INFRARED AND PHOTOELECTRON-SPECTROSCOPY OF AMINO-ACIDS ON COPPER - GLYCINE, L-ALANINE AND BETA-ALANINE

Infrared reflection-absorption spectroscopy and X-ray photoelectron spectroscopy (XPS) are used to characterize adsorbed layers of amino acids on evaporated copper surfaces. Thin layers of glycine, l-alanine, and β-alanine are formed by adsorption from 5 m M aqueous solutions at pH values near their isoelectric points. Glycine is most thoroughly studied, and much attention is paid to a comparison with synthesized complexes of cis-Cu(II)(Gly)2·H2O and trans-Cu(II)(Gly)2·2H2O. A very good agreement is obtained between calculated reflection-absorption spectra based on these model substances and observed spectra of adsorbed glycine on copper. This observation suggests that both the carboxylate oxygens and the amino nitrogens are involved in the bonding to copper and that an ionic lattice (≈10 Å) consisting of Cu ions and glycine is formed on the surface. Further support for an ionic structure is obtained from a preliminary XPS study where differently prepared glycine layers on copper are compared with the cis-Cu(II)(Gly)2·H2O complex. The composition of the ionic lattice is found to vary with the microstructure of the copper surface and with film orientation in particular. Our infrared data indicate that the cis form is more pronounced on copper films with preferred (111) orientation, whereas the trans form appears to dominate on "polycrystalline" copper. l-Alanine and β-alanine also react with copper via the carboxylate-oxygen and amino-nitrogen atoms. However, the layer thicknesses for l- and β-alanine appear to be smaller than those obtained for glycine. © 1990.