Formation of polypeptide-dye multilayers by electrostatic self-assembly technique

Abstract

To demonstrate the feasibility of preparing ordered multilayers composed of charged macromolecule/dye bilayers, we used an electrostatic self-assembly technique to prepare polypeptide/dye multilayers. We immersed previously protonated silanized glass slides in aqueous solutions (1 mM) of two representative anionic dyes : rod-shaped congo red or plate-shaped copper phthalocyanine tetrasulfonic acid sodium salt and cationic polypeptidepoly(L-lysine) solutions (15 mM monomer). Films resulting from up to 100 dipping cycles were characterized by UV-vis absorption spectra, ellipsometry, CD, and FT IR. FT IR spectra of film material embedded in KBr pellets showed an amide I band at 1654 cm -1 , suggesting thin film α-helical conformation. In both systems, the dye absorbance increased with the number of layers. In the copper phthalocyanine system, the absorption spectrum was a combination of phthalocyanine monomer and dimer contributions, with no evidence of higher aggregates. The congo red film dye absorption maximum was a function of the number ofbilayers, suggesting congo red resided in a polar, hydrophilic environment that became less polar with increasing bilayer number. From ellipsometry measurements, the bilayer thickness in both films was determined to be 20 A. The phthalocyanine Q band showed negative-induced CD, while the congo red ππ * band exhibited positive-induced CD. The CD measurements gave evidence for ordered dye-polypeptide complexes in films greater than 100-A thickness. The current work suggested a bilayer structure where monomeric congo red or dimeric copper phthalocyanine tetrasulfonate were randomly distributed among excess poly(K) monomer bindings sites, giving clear films with no scattering centers.