The review presents the theoretical basis for the origin of optical activity of nucleic acids. The optical properties of nucleic bases are discussed in terms of theoretical calculations by Hug and Tinoco. Although nucleic bases are themselves optically inactive, the in-plane (pŽp*) as well as out-of-plane (nŽp*) transition dipoles induced in their rings are responsible for optical activity of nucleosides, nucleotides and polynucleotides. In a case of nucleosides and nucleotides, which are called "monomers", the optical activity originates from the disturbance of the electronic system of a base caused by the presence of a sugar ring. The main factor influencing the character of CD spectra is the torsion angle about the glycosyl bond. The experimental spectra of monomers are dominated by in-plane transitions. In a case of cytidine and guanosine (Rys. 3) the lowering of pH reveals the presence of the out-of-plane transition.
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