CORRELATIONS BETWEEN THE DUPLEX WINDING ANGLE AND THE CIRCULAR-DICHROISM SPECTRUM OF CALF THYMUS DNA

Circular dichroism spectra have been obtained on linear calf thymus DNA under conditions where information on the effect of these conditions on the duplex winding angle of circular PM-2 DNA is available from the studies of others [Anderson, P., & Bauer, W. (1978) Biochemistry 17, 594], Our experiments reveal that the magnitude of the positive band above 260 nm decreases in a linear manner as the duplex winding angle increases, as a result of decreases in temperature and/or increases in the content and variations in the nature of the cation concentration of the aqueous medium. The character of the changes in the spectral properties as well as the direction of the changes in the winding angle is consistent with the interpretation that DNA moves along a conformational continuum, from a more B-like to a more C-like state as the temperature decreases or the cation concentration increases. Temperature appears to have an additional discriminating effect on the relative weights which the GC and the AT pairs make to the spectral properties; the contributions of the latter appear to decrease with increasing temperature. These results support our previous conclusion that the specific cation effect on the circular dichroism spectrum of DNA is a real conformational effect rather than a perturbation of the spectral properties of the base chromophores [Hanlon, S., Brudno, S., Wu, T. T., & Wolf, B. (1975) Biochemistry 14, 1648]. © 1979, American Chemical Society. All rights reserved.