LUMINESCENCE OF 2 HYDRATED FORMS OF ADENOSINE

The fluorescence excitation spectrum of dilute solutions of adenosine (5 x 10(-5) M) mixtures of ethylene glycol and H2O at 77 K and pH 7.0 was found to be red shifted by 10 nm compared with the absorption spec trum. This is explained by the existence of two monomeric species. The major monomeric species has an absorption maximum at 260 nm and a flu orescence quantum yield of less than 2 x 10(-4) and is assigned to the adenosine molecules cyclically hydrogen bonded through the N7 atom an d the amino group to water molecules. The minor monomeric species exhi bits an absorption maximum at 270 nm and a fluorescence quantum yield of greater than 2 x 10(-2) and is assigned to molecules without a cycl ic hydrogen bond. The concentration dependence of the fluorescence exc itation and emission spectra of adenosine also indicates the existence of aggregates with fluorescence properties similar to those of adenyl yl (3'-->5') adenine (ApA). A mechanism of S1 state quenching for ade nosine molecules cyclically hydrogen bonded to water molecules is prop osed.