Systematic Analysis of the Role of Substituents in Oxiranes, Oxetanes, and Oxathietanes Chemical Shifts.

The important role substituents play on proton chemical shifts in heterocyclic compounds was investigated in detail. For this purpose, a considerable number of model oxiranes, oxetanes, and oxathietanes with different substituents were studied in a systematic way. In addition, the oxygen and sulfur heteroatom influence on the chemical shift values was analyzed. The density functional theory (DFT) approximation was employed together with the M06 and the B3LYP functionals and the aug-pcS-1 and the 6-311++G** basis sets. We carried out a careful analysis of the shift values and the changes in the corresponding molecular electrostatic potential surfaces due to substitution. We observed that chemical shift values for the protons closest to the substituents are larger for the chloro and fluoro derivatives than those for the cyano and ethynyl ones. The presence of oxygen as well as sulfur in the ring causes an increase of the chemical shift values, most pronounced for the atom closest to the substituent. A large decrease of the proton shifts was observed when going from methylenecyclopropane to methyleneoxirane that can be attributed to π-electron resonance. Protons diagonal to the substituents behaved in a different way depending on their cis or trans disposition with respect to them. The conclusions of the present study will be useful in theoretical and experimental work on NMR spectra of heterocyclic compounds.

Towards understanding the interaction of (S)-thalidomide with nucleobases.

Interaction of (S)-thalidomide molecule with four nucleobases: adenine, guanine, cytosine and thymine, is investigated in details employing density functional theory methods. Different mutual positions of the molecules are considered, with the starting geometries enabling hydrogen bond interactions between the monomers. Optimization of geometrical parameters is carried out within the B3LYP/6-311G** approximation and followed by evaluation of vibrational frequencies. Binding and interaction energies are calculated employing exchange-correlation functionals including long-range corrections and properly diffuse basis sets. The strongest interaction exists within the (S)-thalidomide-guanine complex. Interestingly, in one of the investigated (S)-thalidomide-guanine complexes two bifurcated hydrogen bonds are observed. The two hydrogens involved in one of them are bonded to a carbon atom in the α position relative to carbonyl group. The present study can be useful in the design of new anticancer and antiviral drugs interacting selectively with DNA or RNA.

Tribological efficacy and stability of phospholipid-based membrane lubricants in varying pH chemical conditions.

In this study, the authors examine the influence of joint chemical environment by measuring changes in the tribological properties (friction coefficient and charge density) of contacting surfaces of normal and degenerated cartilage samples in bath solutions of varying pH (2.0-9.0). Bovine articular cartilage samples (n = 54) were subjected to several surface measurements, including interfacial energy, contact angle, and friction coefficient, at varying pH. The samples were delipidized and then subjected to the same measurement protocols. Our results reveal that the interfacial energy and charge density, which have been shown to be related to friction coefficient, decrease with pH in the acidic range and approach constant values at physiological (or synovial fluid) pH of 7.4 and beyond it, i.e., toward basic pH domain. The authors conclude that this rather complex response explains the long-term efficacy with respect to ageing and associated pH changes, of the phospholipid layers that facilitate the almost frictionless, hydration-lubrication involving contact in the mammalian musculoskeletal system.