Micro-solvation of the Zn2+ ion-a case study.

The structures and thermodynamic parameters of hydrated zinc ion clusters incorporating a single zinc ion and up to eighteen water molecules have been determined with a quantum mechanical hybrid density functional, namely B3LYP using cc-PVDZ basis functions for H and O and a split valence 6-31G (d, p) basis function for Zn. The geometries for all the zinc ion water clusters are optimized with several initial guess structures and without imposing any initial symmetry restriction. Zinc metal ion is found to be preferably four coordinated for smaller sizes of hydrated cluster but attains an octahedral coordination for larger sizes (n >or= 11) of hydrated cluster. Structures with seven or more than seven water molecules attached directly to the central zinc ion are not found. The calculated gas phase coordination number in the first solvation sphere of a large hydrated zinc metal ion is found to be six and the same is also confirmed in the force field based classical molecular dynamics simulation study for an aqueous zinc ion and thus confirms the experimental findings. The equilibrium zinc-oxygen distance of 2.09328 A at the present B3LYP level of study is in excellent agreement with the X-ray diffraction result of 2.093 +/- 0.002 A for a hexahydrated zinc cluster.

Beneficial effect of hydrotropes on partitioning behaviour of proteins in aqueous two phase systems.

Proteins have been partitioned in poly(ethylene glycol) (PEG)-salt systems containing hydrotropes. Hydrotropes are surface active compounds with strong ionic nature with a smaller hydrophobic part as compared to surfactants. The effect of pH, hydrotrope concentration, polymer molecular weight and protein molecular weight on partitioning of proteins including enzymes and their separation has been investigated. The effects of hydrotropes may be explained on the basis of interaction of PEG and the hydrotropes. This is substantiated by measuring the concentration of hydrotropes in both the phases. The practical utilization of the described effect is to enhance the separation of a mixture of proteins by many folds. Preliminary experiments have shown that hydrotropes at low concentrations do not affect enzyme activity and do not have any bactericidal activity.