Intramolecular interactions and intramolecular hydrogen bonding in conformers of gaseous glycine

Ab initio calculations at the MP2/6‐311++G** level of theory led recently to the identification of 13 stable conformers of gaseous glycine with relative energies within 11 kcal/mol. The stability of every structure depends on subtle intramolecular effects arising from conformational changes. These intramolecular interactions are examined with the tools provided by the Atoms In Molecules (AIM) theory, which allows obtaining a wealth of quantum mechanics information from the molecular electron density ρ( r ). The analysis of the topological features of ρ( r ) on one side and the atomic properties integrated in the basins defined by the gradient vector field of the density on the other side makes possible to explore the different intramolecular effects in every conformer. The existence of intramolecular hydrogen bonds on some conformers is demonstrated, while the presence of other stabilizing interactions arising from favorable conformations is shown to explain the stability of other structures in the potential energy surface of glycine. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 702–716, 2001     

Fibrillar morphologies of hydrogels obtained from a lamellar medium

Hydrogels are suitable for multiple applications and their properties are strongly dependent on their morphology. Sponge-like morphologies are obtained in conventional hydrogels when the polymerization is performed in isotropic media. Fibrillar morphologies imparting new properties to hydrogels are expected when the reacting medium is anisotropic. Here, we synthesize such fibrillar hydrogels by polymerization in a lamellar medium formed by 1,4-bis(2-ethylhexyl)sodium sulfosuccinate (AOT) and water. At high surfactant content this objective is achieved, and the thickness of the obtained fibrils (0.1–1 μm) can be correlated with the physical properties of the lamellar system. At intermediate AOT concentrations, the morphology is hierarchical, with primary closed pores containing a secondary fibrillar structure that fills the pores. For low AOT concentrations, the crosslinking process is unhindered, and the hydrogels are mechanically consistent with a sponge-like morphology having large (10–10² μm) void pores. The time evolution of the mesophase as polymerization advances is followed by small angle X-ray scattering.     

LocaPep: localization of epitopes on protein surfaces using peptides from phage display libraries

The use of peptides from a phage display library selected by binding to a given antibody is a widespread technique to probe epitopes of antigenic proteins. However, the identification of interaction sites mimicked by these peptides on the antigen surface is a difficult task. LocaPep is a computer program developed to localize epitopes using a new clusters algorithm that focuses on protein surface properties. The program is constructed with the aim of providing a flexible computational tool for predicting the location of epitopes on protein structures. As a first set of testing results, the localization of epitope regions in eight different antigenic proteins for which experimental data on their antibody interactions exist is correctly identified by LocaPep. These results represent a disparate sample of biologically different systems. The program is freely available at http://atenea.montes.upm.es.     

Novel ZnO nanostructured electrodes for higher power conversion efficiencies in polymeric solar cells

1-Dimensional nanostructured ZnO electrodes have been demonstrated to be potentially interesting for their application in solar cells. Herein, we present a novel procedure to control the ZnO nanowire optoelectronic properties by means of surface modification. The nanowire surface is functionalized with ZnO nanoparticles in order to provide an improved contact to the photoactive P3HT:PCBM film that enhances the overall power conversion efficiency of the resulting solar cell. Charge extraction and transient photovoltage measurements have been used to successfully demonstrate that the surface modified nanostructured electrode contributes in enhancing the exciton dissociating ratio and in enlarging the charge lifetime as a consequence of a reduced charge recombination. Under AM1.5G illumination, all these factors contribute to a considerably large increase in photocurrent yielding unusually high conversion efficiencies over 4% and external quantum efficiencies of 87% at 550 nm for commercially available P3HT:PCBM based solar cells. The same approach might be equally used for polymeric materials under development to overcome the record reported efficiencies.     

