The interaction between two associating hydrophobic particles has traditionally been explained in terms of the release of entropically frustrated hydration shell water molecules. However, this picture cannot account for the kinetics of hydrophobic association and is therefore not capable of providing a microscopic description of the hydrophobic interaction (HI). Here, Monte Carlo simulations of a pair of molecular-scale apolar solutes in aqueous solution reveal the critical role of collective fluctuations in the hydrogen bond (HB) network for the microscopic picture of the HI. The main contribution to the HI is the relaxation of solute-water translational correlations. The existence of a heat capacity maximum at the desolvation barrier is shown to arise from softening of non-HB water fluctuations and the relaxation of many-body correlations in the labile HB network. The microscopic event governing the kinetics of hydrophobic association has turned out to be a relatively large critical collective fluctuation in hydration water displacing a substantial fraction of HB clusters from the inner to the outer region of the first hydration shell.
COBISS.SI-ID: 36953605
Understanding the factors that affect the conformational stability of the polypeptide main chain provides insight not only into the molecular basis of unfolded states but also into the earliest event that occurs during the protein folding. The presented study was concentrated on finding the conformational distributions of poly-L-lysine (PLL) by applying infrared spectroscopy. We assigned the amide bands for different conformations of PLL in water. At low pH values PLL mainly possesses the PII and β structures while at higher pH values and low temperatures characteristic bands for the α-helical conformation are found. The increase in temperature induces the formation of β structures. The obtained assignment of the infrared bands for various conformations was used to determine the conformational populations of PLL in non-aqueous solvents. In TFE, PLL possesses an α-helix structure that is after heating partially transformed into the PII conformation. DMSO enables a uniform α-helical conformation of PLL. A similar uniform conformation (PII, 88%) was found for PLL dissolved in ethylene glycol, suggesting that the PII structure is not limited to the presence of water molecules or charged side chains. The role of intermolecular interactions between the solvent molecules and PLL in stabilizing the PII conformation is discussed.
COBISS.SI-ID: 5211930
DFT calculations were employed to investigate the nature of the binding between the physiological form of glutathione (GSH) and cadmium dication (Cd2+) in aqueous solution. The results revealed that, upon complexation, the cysteine –SH group gets deprotonated by the neighbouring glycine carboxylate, reverting the latter to its unionized form with not enough nucleophilicity to coordinate Cd2+. This proton transfer is facilitated by the formation of favourable Cd2+…S coordination, which reduces cysteine pKa(SH) value by around 13 pKa units, and produces adduct in which GSH interacts with Cd2+ only through the cysteine thiolate anion and the ionized glutamine carboxylic group. Subsequent deprotonation of the glycine –COOH moiety to the bulk solvent yields a system 15.9 kcal mol -1 higher in energy, which makes this process unfeasible. We found our results to be fully consistent with vibrational and NMR spectroscopic measurements reported earlier.
COBISS.SI-ID: 5157914
We investigated the housekeeping cytochrome P450 CYP51A1 encoding lanosterol 14-demethylase from cholesterol synthesis that was so far not directly linked to human disorders. By direct sequencing of CYP51A1 in 188 women with spontaneous preterm delivery and 188 unrelated preterm infants (gestational age (37 weeks) we identified 22 variants where 10 are novel and rare. In infants there were two novel CYP51A1 variants where damaging effects of p.Tyr145Asp from the substrate recognition region, but not p.Asn193Asp, were predicted by PolyPhen2 and SIFT. This was confirmed by molecular modeling showing that Tyr145Asp substitution results in changed electrostatic potential of the CYP51 protein surface and lengthened distance to the heme which prevents hydrogen bonding. The CYP51 Tyr145Asp mutation is rare and thus very interesting for further structure/function relationship studies. From the 12 identified known variants rs6465348 was chosen for family based association studies due to its high minor allele frequency. Interestingly, this CYP51A1 common variant associates with small for gestational age weight in newborns (p = 0.028) and lower blood total cholesterol and low density lipoprotein cholesterol levels in mothers in 2nd trimester of pregnancy (p = 0.042 and p = 0.046 respectively). Our results indicate a new link between a cholesterol synthesis gene CYP51A1 and pregnancy pathologies.
COBISS.SI-ID: 31046873
The aim of this study was to develop a sirolimus (BCS class II drug substance) solid oral dosage form containing a precipitation inhibitor, which would result in an improved sirolimus absorption in humans compared to the formulation containing nanosized sirolimus without a precipitation inhibitor, i.e., Rapamune. The selection of the precipitation inhibitor was based on the results of a screening campaign that identified two “hit” excipients: HPMC 603 (i.e., Pharmacoat 603) and Poloxamer 407. However, in a confirmatory precipitation inhibitor study using biorelevant media (Fa/FeSSIF) HPMC 603 more effectively inhibited sirolimus precipitation than Poloxamer 407. In the PAMPA assay, HPMC 603, but not Poloxamer 407, significantly increased the flux of the sirolimus across the membrane lipid layer. Additionally, a differential scanning calorimetry (DSC) and an infrared (IR) spectroscopy study revealed that interactions between the sirolimus and HPMC 603 were developed that could lead to the observed precipitation inhibition effect. Based on the above data, two formulations with HPMC 603-coated sirolimus particles were developed, namely, formulation A (d (0.5) = 0.21 μm) and formulation B (d (0.5) = 1.7 μm). A human pharmacokinetic study outlined that significantly higher AUC and Cmax were obtained for formulations A and B in comparison to Rapamune. This result could be attributed to the HPMC 603 (Pharmacoat 603) mediated sirolimus precipitation inhibition resulting in improved sirolimus absorption from the gastrointestinal tract in humans.
COBISS.SI-ID: 1533276