Sorption of water vapour by poly-L-glutamic acid, poly-L-lysine and their salts and some chemically modified derivatives

Abstract

Isotherms for the sorption of water by poly-L-lysine and poly-L-glutamic acid, their hydrobromide and sodium salts respectively, and six chemically modified derivatives of the polypeptides have been measured gravimetrically.

At low humidities sorption primarily involves direct interaction of water with charged or polar side-chain groups. At moderate humidities aggregates of water molecules develop around the side chain groups to give structures akin to hydration shells. The relative abilities of charged or polar groups to sorb water is in the sequence COO^– > NH₃⁺ NH₂ > COOH. Cross-linking interactions between adjacent side chain groups hinder the uptake of water. At high humidities the further build up of the aggregates of water molecules around side chain groups is probably not much influenced by the precise nature of the latter. Sorption of water by peptide groups in the main chains of the polypetides also occurs at moderate and high vapour pressures. The release of peptide groups from the α-helical conformation increases their ability to sorb water.