Changes Produced by Ultraviolet Light in the Presence and in the Absence of Oxygen on the Physical Chemical Properties of Deoxyribonucleic Acid

Abstract

Dilute solutions of DNA were irradiated with a mercury lamp in a quartz envelope that allowed radiations of low wavelength to penetrate, and with a filtered lamp (Vycor) with which the effects were due solely to light of 254-m[mu] wavelength. In the absence of oxygen, irradiation with the Vycor lamp reduced the viscosity and radius of gyration of the DNA. Initially there was no effect on the molecular weight, which fell only after a threshold period. Evidence from the absorption spectrum of the irradiated DNA and its stability at elevated temperatures suggests that the primary effect of 254-m[mu] light is to disrupt hydrogen bonds and thereby make the molecule more flexible so that it assumes a more coiled configuration in solution. In the presence of dissolved oxygen, the rate of coiling by 254-m[mu] light is increased and main-chain scission (estimated from molecular weight changes) occurs immediately. With the unfiltered UV light both coiling and main-chain scission occur in the absence of oxygen. In its presence the number of breaks is greatly enhanced. The oxygen effect is probably due to the addition of oxygen to activated DNA to give a peroxide which is decomposed with the release of OH radicals. Hydroxyl radicals were shown to produce main-chain breaks but did not give rise to coiling. Ozone produced by short-wavelength UV light produces main-chain breaks relatively ineffectively. Decomposition products of ozone are probably the principal degradative agents when DNA is irradiated with the unfiltered lamp. Glutathione, dimethylaniline, and glucose (in relatively high concentrations) protect DNA against the effect of UV light in the presence of oxygen but not in the absence of oxygen.