The adsorption of microperoxidase-8 and -11 (MP-8 and - 11) on hydrophilic platinum and gold surfaces was studied by in situ ellipsometry. Neither peptide adsorbed on gold regardless of the composition of the aqueous solution. Electrochemical preoxidation of Pt decreased both the rate constant of adsorption (k(ads)) and the maximum surface concentration (GAMMA(max)) of MP-8 by a factor of 2.7 and 1.6, respectively. Thus, 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 the pH-induced ionization of the MP-8 and -11 molecules, i.e., on the structure of the peptide chain. The experimental and calculated GAMMA(m) values for the different pH-dependent species of heme peptides indicated formation of a monolayer. Electrochemical studies on Pt revealed that 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 Pt 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 the concentration of H+ at the electrode surface. (C) 1994 Academic Press, Inc.