Possible models of adsorbed deoxyadenosine-5'-monophosphate on a gold electrode derived from the characteristic specular reflectance data.

Abstract

The adsorption behavior of deoxyadenosine-5'-monophosphate (dAMP) on a gold electrode has been studied by specular reflectivity measurement in 0.1 M NaClO4 solutions ranging in concentration from 2.5×10-6 to 2.5×10-4 M. The reflectivity change due to the adsorption of dAMP was suggestive of the formation of two types of adsorbed layer depending on both the bulk concentration and the electrode potential ; one of them, termed the type I layer, appeared at low concentrations at potentials around the point of zero charge (pzc) as well as over the whole concentration range at rather negative potentials, and the type II layer appeared at high concentrations around pzc. The results obtained by analysis of the isotherms corresponding to these layers suggest that the adsorbed molecule is oriented with its adenine moiety nearly flat on the surface in the type I layer, but perpendicular to it in the type II layer. Such a change in orientation was not observed for adenine, deoxyadenosine, or deoxyadenosine-5'-diphosphate or -triphosphate. The characteristics of the adsorption behavior of dAMP were reasonably explained on the basis of its structural effect, i.e., the ease of orientational change of the adsorbed molecules and the resulting mutual interaction between adjacent adsorbates in the type II layer.