are shown for the backbone amides of Leu69-Arg90 in a 1.2 mM sample of [15N]calmcdulin (CaM) complexed with a peptide fragment, known as M 13, of skeletal muscle myosin light chain kinase. Figure 2A shows the decrease in IH-lSN HSQC correlation intensity observed using H20 presaturation (filled circles) or a scrambling pulse3 (open circles) relative to the water flip-back HSQC experiment of Figure 1A. In theNMR structure of the CaM/M13 complex,lo residues 61-13 and 83-92 are in a a-helical arrangement, whereas Arg74-G1~82 form a solvent-

Molecular modeling of the kinetic isotope effect for the [1,5]-sigmatropic rearrangement of cis-1,3-pentadiene

Abstract: The primary kinetic isotope effect for the sigmatropic rearrangement reaction of cis-1,3-pentadiene is studied by the direct dynamics method. The calculations are carried out with the computer code MORATE, which combines the semiempirical molecular orbital package, MOPAC, and the polyatomic dynamics code, POLYRATE, developed previously by the authors' research group. Dynamics calculations are based on canonical variational transition-state theory including multidimensional tunneling corrections. The force field is obtained by molecular orbital theory with the AM1, PM3, and MINDO/3 parameterizations. The kinetic isotope effects calculated with the MINDO/3 and PM3 Hamiltonians agree with those calculated by AM1 within 13%, and the latter agree with experiment within 13%. The tunneling contributions to the kinetic isotope effects are analyzed, and the nature of the vibrationally assisted tunneling process is discussed. General features of the dynamics from all three parameterizations are similar, and the quantitative differences in the predictions of the three calculations can be understood in terms of global characteristics of the potential energy functions that they predict. 41 refs., 8 figs., 6 tabs.

Porphyrin Assemblies On DNA As Studied By A Resonance Light-Scattering Technique

Abstract: Under appropriate conditions of concentration and ionic strength, trans-bis(N-methylpyridinium-4-yl)-diphenylporphine and its copper(II) derivative produce very large, bisignate circular dichroism signals in the Soret region when bound to DNA. The processes leading to these signals are shown to be highly cooperative. A new light-scattering experimental approach is described for detecting such extended aggregates of chomophores in which the radiation used is within an absorption maximum, specifically in the porphyrin Soret absorption region. Results from such resonance light-scattering experiments confirm that extended porphyrin aggregates form on the DNA, which, in contrast, remains dispersed

Molecular engineering of organic semiconductors: design of self-assembly properties in conjugated thiophene oligomers

Abstract: In order to analyze the correlation between charge transport and structural properties in conjugated oligomers, sexithiophene, 6T, was substituted by hexyl groups, both on the terminal α positions (α,ωDH6T) and as pendant groups in the β position (β,β'DH6T). Structural characterizations by X-ray diffraction show that vacuum-evaporated thin films of 6T and α,ωDH6T consist of layered structures in a monoclinic arrangement, with all-trans planar molecules standing on the substrate. When compared to 6T, α,ωDH6T is mainly characterized by a very large increase of molecular organization at the mesoscopic level, evidenced by a much longer range ordering

Tocopherol-mediated peroxidation. The prooxidant effect of vitamin E on the radical-initiated oxidation of human low-density lipoprotein

Abstract: Oxidation of human low-density lipoprotein (LDL) is implicated as an initiator of atherosclerosis. α-Tocopherol (α-TocH) may thus inhibit atherosclerosis beause it is the major and most active chain-breaking antioxidant in extracted LDL lipid. Our studies show, however, that α-TocH can be a strong prooxidant for the LDL itself, i.e., an aqueous dispersion of lipid-bearing particles. Thus, a steady flux (R g ) of alkylperoxyl radicals (ROO . ) generated from a water-soluble azo initiator induced lipid peroxidation in LDL which was faster in the presence of α-TocH than in its absence (for R g <2 NM S -1 ), insensitive to R g and [O 2 ], and inhibited by vitamin C, ubiquinol-10 (normally present in fresh LDL), and small phenolic antioxidants but not inhibited by the aqueous radical scavenger uric acid

