Optical rotatory dispersion and circular dichroism of sulfur-containing nucleosides and nucleotide and of the ribonuclease-thionucleotide complex

Optical rotatory dispersion (ORD) and circular dichroism (CD) of sulfur-containing nucleosides (4-thiouridine, 6-thioguanosine and 6-thioinosine) and a nucleotide (4-thiouridine 2′(3′)-monophosphate) as well as a complex between ribonuclease (EC 2.7.7.16) and the nucleotide were measured in the near-ultraviolet and ultraviolet regions. All thionucleosides and the nucleotide displayed multiple Cotton effects which were correlated with their absorption maxima. Their CD spectra corresponded to the multiple Cotton effects except for 6-thioguanosine and 6-thioinosine which exhibited such low intensities that their bands could not be detected. The Cotton effects and CD bands were displaced toward shorter wavelengths upon alkaline treatment. The ribonuclease-thionucleotide complex showed ORD and CD near the absorption maximum at 331 mμ of the nucleotide quite different from the curves computed on the basis of the constituents. This suggests that the change in dissymmetry in the nucleotide molecule occurred on the formation of the complex. The ultraviolet ORD and CD were almost the same as those of the constituents, indicating the absence of any drastic conformational change in the protein molecule accompanying complex formation.