Preferential binding to poly(α-L-lysine HBr) from polar solvent mixtures

Abstract

The preferential binding behaviour of one of the components of aqueous organic solvents or organic solvent binary mixtures to poly(α-L-lysine HBr)(PLLHBr) has been studied by differential refractometry. The aqueous mixtures have dimethyl sulphoxide (DMSO), N,N-dimethylformamide (DMF), N-methylformamide (NMF), 1,2-ethanediol (EG), 1-methyl-2-pyrrolidinone (NMP) and 2-propanol (2PrOH) as their second component, while nonaqueous mixtures comprise DMF + DMSO, EG + DMSO and NMP + DMSO. Aqueous mixtures show different behaviour; large positive binding of DMSO at lower concentrations is followed by a decrease and eventually by an inversion at higher concentrations, while the binding of 2PrOH is increasingly negative with concentration. The behaviour pattern of DMSO binding from nonaqueous mixtures is the same as that of aqueous mixtures, though the extent is different. In DMF + DMSO, the helix formation of PLLHBr is observed. The data obtained is interpreted invoking an operational model of PLLHBr, in which polar and less polar portions of the polymer residue undergo solvation separately. PLLHBr is exclusively hydrated in aqueous 2PrOH, if the polymer assumes no helix. It is suggested that the helix formation in DMF + DMSO mixtures is accompanied by the release of solvent molecules from around the less polar portion of the polymer.