DNA binding studies and site selective fluorescence sensitization of an anthryl probe

Abstract: (9-Anthrylmethyl)ammonium chloride (AMAC, 1) binds to natural and synthetic DNA sequences with a high affinity, as deduced from the absorption and fluorescence spectral data. Scatchard plots constructed from these data gave binding constants in the range (2-8)×10 4 M -1 of base pairs. Extensive hypochromism, broadening, and red shifts in the absorption spectra were observed when AMAC binds to various sequences of synthetic and natural DNA. Upon binding to DNA, the fluorescence from the anthryl chromophore was efficiently quenched by the DNA bases and the fluorescence spectra at high concentrations of CT DNA show significant broadening of the vibronic bands

Dna-binding of delta- ru(phen)2dppz 2+ and lambda- ru(phen)2dppz 2+

Abstract: Linear dichroism (LD) spectroscopy and steady-state as well as time-resolved luminescence spectroscopy have been used to investigate the interaction of the DELTA and LAMBDA enantiomers of Ru(phen)2DPPZ2+ (phen = 1,10-phenanthroline; DPPZ = dipyrido[3,2-a:2',3'-c]phenazine) with DNA. The pure enantiomers, which were difficult to separate by traditional resolving methods, were synthesized via a chiral precursor. Changes in luminescence, isotropic absorption and excited state lifetimes upon binding, and the LD observed in flow-oriented DNA systems provide detailed information about the DNA binding of the enantiomers. Flow LD shows that both enantiomers bind to DNA in a well-defined manner with an orientation of the dipyridophenazine chromophore consistent with intercalation of this moiety between base-pairs. Both enantiomers are found to show luminescence in the presence of DNA to which they bind very strongly (K almost-equal-to 10(8) M-1); however, the relative luminescence quantum yield of the bound DELTA enantiomer is 6-10 times larger than that of the bound LAMBDA enantiomer. Furthermore, for each enantiomer two distinct excited state lifetimes are found in varying proportions depending on the binding ratio. The large difference in luminescence quantum yield between the enantiomers is interpreted in terms of slightly different intercalation geometries of the dipyridophenazine ligand, resulting in different protections from quenching by solvent water and diastereomeric differences in the interactions between enantiomers bound in contigue on DNA.

Self-orienting head-to-tail poly(3-alkylthiophenes): new insights on structure-property relationships in conducting polymers

Abstract: It is found that head to tail poly(3-alkylthiophene)s (PAT) undergo solid-state macromolecular self-assembly and give self-oriented structures, as determined by X-ray diffraction studies. In addition, it is found that PATs with longer alkyl side chains (e.g. C 12 H 25 ) produce the structures with the most desirable electronic, electrochemical and optical properties and induce the formation of planar main-chain structures

Hydrolysis of tri-tert-butylaluminum: the first structural characterization of alkylalumoxanes [(R2Al)2O]n and (RAlO)n

Abstract: tert-Butyl-substituted alumoxanes have been prepared and characterized by multinuclear magnetic resonance spectroscopy, mass spectrometry, and X-ray crystallography. The low-temperature (-78 o C) hydrolysis of Al( t Bu) 3 in pentane results in the formation of the trimeric hydroxide [( t Bu) 2 Al(μ-OH)] 3 (1) as the major product. Hydrated salt hydrolysis of Al( t Bu) 3 in toluene followed by thermolysis of the reaction mixture yields the tetrameric alumoxane [( t Bu) 2 Al{μ-OAl( t Bu) 2 }] 2 (2) and the octameric alumoxane [( t Bu)Al(μ 3 -O)] 9 (5)

Photoinduced biochemical activity of fullerene carboxylic acid

Abstract: Here we report the preparation of a water-miscible fullerene carboxylic acid (2) and its biological activity-cytotoxicity and G-selective DNA cleaving ability. What is truly remarkable is that the biological activity of C{sub 60} was observed only under irradiation with visible light and not in the dark, suggesting that fullerenes may serve as useful photosensitive biochemical probes. We have found, for the first time, that even low-energy visible light is surfficient to induce biological activity in fullerene derivatives. Among the numerous implications of the present findings, the most exciting prospect includes the use of fullerene derivatives for photodynamic therapy. 18 refs., 2 figs., 1 tab.

