Monolayers of neutral and charged polyamino-acids spread on silicic acid substrates

Abstract

Surface pressure-area isotherms (π-A) of poly(L-alanine) (PA), poly(L-glutamic acid) (PG), and poly(L-arginine) (PArg) monolayers spread on substrates of varous pH values containing silicic acid were recorded and compared with those obtained on silica-free substrates. The compression curves of PA showed a pH-independent plateau region which was assumed to correspond to a monolayer to bilayer transition. The inflection observed in the π-A curves of PG was attributed to the formation of loops made up of the terminal heads of the polymer and lying under the monolayer. Polysilicic acid interacted with the PArg films at pH 6; such films were more rigid and stable and had larger specific areas than those obtained in the absence of silica, which prompts the occurrence of an ionic interaction. The other two polyamino-acids did not interact with silicic acid at any pH value, so we may rule out the formation of hydrogen bonds between the silanol groups of polysilicic acid and the peptide or carboxyl groups of these amino acids.