Atomic radii scales and electron properties deduced from the charge density

A procedure to represent Hartree-Fock electron densities in atoms [L. Fernandez Pacios, J. Comp. Chem., 14 , 410 (1993)] defines ρ(r) as a reduced expansion of exponential functions. These analytically modeled densities (AMDs) are used in this article to develop a simple computational procedure for analyzing different atomic radii scales implemented in the commercial software system MATHEMATICA. The analysis is focused on the physical information associated to a given atomic radius as deduced from calculations depending on ρ(r). The amount of electron charge contained in the sphere of the given radius as well as the distinct contributions to the potential energy integrated up to that radius are obtained within the AMD formulation for main-group atoms H—Kr. The ASCII file needed to run the procedure within MATHEMATICA is also presented. © 1995 by John Wiley & Sons, Inc.     

Fragmentation of the lamellae and fractionation of polymer coils upon mixing poly(dimethylacrylamide) with the lamellar phase of aerosol OT in water

The lamellar mesophase formed by surfactant 1,4-bis(2-ethylhexyl) sodium sulfosuccinate (AOT) in deuterated water is mixed with poly(dimethylacrylamide) (PDMAA) polymers of low molecular weight (Mn= (2-20) x 10(3)). The mixtures separate into microphases (lamellar plus isotropic polymer solution). Their microstructures are studied by microscopy, small-angle X-ray scattering (SAXS), and deuterium NMR (2H NMR). According to SAXS, the lamellar phase fractionates the molecular weight distribution of the polymer, by dissolving only chains with coil sizes smaller than the thickness of the water layers between lamellae, and keeping larger chains segregated from the lamellar phase. The fraction of polymer that is segregated from the lamellar phase grows with Mn of the polymer. In 2H NMR, there are two signals, a quadrupolar doublet (water molecules hydrating the anisotropic lamellar phase contribute to this doublet) and a singlet (water molecules in the isotropic polymer solution contribute to this singlet). These two signals are deconvoluted to analyze the phases. Mixing with the polymer produces the partial dispersion of the lamellar phase into small fragments (microcrystallites). The structure of these microcrystallites is such that they conserve the regular long period spacing of the macrophase, and are thus identified in SAXS, but they are smaller than the minimum size required to produce quadrupolar splitting (about 4 microm), and therefore, in 2H NMR, they contribute to the singlet. 2H NMR can thus not distinguish between small microcrystallites and an isotropic polymer solution segregated from the lamellar phase; instead small microcrystallites are detected as an apparent increase of the isotropic solution. The degree of dispersion produced by the polymer in the lamellar phase is correlated with the degree of segregation that the polymer suffers. Thus, much greater dispersion into microcrystallites is produced by the higher Mn polymers than by the lower Mn polymers (in the range covered by the present samples, although with a much higher molecular weight sample (3 x 10(6)) that is totally segregated no such microcrystallites were detected).     

Porosity inherent to chemically crosslinked polymers. Poly(N-vinylimidazole) hydrogels

Swollen polymer networks exhibit multiscale pores filled with solvent. Such porosity, inherent to cross-linked polymers, determines some of their most relevant physical properties and applications. In this research, several samples of chemically crosslinked poly(N-vinylimidazole) were synthesized with the same permanent crosslinking density at two different conversions, and their inherent porosity was characterized on freeze-dried specimens by SEM, TEM and nitrogen physisorption. It was thus found that all of the samples showed pores, both on the nanometer and the micrometer scales, whose dimensions were mostly equal to or larger than the mesh size of the primary polymer network (22 nm) and whose volume and specific surface decreased with increasing conversion. Micropores have, in all cases, a very minor contribution. Samples synthesized with the largest comonomer concentrations show quasi-spherical mesopores (90 nm average diameter at any conversion) and macropores (from 5 to 10 microm with increasing conversion), whereas the mesopores of samples synthesized with the largest crosslinker ratios were channel-like (150 nm) and the macropores were interconnected contiguous voids (3 microm). Samples with intermediate compositions exhibit the lowest porosity due, mostly, to interconnected mesopores. The differences in shape were ascribed to the mechanism of phase separation, taking place during polymerization, even for samples that are transparent following polymerization. The inherent porosity is a significant source of spatial inhomogeneity, which contributes to the increase in turbidity. Light scattering decreases with increasing ionization when the degree of protonation is greater than 10%. An important consequence of the inherent porosity is that the degrees of swelling determined either gravimetrically or through size measurements are not equivalent.