Electronic-Transitions in the Near-Ultraviolet Spectra of Uracil, Thymine, Uridylyl(3'-5')uridine and Thymidylyl(3'-5')thymidine

Abstract

The absorption spectra in the near-ultraviolet region of uracil and thymine, and of their derivatives may be deconvoluted into two vibronic progressions; the higher-energy progression has a relatively low intensity. The corresponding component spectra coincide with the two separate transitions manifested in the circular dichroism of the relevant nucleoside derivatives. A series of apparent contradictions may be resolved by the assumption that the higher-energy transition in absorption is a vibronically allowed (π*,n) transition. This interpretation is further supported by an analysis of the spectra of uridylyl (3′-5′) uridine and thimidylyl (3′-5′) thymidine in terms of exciton interactions. The magnitude of exciton coupling in the dinucleoside phosphates is found to be small but significant in terms of energy transfer rates.