Preparation and use of C2-symmetric bis(phospholanes): production of .alpha.-amino acid derivatives via highly enantioselective hydrogenation reactions

Abstract: A new class of chiral C 2 -symmetric bis(phospholane) ligands has been prepared and used in rhodium-catalyzed asymmetric hydrogenation reactions. We describe a practical, one-pot procedure which utilizes enantiomerically pure 1,4-diol cyclic sulfates 4 of the preparation of a homochiral series of 1,2-bis(phopholano)ethanes 1 and 1,2-bis(phospholano)benzenes (DuPHOS) 2. Cationic rhodium complexes bearing these new ligands behave as very efficient catalyst precursors for the asymmetric hydrogenation of a broad range of α-(N-acylamino)acrylate (enamide) substrates 5

Synthesis of copper metallic clusters using reverse micelles as microreactors

Abstract: Reverse micelles are used to synthesize in situ nanometallic copper particles. In mixed reverse micelles with hydrazine as a reducing agent, metallic copper particles are formed. At low water content, small particles are obtained. With increasing water content, the size of the metallic cluster increases from 2 to 10 nm. The use of pure copper surfactant molecules instead of mixed micelles favors the formation of cylindrical metallic particles. By using sodium borohydride as a reducing agent, in the absence of oxygen and at low water content, large metallic copper particles (from 20 to 28 nm) are formed. With increasing water content, the metallic clusters progressively disappear, forming instead copper oxide particles

Structural comparison of self-assembled monolayers of n-alkanoic acids on the surfaces of silver, copper, and aluminum

Abstract: Self-assembled monolayers of n-alkanoic acid, (CH 3 (CH 2 ) m COOH, m=2-18, 22), were formed on the native oxide surfaces of Ag, Cu, and Al. The structure of the monolayers was probed with optical ellipsometry, contact angle measurement, and reflection absorption IR spectroscopy. The results suggest that the structure of the absorbed film strongly depends on the metal substrate, as well as the chain length of the acid involved. The metal substrate dominates the binding geometry of the head group and probably the packing density. The chain length also affects the packing density and the conformation of the molecular chain

Bis(oxazoline)-copper complexes as chiral catalysts for the enantioselective aziridination of olefins

Abstract: A recent study from this laboratory suggests that soluble copper complexes are the metal catalysts of choice2 in the aziridination of olefins with (N-@-toluenesulfonyl)imino)phenyliodinane, PhI=NTs ( l ) . 3 Preliminary evidence also indicates that chiral 4,4'-disubstituted bis(oxazolines), which are excellent ligands for the copper-catalyzed cyclopropanation of olefins, might also be effective in enantioselective aziridination reactions.495 The purpose of this communication is to report our interim studies directed toward the development of this enantioselective process. Our initial results demonstrate that copper complexes of bis(oxazolines) 2a-e are efficient catalysts for the asymmetric aziridination of olefins (eq 1). We report here our evaluation of

Synthesis and RGD peptide modification of a new biodegradable copolymer: poly(lactic acid-co-lysine)

Abstract: Poly(L-lactic acid-co-L-lysine) [OCH(CH 3 )COOCH(CH 3 )CO] m [OCH(CH 3 )CONHCH([CH 2 ] 4 NH 2 )CO] n was synthesized by ring opening copolymerization of L,L-lactide with an urethane-protected 2,5-morpholinedione monomer. The polymer films degraded to half their original M w in 5 weeks when exposed to pH 7.1 in phosphate-buffered saline at 37°C with agitation.