The optical rotatory dispersion, absorption, and, in some cases, the circular dichroism of some adenine nucleosides having diverse substituents in the carbohydrate ring have been examined. Solvent and temperature studies have also been performed when possible. Explanations for the changes in rotation with substituent are sought through theoretical means in correlation with available structural information. The changes in the 260-mfi Cotton effect lxoduced by the introduction of new s-electron systems can be accounted for quantitatively.
The circular dichroism data on 38 pyrimidine nucleosides, selected to provide a basis for a reliable index of furanose conformation as a function of its optical activity, are presented. The functional dependency of the B2u Cotton effect on the sugar-base torsion angle and on the specific pentose conformation is determined from theory and compared with experimental data. The theoretical description of the rotational strength as a function of the torsion angle agrees in every respect with available experimental data. Substituent effects on the signed magnitudes of the B2u rotational strength generally correlate quite well with theoretical expectations. The success of the theoretical calculations depend on giving up the Kirkwood-Tinoco coupled oscillator scheme where the transition moment vector is located at the center of gravity of the chromophore and, instead, breaking down the electric transition moment into bond contributions.In paper VIII of this series,1 it was shown that an analysis of the -* * Cotton effects of cyclopyrimidine nucleosides and several other rigid systems predicts their absolute stereochemical configuration. The method of analysis is an extension of the classical coupled oscillator theory of optical activity of Kuhn,2 which was reformulated in quantum mechanical terms
In the past few years the investigation of the properties and function of nucleic acids has become a central problem in molecular biology. The double-helix configuration of the crystalline salts of deoxyribonucleic acid (DNA) has now been well established. However, the secondary structure of DNA and RNA in aqueous solutions remains obscure. Structural investigations on the nucleic acids and polynucleotides based on the interpretation of optical rotatory dispersion and circular dichroism measurements by means of excitation theory requires quite complete information on the ultraviolet optical properties of the isolated bases. We have studied the optical rotatory dispersion (ORD), circular dichroism (CD), and absorption properties of nucleosides that are related to the constituents of the nucleic acids and polynucleotides. Close examination of these measurements reveals considerably more detail than was evident in preliminary absorption studies on similar systems.1-3 Our investigation hopefully will provide useful reference material for future work on the conformation and structure of the nucleic acids and related polynucleotides. The recent paper of Clark and Tinoco3 should be consulted for convincing evidence that the electronic states of all the bases are simply derived from those of benzene. Following Tinoco,3 we shall label bands which appear to be derived from the benzene transition Alg-B2U and Aig-Bl as B2. and Bl,, respectively. The more intense bands in the 180to 220-mA region will be related to the El. band in benzene. (Alternatively we could use the more descriptive Platt symbols.4) These bands are, of course, ir-+r'* transitions which are polarized in the plane of the base. The purine and pyrimidine chromophores, due to the nonbonding electron pairs of the N and 0 atoms, also exhibit n-7r* transitions which are polarized perpendicular to the base plane. Ultraviolet absorption studies' 3' , of various substituted purines and pyrimidines indicate that the strong absorption band in the 250to 280-mA region is complex containing the B2U and one or more n-7r* transitions. Furthermore, the studies reveal that a second in-plane transition (Bl.) is also located in the same region in some purines such as adenine. The two bands derived from the doubly degenerate Aig El. benzene transition are usually found in the 180to 220-m/A region. The fact that ir 7r* and n 7r* bands usually show opposite substituent, pH, and solvent effects helps to determine which type of transition is responsible for a particular Cotton effect or absorption band.1 6-8 For example, the ir-7r* bands are usually not greatly affected by protonation, whereas the no lr* bands undergo significant blue shifts. The absence of a significant pH effect and the close correlation between the observed Cotton effects and the intense absorption bands may be taken as evidence that a particular Cotton effect arises from one of the 4rw* transitions.
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