Interfacial Behavior of D‐Ribose, 2′‐Deoxy‐D‐Ribose, and D‐Ribose 5′‐Phosphate

Abstract

The adsorption of D‐ribose, 2′‐deoxy‐D‐ribose, and D‐ribose 5′‐phosphate at the mercury electrode‐electrolyte solution interface at pH 8.0 has been studied by electrocapillary measurements. Each of these compounds is only weakly adsorbed with the free sugars being adsorbed at potentials close to the potential of zero charge (electrocapillary maximum potential). D‐ribose 5′‐phosphate, being extensively ionized at pH 8.0, is adsorbed mainly at a positively charged electrode. The areas occupied by these molecules at monolayer surface saturation suggest that they are all adsorbed with the average plane of atoms of the 5‐membered sugar ring parallel to the electrode surface, i.e., in a flat surface orientation. Information about the interfacial behavior of these sugars and sugar phosphate has been used to interpret in more detail the interfacial conformation of purine and pyrimidine nucleosides and nucleotides adsorbed at an electrode surface. It appears that pyrimidine nucleosides and nucleotides, such as uridine and uridine 5′‐monophosphate, are adsorbed with the planar pyrimidine base adsorbed in a flat orientation on the electrode. The sugar residues must be adsorbed perpendicular to the electrode surface. Since pyrimidine nucleosides and nucleotides preferentially adopt an anti conformation, these molecules occupy significantly more surface area than the base. On the other hand, purine nucleosides and nucleotides, such as those of adenine, adopt a synconformation in the adsorbed state at monolayer surface saturation. Again the purine base is adsorbed in a flat orientation on the electrode. This results in the sugar or sugar phosphate residues being largely rotated out of the plane of the electrode surface so that the adenine moieties can pack closely together. It is for this reason that the areas occupied by adenine, deoxyadenosine, and deoxyadenosine 5′‐monophosphate are close to those observed for the free purine base.