ROLE OF HYDROGEN-BOND AND METAL-COMPLEX FORMATION FOR CHIRAL DISCRIMINATION IN AMINO-ACID MONOLAYERS STUDIED BY INFRARED REFLECTION - ABSORPTION-SPECTROSCOPY

Comparison between the macroscopic order (inferred from surface pressu re/area isotherms and fluorescence microscopy) and the molecular order [determined by infrared reflection-absorption spectroscopy (IRRAS)] f or N-acylamino acid monolayers shows that hydrogen bond formation via the NH, COOH, and p-hydroxyphenyl groups, respectively, may lead to pr onounced chiral discrimination. On a pure water surface, N-hexadecanoy lalanine films exhibit preferential homochiral interactions, which may be strengthened by Pb2+ in the aqueous subphase, while Zn2+ disturbs this structure and suppresses the chiral discrimination. In N-octadeca noyltyrosine and N-octadecanoyltyrosine methyl ester monolayers, both homo- and heterochiral discrimination may be observed, depending on th e available area per molecule, where the free fatty acid carboxyl grou p gives rise to additional film compression and, thus, to a higher con formational order for the alkyl chains. However, in the presence of bo th PbCl2 and ZnCl2 in the aqueous subphase, the pronounced chiral disc rimination effect disappears. Analysis of the methylene scissoring del ta(CH2) bands shows that the different effects of Pb2+ and Zn2+ cannot be explained on the basis of different subcell structures (in both ca ses an orthorhombic structure is prevailing), but on the basis of diff erent complex formations between the bivalent cations and the carboxyl ic acid headgroup as inferred from the separation Delta between the an tisymmetric and the symmetric carboxylate vibrations, Delta = v(a)(COO ) - v(s)(COO). The results summarized in the present paper suggest tha t theories like Andelman's tripode theory should be expanded to accoun t for potential metal complex formations.