| Abstract: | Theoretical calculations of the circular dichroism of double-helical DNA and RNA by the method of Johnson and Tinoco were performed in order to investigate the origins of the optical activity spectral differences between these polynucleotides. Calculations were performed using transition moment directions arising from molecular orbital calculations as well as a transition moment directions in agreement with experimental directions. The results of these calculations indicate that the conservative circular dichroism spectrum of B-form DNA and the nonconservative spectrum of RNA (and A-form DNA) arise as a consequence of the distance between the paired bases and the helix axis. The negative nonconservative spectrum of C-form DNA was calculated and shown also to result from the distance of the paired bases from the helix axis. Several other conspicuous geometric parameters of DNA and RNA were investigated and were found to be less significant in their effects upon the spectral differences. Theoretical calculations on a four-stranded DNA model which has paired bases similarly related to the helix axis as RNA and A-form DNA was found to yield a low intensity, nonconservative circular dichroism spectrum. |
