Luminescence of two hydrated forms of adenosine

Abstract

The fluorescence excitation spectrum of dilute solutions of adenosine (5 × 10 −5 M) mixtures of ethylene glycol and H 2O at 77 K and pH 7.0 was found to be red shifted by 10 nm compared with the absorption spectrum. This is explained by the existence of two monomeric species. The major monomeric species has an absorption maximum at 260 nm and a fluorescence quantum yield of less than 2 × 10 −4 and is assigned to the adenosine molecules cyclically hydrogen bonded through the N 7 atom and the amino group to water molecules. The minor monomeric species exhibits an absorption maximum at 270 nm and a fluorescence quantum yield of greater than 2 × 10 −2 and is assigned to molecules without a cyclic hydrogen bond. The concentration dependence of the fluorescence excitation and emission spectra of adenosine also indicates the existence of aggregates with fluorescence properties similar to those of adenylyl (3′ → 5′) adenine (ApA). A mechanism of S 1* state quenching for adenosine molecules cyclically hydrogen bonded to water molecules is proposed.