PHOSPHORESCENCE ANALYSIS OF THE TRIPLET-STATE OF PTERINS IN CONNECTION WITH THEIR PHOTORECEPTOR FUNCTION IN BIOCHEMICAL SYSTEMS

The triplet states of two pterin derivatives with different chain subs tituents, 2-amino-4-hydroxy-6,7-dimethylpteridine (DMP) and -4-hydroxy -6-tetrahydroxybutyl-(D-arabo)-pteridine (TOP) have been studied in et hanol-buffer mixtures using phosphorescence and fluorescence measureme nts at 77 degrees K. The phosphorescence yield and lifetime were 2-6% and 0.9-1.2 sec, respectively. The main phosphorescence maximum was ob served at 505 nm that corresponds to the triplet-level energy of 2.45 eV. Excitation spectra showed that phosphorescence originated from mon omeric neutral pterin molecules. Phosphorescence emission from ionized and reduced pterin molecules was not detected. Fluorescence spectra o f the neutral pterin forms had at 77 degrees K maxima at 427 nm for DM P and at 432 nm for TOP. This shows that the energy of the pterin exci ted singlet state is 2.87-2.90 eV and the singlet-triplet energy gap i s 0.42-0.45 eV. At room temperature, no pterin phosphorescence was obs erved. Instead, in aerobic aqueous (D2O) solutions, pterin-photosensit ized phosphorescence of singlet oxygen was detected. The quantum yield of pterin-generated singlet oxygen calculated from the phosphorescenc e measurements was 10-17%. The data indicate that the pterin triplet s tate and singlet oxygen might be involved in photobiochemical reaction s of pterins. As the intensity of pterin phosphorescence is very high, its detection can be used as a sensitive assay for studies of pterins in model and biological systems.