Evaluation of configuration coordinate parameters from high pressure optical data. II. Purine and pyrimidine bases and nucleosides

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Abstract
High pressure optical absorption measurements have been made on a series of purine and pyrimidine bases and the corresponding nucleosides in the crystalline state, in neutral aqueous solution, in acidic solution, and in basic solution. The results are discussed in terms of the configuration coordinate parameters as developed in the previous paper. For the crystalline compounds, the purine bases exhibit a volume decrease (Δ) like that of phenanthrene upon excitation. The pyrimidines show slightly smaller contractions. The addition of the ribose group inhibits the volume decrease accompanying excitation. The coupling coefficients β are of the same order of magnitude as those for the crystalline hydrocarbons. In neutral aqueous solution larger volume changes occur upon excitation, and the ribose group is less inhibiting. The substituent effects are significant. In acidic solutions the magnitude of Δ is smaller and that of β is larger than in neutral solutions. The nucleosides show smaller changes from the neutral solution than do the corresponding bases. In basic solution one observed either a small negative Δ or a positive Δ. The ribose group again inhibits the change from neutral solution. Thymine and uracil exhibit very large blue shifts of the peaks with pressure. It is possible that they are doubly ionized.

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