Structure and architecture of the bacterial virus fd. an infrared linear dichroism study

Oriented gels of intact bacterial virus fd have been investigated by infrared linear dichroism. Infrared absorption hand maxima and dichroism indicate an α-helix content of the major coat protein of 95-100%. The α-helical rods of the coat protein are aliened parallel to the long axis of the virion with an inclination roughly estimated to ≈37°. The presence of DNA infrared bands at 968, 885. 830 and 799 cm^-, the absence of a hand at 860 cm^-1 and the perpendicular polarization of the symmetric PO₂^- stretching vibration at 1085 cm^-1 are all indicative of a B-type backbone conformation in the single-stranded DNA. We find no evidence for specific interaction between aromatic side groups (phenylalanine. tyrosine) and the DNA bases. Our results independently confirm most features of the model of Marvin and co-workers [2.15] based on low-resolution X-ray diffraction studies. However, our findings contradict their suggestion of an A-type DNA in the bacterial virus fd. Two results are consistent with rigid and stable order in the virus. First, over a 4-day period. 65% of the peptide hydrogens remain unexchanged with deuterium. Second, changes in the relative humidity of the sample do not result in any sliifts in the DNA spectrum that are characteristic of free DNA.