X-ray diffraction study of three DNA fibres with different base compositions.

Abstract

An X-ray diffraction study on three DNAs with guanine + cytosine contents of 26%, 42% and 72%, respectively has been made in order to establish a possible correlation between base composition and the conformations of double helices. This work shows that the helical parameters in the A and B forms in the presence of sodium and lithium salts are independent of the base composition. The experimental intensities are compared to calculated values, taking into account the variations of the parameter α in the hexagonal lattice of the B form. The three DNAs show no noticeable difference. Theoretical calculations with a ratio of (A + T)/(C + G) varying from 4 to 0·25 confirm that the base composition has a negligible effect on the diffracted intensities, provided the helical parameters remain constant. A complementary calculation made with a tilt of the bases shows that only the intensities corresponding to the higher layers are affected. One may conclude that the conformation of the sugar-phosphate chain is not affected by the base composition, and that the proportion of (G + C) bases cannot be detected by X-ray analysis.