The degradation and denaturation of sodium deoxyribonucleate by γ‐rays

Abstract

AbstractAn earlier investigation of the effects of γ‐irradiation on aqueous solutions of sodium deoxyribonucleate showed that a plot of log [η] against log R (where [η] = intrinsic viscosity, R = radiation dosage) was linear at high R with a negative slope of 1.85. According to the theory of degradation of macromolecules, this slope should equal ( −α), where α is defined by [η] = KMα (K = constant, M = molecular weight). Determination by light scattering measurements of the M values of γ‐degraded nucleate samples has now shown that [η] = KM1.0; the slope of the log [η] against log R plot was therefore approximately twice that to be expected for random degradation of a single chain. Moreover, M−1 was a linear function of R2 over the whole range from R = 0 and this suggested that the initial nucleate had a random distribution of molecular weights. These observations consitute more positive evidence for the earlier proposal that the decrease in molecular weight of the double‐helical nucleate structure during γ‐irradiation was caused by two independent and approximately opposite breaks, one in each of the polynucleotide strands, and that the formation of such double breaks was proportional to R2. Heating ruptures these hydrogen bonds and, in accordance with the mechanism proposed, caused a relatively greater decrease in M the larger the previous radiation dosage (R). Titration at 25°C showed that γ‐irradiation ruptured the hydrogen bonds, although half of them were still present in the samples of lowest [η]. This change has now been re‐investigated by titrations at −0.75°C. which allow the determination of the reversible dissociation curves of the double helices and of the denatured form. The low temperature titrations showed that, unlike the main chain breakdown, the rupture of hydrogen bonds was non‐random, those linking adenine and cytosine being broken more easily than those linking guanine and cytosine.