[Oligonucleotides and their analogs. III. Circular dichroism and conformation of analogs cytidylyl-(3'--5')-adenosine].

The CD spectra of 11 analogues of CpA, containing modified carbohydrate residue, achiral hydroxyalkyl- and optically active hydroxymethylene substituents have been studied. On the basis of the dependence of the CD spectra on temperature, the constants of U in equilibrium S equilibria and thermodynamic parameters in the approximation of dilute solution was calculated. The calculation was carried out on a computer in the terms of a two-state model. The influence of the modifications of the carbohydrate on the stability of the S-conformation is discussed. The CD spectra of S-conformation was calculated and the assumption of the origin of oligonucleotides CD is considered. Possible mechanisms of how water participates in the stabilization of the S-conformation of dinucleoside phosphate are analyzed.