Infrared studies of molecular configurations of DNA

Abstract

The experimental techniques involved in an infrared study of films of biological polymers, whose configurations depend on the water content of the specimen, are described in detail. Polarized infrared spectra of oriented sheets of the sodium and lithium salts of DNA, in both hydrogenated and deuterated states, have been recorded over a range of humidities between 0 and 94% r.h. (relative humidity). For sodium DNA two forms have been shown to exist, one at humidities greater than 90% r.h. where the bases are perpendicular to the helix axis (B form) and the other at humidities between 70 and 80% r.h. where the bases have tilted by an angle not less than 13° to the normal to the helix axis (A form). For lithium DNA the B form is found to exist for humidities greater than 66% r.h. and a second form, different from the A form of NaDNA, exists between 44 and 56% r.h. where the bases are tilted by about 4° from the perpendicular (C form). The orientation of the phosphate groups, inferred from the dichroic effects of the phosphate bands, is also discussed. The deuteration rates of both salts of DNA have been measured and found to be extremely fast—less than two minutes for complete deuteration.