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

Abstract

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.

On the Photochemistry of Purine Nucleobases

“…This is also consistent with the observation that in solution at low temperatures the lifetime increases. 35,36 …”

The photophysics of 7H-adenine: A quantum chemical investigation including spin–orbit effects

“…The luminescence spectra of adenine in glassy matrices of alcohol/water mixtures, recorded by Longworth et al [9] at low temperatures, are broad but show some vibrational structure. While there is evidence that the adenine fluorescence stems from the minority component 7H-A [3,6], there is no indication that this applies equally to the phosphorescence bands of adenine.…”

“…According to Longworth et al [9], the fluorescence and phosphorescence quantum yields of adenine are nearly identical. A similar conclusion was drawn by Wilson and Callis [6] who observed strong phosphorescence bands in 7-MA.…”

“…As 7H-A exhibits a considerably larger dipole moment than 9H-A, it is preferentially stabilized by solvent effects, increasing its relative concentration to values between 6% in alcohols and 22% in water [3][4][5][6][7][8]. While the broad and structureless absorption spectra of adenine and its derivatives substituted in 9-position are very similar, this is not the case for the emission spectra [9]. Although being the minority component, 7H-A has been made responsible for most of the luminescence observed for adenine in glassy matrices [3,6].…”

Biology and the Exploration of Mars

“…There is need for a good deal more exploratory work on phosphorescence in biologically important systems. One promising area is the nucleic acids, which are virtually non-fluorescent but do display considerable phosphorescence [Longworth, 1962]. Furthermore, certain dye complexes of nucleic acids show particularly interesting delay-emission characteristics [Isenberg et al, 1964].…”