A circular dichroism study of modified nucleosides

The conformation of 5-methoxycarbonylmethyluridine and 5-methoxycarbonylmethyl-2-thiouridine was studied by means of circular dichroism in various solvents. In order to calculate the accurate spectral parameters of the Cotton effects, the circular dichroism spectra were resolved into component Gaussian functions which simultaneously fit the adsorption spectra. On the basis of circular dichroism and proton magnetic resonance spectra, these nucleosides were found to occur in the β-configuration with the ³E-gg-anti conformation preferred. Due to the fact that the long-wavelength Cotton effect of mcm⁵s²U is not masked by the Cotton effects of the other nucleic acid monomers, the molecular parameters of this band may be useful for the conformational analysis of tRNA segments.