Luminescence of Pyrimidines, Purines, Nucleosides, and Nucleotides at 77°K. The Effect of Ionization and Tautomerization

The luminescence of purines and their corresponding nucleosides and nucleotides has been studied at 77°K for different states of ionization and for some alterations of tautomeric forms. To a first approximation, the excitation spectra always coincided with the absorption spectra. Furthermore the emission profiles were independent of the exciting wavelength and, except for the protonated forms of guanine and its derivatives, phosphorescence always consisted of a single exponential decay. The naturally occurring pyrimidines, uracil, thymine, and cytosine, fluoresced over a wide pH range but only phosphoresced at high pH where they have lost a proton. From a study of methylated pyrimidines it was shown that the lactim forms fluoresced at 300°K and phosphoresced at neutral pH at 77°K. In these two respects they differed from the natural pyrimidines and support the commonly accepted lactam structures for the natural pyrimidines. The purines, adenine and guanine, and their derivatives fluoresced and phosphoresced in neutral and basic solutions at 77°K. Upon protonation at pH∼4 the emission of AMP was completely quenched, adenosine had its phosphorescence considerably reduced, while the luminescence of adenine was essentially unchanged. The quantum yields, luminescence spectra, and phosphorescence decay times were measured for all the compounds.