Conformational Properties of Some Viral DNAs

Oriented films of bacteriophage DNAs of low and intermediate (G + C) content and of different chemical composition were investigated by infrared linear dichroism: PBS 1 , DNA (28% G + C, uracil replaces thymine), T 4 , DNA (34%, G + C, glucosylated), T 4 * DNA (34% G + C, unglucosylated), T 5 , DNA (39% G + C). None of these phage DNAs exhibit a transition to a predominantly pure A form when relative humidity is lowered. At lower relative humidity, the infrared dichroism of the 1230‐cm‐ 1 band changes to mainly parallel orientation with respect to the DNA axis. The pattern of the tansition as a function of relative humidity indicates the presence at low relative humidity of a well‐defined form for which the O2–O3 line of the phosphate group was found to be directed approximately at 52°C with respect to the helix axis. This allows one to assign the lower‐humidity form of these phage DNAs as a predominant C‐like form. The comparison of T 4 with T 4 * DNA indicates that glucosylation has no effect on conformational properties of phage DNAs, and particularly on the absence of the B to A structural change. The replacement of the thymine by uracil in DNA of PBS 1 , has not affected the structural properties of the B form at high humidity nor the absence of the main A form at lower humidity; the transition from the B form to a predominant C‐like form was observed.