TOPOGRAPHY OF NUCLEIC ACID HELICES IN SOLUTIONS .12. ORIGIN OF OPPOSITELY INDUCED CIRCULAR DICHROISM OF REPORTER MOLECULES BOUND TO RIBO- AND DEOXYRIBONUCLEIC ACID

A number of reporter molecules I, R(CH2)nN +(CH3)2(CH2)3N +(CH3)3 • 2Br¯, where R is a chromophore absorbing in the 300-500-mµ region, have been synthesized. The effect of DNA and RNA on the absorption spectra of I is reported. A red shift and a hypochromic effect on the absorption spectra of the bound chromophore are observed. Moreover, it is found that DNA and RNA induce an opposite CD in the absorption band of the “unsymmetrical” reporter molecules, I. The origin of this effect appears to be the orientation of the chromophore with respect to the helix axis. The results are interpreted in terms of coupling of the transition moments of the reporter molecules, I, with the transition moments of the bases of the nucleic acids. Moreover, the evidence seems to indicate that the chromophore ring of I in the nucleic acid complex lies in the minor groove of the helix. The substituent of the “ unsymmetrical” 4-nitroaniline reporter molecules appears to be pointing in and out for DNA and RNA complexes, respectively. Molecular models of the DNA- and RNA-reporter systems are formulated and discussed. © 1969, American Chemical Society. All rights reserved.