Hydrophobicity plays an important role in numerous physico-chemical processes, from the process of dissolution in water to protein folding, but its origin at the fundamental level is still unclear. The classical view of hydrophobic hydration is that in the presence of a hydrophobic solute, water forms transient microscopic ‘icebergs’ arising from strengthened water hydrogen bonding, yet there is no experimental evidence for enhanced hydrogen bonding and/or ‘icebergs’ in such solutions. Here we have used the redshifts and line-shapes of the isotopically decoupled infrared O-D stretching mode of small, purely hydrophobic solutes (methane, ethane, krypton, xenon) in water to study hydrophobicity at the most fundamental level. We present the first unequivocal and modelfree experimental proof for the presence of strengthened water hydrogen bonds near four hydrophobic solutes, matching those in ice and clathrates. The water molecules involved in the enhanced hydrogen bonds display extensive structural ordering resembling that in clathrates. The number of ice-like hydrogen bonds is 10 to 15 per methane molecule. Abinitio molecular dynamics simulations have confirmed that water molecules in the vicinity of methane form stronger, more numerous and more tetrahedrally oriented hydrogen bonds than those in bulk water, and that their mobility is restricted. We demonstrate the absence of intercalating water molecules that cause the electrostatic screening (shielding) of hydrogen bonds in bulk water as the critical element for the enhanced hydrogen bonding around a hydrophobic solute. Our results confirm the classical view of hydrophobic hydration.
COBISS.SI-ID: 6068250
Low temperature Raman spectra of oxalic acid dihydrate (8-300 K) for both the polycrystalline and single crystal phase show strong variation with temperature in the interval from 1200 to 2000 cm-1. Previous low temperature diffraction studies all confirmed the stability of the crystal P21/n phase with no indications of any phase transition, reporting the existence of a strong hydrogen bond between the oxalic acid and a water molecule. A new group of Raman bands in the 1200-1300 cm-1 interval below 90 K is observed, caused by possible loss of the centre of inversion. This in turn could originate either due to disorder in hydroxyl proton positions or due to proton transfer from carboxylic group to water molecule. The hypothesis of proton transfer is further supported by the emergence of new bands cantered at 1600 and 1813 cm-1, which can be explained with vibrations of H3O+ions. The agreement between quantum calculations of vibrational spectra and experimentally observed Raman bands of hydronium ions in oxalic acid sesquihydrate crystal corroborates this hypothesis.
COBISS.SI-ID: 6072602
Experiments show that polymeric nanoparticles often self-assemble into several non-close-packed lattices in addition to the face-centered cubic lattice. Here, we explore theoretically the possibility that the observed phase sequences may be associated with the softness of the particles, which are modeled as elastic spheres interacting upon contact. The spheres are described by two finite-deformation theories of elasticity, the modified Saint-Venant–Kirchhoff model and the neo-Hookean model. We determine the range of indentations where the repulsion between the spheres is pairwise additive and agrees with the Hertz theory. By computing the elastic energies of nine trial crystal lattices at densities far beyond the Hertzian range, we construct the phase diagram and find the face- and body-centered cubic lattices as well as the A15 lattice and the simple hexagonal lattice, with the last two being stable at large densities where the spheres are completely faceted. These results are qualitatively consistent with observations, suggesting that deformability may indeed be viewed as a generic property that determines the phase behavior in nanocolloidal suspensions.
COBISS.SI-ID: 30267175
Cholesterol is essential for development, growth, and maintenance of organisms. Mutations in cholesterol biosynthetic genes are embryonic lethal and few polymorphisms have been so far associated with pathologies in humans. Previous analyses show that lanosterol 14[alpha]-demethylase (CYP51A1) from the late part of cholesterol biosynthesis has only a few missense mutations with low minor allele frequencies and low association with pathologies in humans. The aim of this study is to evaluate the role of amino acid changes in the natural missense mutations of the hCYP51A1 protein. We searched SNP databases for existing polymorphisms of CYP51A1 and evaluated their effect on protein function. We found rare variants causing detrimental missense mutations of CYP51A1. Some missense variants were also associated with a phenotype in humans. Two missense variants have been prepared for testing enzymatic activity in vitro but failed to produce a P450 spectrum. We performed molecular modeling of three selected missense variants to evaluate the effect of the amino acid substitution on potential interaction with its substrate and the obligatory redox partner POR. We show that two of the variants, R277L and especially D152G, have possibly lower binding potential toward obligatory redox partner POR. D152G and R431H have also potentially lower affinity toward the substrate lanosterol. We evaluated the potential effect of damaging variants also using data from other in vitro CYP51A1 mutants.
COBISS.SI-ID: 33314265
The influence of a stepwise hydration on the secondary structure of Concanavalin A, a 273 amino acid residues long lectin protein, was monitored by infrared spectroscopy. An analysis of Amide I and Amide III bands, assignment of model bands and determination of the populations of secondary structure elements using computed integral intensity of particular amino bands, was used to determine the proportion of b-sheet in protein films recorded under various steps of hydration and solution in water. In dry protein film 53% of amino acid residues are in b-sheet conformation. The hydration increases a population of b-sheet to 57% determined in fully hydrated film which finally reached 61% in water solution. On the basis of characteristic differential spectra calculated from the various hydration levels, we established that in the initial stage of hydration water molecules bind through hydrogen bonds directly to the main and side chains of protein. Hydration of side chains mainly occupies COO' and COOH groups. The increase of b-sheet population induced by hydration rearranged the water molecules attached to COOH side chain groups. A parallel analysis of Amide III bands shows that the Amid III region provides more complete information regarding the protein structure. Contemporary analysis of this region is very supportive, because it offers additional structural parameters which significantly contribute to reliability of secondary structure analysis by applying Amide I mode. Moreover, besides the comparable information about the population of secondary structure elements, the analysis of the Amide III area provides also the distribution of conformations of amino acids which are found in unstructured parts of protein.
COBISS.SI-ID: 6089242