Spectroscopic Studies of Purines—i. Factors Affecting the First Excited Levels of Purine*

Abstract

Abshct-Studies of purine absorption and emission in seven solvents differing greatly in dielectric constant and hydrogen bonding potential, reveal a variety of solvent effects. For example, the resolution of structure in the absorption spectrum, the position and/or intensity of the X, absorption band, the intensity of fluorescence, the magnitude of the long wavelenth tail, and the position of the XI absorption band are differentially affected-in the order listed-by the solvents tested. Even though it is possible to correlate the extent of decrease in the n-a* tail with increasing solvent dielectric constant, probably alterations in all of these spectroscopic parameters depend most critically upon the ability of the various solvents to form hydrogen bonds with the hydrogen on N9 andfor with the non-bonding electrons on the purine nitrogens: it is tentatively concluded that the probability of hydrogen bonding is directly correlated with the electronegativity of the aza nitrogens (N7 > N3 > Nl).In solvents like isopropanol not all of the non-bonding electrons must be solvated maximally in most purine molecules since there is appreciable fluorescence under conditions where a long wavelength tail is readily observed in the absorption spectrum (alternatively some noabonding electrons may not t e relevant to fluorescence quenching.) Decreases in fluorescence yield are associated with red shifts in the fluorescence maximum, and in the solbents of highest polarity the fluorescence yield is again small indicating that glycerol and water can enhance radiationless tunneling-presumably by altering Franck-Condon configurations and/or improving electronic-vibrational coupling betwen solute and solvent. The quantum yield is uniform throughout the atsorption band for a given solvent, but studies in aqueous buffers varying from pH 1 to 11 show that the fluorescence yield is greater for charged than for neutral molecules. Further, the fluorescence excitation ~eak is red shif.ed in powders. Since phosphorescence is the predominant emission at 77'K and increases in fluorescence can be correlated with the presumed solvation of non-bonding electrons, the singlet excited state of lowest energy in 'unperturbed' purine must be n-n* in nature. The shape of the phosphorescence band and the decay lifetime of -1 sec at 77'K lead to the conclusion that the emitting triplet is a P-H* state. The eight vibrational structures in phosphorescence emission can be readily grouped into two progressions: there is an average separation of about 1300 cm-* between peaks within a given progression, and the two sets are mutually displaced by about 500 cm-l. Individual vibrational peaks are favoured in different solvents and the whole band may be shifted up to 500 cm-l. Even larger shifts are observed in charged purine molecules and in powders (up to 3000 cm-l) and the presumed 0-0 band is not observed.

Excited-State Proton Transfer and Phototautomerism in Nucleobase and Nucleoside Analogs: A Mini-Review

“…One simple clue to identification of such systems may be finding of the large Stokes' shifts in their fluorescence in the acidic conditions (see, e.g., refs. [78,106]), suggesting "phototautomerism via the cation." For the canonical purines and pyrimidines, and the corresponding nucleosides, effective excited-state deactivation pathways lead to a very low emission and extremely short excited-state lifetimes, [107,108] making the ESPT difficult to observe experimentally, but the situation is quite different in the fluorescent 8-azapurines and pyrazolopyrimidines, and, possibly, many other analogs.…”

Lowest Excited Triplet‐state Dissociation Constants of Purines in Aqueous Solution. Substituent Effects*

“…However, only a few studies have been undertaken on the acidity of substituted purines and their cations in the excited state (Borresen, 1963;Drobnik and Augenstein, 1966;Longworth er al., 1966). Borresen (1963) measured dissociation constants of several purines and pyrimidines by fluorescence titration at room temperature and found that these pK, values were significantly higher than ground-state pK, values obtained by potentiometric titrations of unsubstituted purine.…”

Intermediates in the Room Temperature Flash Photolysis and Low Temperature Photolysis of Purine Solutions

“…(1971). A phosphorescence lifetime of 3.5 s has been reported for purine in ethylene glycolwater at a pH of 11 (Drobnik and Augenstein, 1966) and in our laboratory using electron paramagnetic resonance (EPR) we have determined a triplet decay lifetime of 3.4 s (M. Rivera and R. Arce, unpublished results).…”