Matrix‐method calculation of linear and circular dichroism spectra of nucleic acids and polynucleotides

Calculations of the optical properties (absorption, linear dichroism, circular dichroism, and anisotropic components of the CD) are presented for polynucleotides of random or regular sequence within the formalism of the matrix method using a set of parameters that includes only the ππ* transitions of the aromatic bases. Experimental solution spectra agree favorably with calculated CD spectra for A‐RNA, A‐DNA, and B‐DNA, when coordinates derived from x‐ray studies on fibers are used. Excessive hypochromicity is predicted when parameters intended to reproduce the vacuum‐uv absorption of the chromophores are included in the calculations, but total elimination of these parameters leads to an insufficient hypochromicity for the long‐wavelength absorption band. Using alternative conformations for DNA in low‐salt aqueous solution did not improve the agreement between experimental and calculated spectra, but some features of the optical properties predicted for these variant structures suggest that the tilt of the bases with respect to the helical axis may be larger than that of the fiber B‐form. In the case of polynucleotides with regular structure, which have been traditionally less easy to understand in terms of the standard nucleic acid conformations, a series of alternative structures has been examined. Unexpectedly, the calculated spectrum for the Z‐DNA structure compares almost quantitatively with the experimental spectrum of poly(dGC·dGC) in low salt. This result, which confirms a recent report [Vasmel, H. & Greve, J. (1981) Biopolymers 20 , 1329–1332], is in contrast with the current identification of Z‐DNA with the high‐salt form of poly(dGC·dGC). Finally, the optical properties of single‐stranded polyribonucleotides appear to be better explained when alternative structures [9 1 ‐helix for poly(rA) and 6 1 ‐helix for poly(rC)] are introduced instead of the A‐RNA form.