Molecular arrangement and polycondensation of octadecyl esters of aromatic amino acids in Langmuir-Blodgett films

Abstract

The molecular arrangements and polycondensation of octadecyl esters of phenylalanine ( L-PheC18), tyrosine ( L-TyrC18) and tryptophan ( L-TrpC18) in Langmuir-Blodgett (LB) films were studied. These LB films adopt good layer structures: the long hydrocarbon chains stand nearly vertical to the film plane; the benzene ring of L-PheC18 lies almost flat; and the phenol group of L-TyrC18 and the indole ring of L-TrpC18 are oriented fairly perpendicular to the film plane. It has been found that the polycondensation is accelerated by regular arrangements of monomer molecules in the LB films, where functional groups can be concentrated in the two-dimensional plane and situated more effectively for the reaction than in the bulk solids or the molten states. The reaction rates in the LB film decreased in the order L-TyrC18 > L-TrpC18 > L-PheC18, which can be ascribed to their distinct steric hindrances caused by the different orientations and sizes of the aromatic rings. Polycondensation in LB films can be improved by mixing them with octadecyl esters of alanine to reduce steric hindrances. The resultant polypeptides take mainly the α-helix conformation.