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: PBS1, DNA (28% G + C, uracil replaces thymine), T4, DNA (34%, G + C, glucosylated), T4* DNA (34% G + C, unglucosylated), T5, 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 T4 with T4* 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 PBS1, has not affected the structural prope