AN ELLIPSOMETRIC AND ELECTROCHEMICAL STUDY OF MICROPEROXIDASE-8 AND MICROPEROXIDASE-11 ADSORPTION ON PLATINUM AND GOLD SURFACES

The adsorption of microperoxidase-8 and -11 (MP-8 and - 11) on hydroph ilic platinum and gold surfaces was studied by in situ ellipsometry. N either peptide adsorbed on gold regardless of the composition of the a queous solution. Electrochemical preoxidation of Pt decreased both the rate constant of adsorption (k(ads)) and the maximum surface concentr ation (GAMMA(max)) of MP-8 by a factor of 2.7 and 1.6, respectively. T hus, the adsorption on Pt is chemical in nature. However, both k(ads) and GAMMA(max) after rinsing (GAMMA(m)) depend to a large degree on th e pH-induced ionization of the MP-8 and -11 molecules, i.e., on the st ructure of the peptide chain. The experimental and calculated GAMMA(m) values for the different pH-dependent species of heme peptides indica ted formation of a monolayer. Electrochemical studies on Pt revealed t hat the protoporphyrin ring of the adsorbed heme peptides is involved in a multiple-electron oxidation reaction. In addition, the adsorbates exert a ''blocking'' effect on the electrochemical reactions of the P t oxides and generation of ''strongly bound'' hydrogen layers. However , through the net charge of the adsorbed heme peptides, the adsorption of ''loosely bound'' oxygen and hydrogen is controlled by changing th e concentration of H+ at the electrode surface. (C) 1994 Academic Pres s, Inc.