Hydration, dehydrative counter-ion binding and helix formation of charged poly(α-amino acid)s in aqueous alcohol as revealed by a preferential binding study

Abstract

The preferential binding behaviour of one of the components of aqueous alcohol solutions to sodium poly(α-L-glutamate)(PLGNa) and poly(α-L-lysine HBr)(PLLHBr) at 25°C has been studied by differential refractometry. The aqueous mixtures have 1-propanol(1-PrOH),2-propanol(2-PrOH), t-butanol (t-BuOH) and 1,2-ethanediol (EG) as their second component. The electrical conductance and the α-helix content of these polymers were also measured to interpret the preferential binding behaviour. Aqueous monohydric alcohols show a characteristic dependence on the alcohol concentration: the initial trend of the increasingly negative binding of the alcohols is reversed at certain alcohol concentrations, giving clear breaks. The results are interpreted invoking an operational model of these polymers, in which polar and less polar portions undergo solvation separately. PLGNa residue is hydrated by 20 water molecules, of which 12 are taken up by the polar portion and the rest by the less polar portion. PLLHBr residue is hydrated by 27 water molecules, of which 15 are taken up by the polar portion. With increase in the alcohol concentration, the hydration of the polar portions of these polymers is lost upon counter-ion binding, which is followed by the helix formation of the polymers. 5 water molecules are released from around the less polar portions upon formation of the helix.