Optical activity and electronic absorption spectra of some simple nucleosides related to cytidine and uridine: all-valence-shell molecular orbital calculations.

Abstract

The circular dichroism and electronic absorption of three simple model systems for cytidine and uridine have been measured to 190 nm. The molecular spectral properties (excitation wavelengths, oscillator strengths, rotational strengths, and polarization directions) and electronic transitional patterns were investigated by using wave functions of the entire nucleoside with the goal of establishing the reliability of the theoretical method. The computed electronic absorption quantities were shown to be in satisfactory agreement with experimental data. It was found that the computed optical rotatory strengths of the B2. and El.electronic transitions and lowest observed n-ir* transition are in good agreement with experimental values. Electronic transitions were characterized by their electronic transitional patterns derived from population analysis of the transition density matrix.The theoretical rotational strengths associated with the B2. and El. transitions stabilize after the use of just a few singly excited configurations in the configuration interaction basis and, hypothetically, are more reliable as indicators of conformation in pyrimidine nucleosides related to cytidine.Spectroscopic studies of the nucleic acid bases and their derivatives have appeared continuously over the past quarter century (1-40). Many of these studies were motivated by the prospect of obtaining stereochemical information from changes in the absorption and optical rotation spectra of the nucleic acids and their models (41-51). More recently, the increasing importance of slightly modified versions of the nucleic acid monomers as potential therapeutic agents has accentuated the importance of optical studies in this area, particularly circular dichroism (CD) studies, which indirectly give one stereochemical information (10-18, 31-41, 52-61). The biological significance of the conformational aspects ofnucleosides and nucleotides has been sufficiently documented (60-63).Of the two main aspects of CD studies, stereochemical and spectroscopic, the spectroscopic problem is currently the more fundamental because incomplete knowledge of the electronic transitions that give rise to the absorption of radiation and the rotatory power has limited attempts to determine optically the chirality of nucleosides. Along with difficulties associated with treating the weak perturbation effects of the nonchromophoric sugar, this incomplete knowledge has hindered the development of theoretical models for calculating the rotatory parameters and extracting stereochemical information from them. In spite of these problems, CD studies of nucleosides have produced some useful rules and diagrams relating the sign of the first CD band to anomeric configuration and to conformation (10, 31-41, 52-61).The major goal of this study is to use the experimental data on 3-deaza-4-deoxyuridine, 3-deazauridine, and 3-deazacytidine to evaluate a new theoretical method for calculating and interpreting the optical properties of nucleosides. The method uses wave functions of...