The Symmetry and Spectral Properties of Helical Polynucleotides

The results of applying the theory of space groups to a model double- stranded helix were used to describe the electronic states (exciton states) of helical polynucleotides. The role of symmetry in determining the nature of the pi pi /sup */ and n pi /sup */ exciton bands of the helix is considered. Selection rules are given for electric-dipole transition between the ground state and exciton bands and for the mixing of zeroth-order states which results from interactions within the helix. Perturbation theory is used to determine quantitatively the effect of pi pi /sup */ mixing on the intensity of an n yields pi /sup */ transition which presumably is present on the lowenergy side of the 2600-A pi yields pi /sup */ absorption band. Since pi pi /sup */- n pi /sup */ mixing to first order is zero, it is necessary to use third-order perturbation theory. Results show that on formation of the helix such mixing may be sufficient to account for the observed increase in intensity in the region 2800 to 2900 A; however, the stolen intensity should appear as a perpendicularly polarized component. Other kinds of interaction which may account for the observed intensity increase aremore » discussed. (auth)« less