On the dynamics of DNA in aqueous solution. Pulse radiolysis studies using the light scattering detection method

Abstract

The kinetics of DNA chain breakage in solution induced by 2 µs pulses of 15 MeV electrons were investigated by light scattering. On irradiating native calf thymus DNA at room temperature the decrease of light scattering intensity (LSI) - due to double strand ruptures - shows a fast decay with a half lifeτ 1/2 of about 30 ms as well as a slow decay withτ 1/2 of about 10 s. With increasing temperature (20–40° C) both the total degree of degradationα and the fraction of the fast decay increase due to the facilitated melting of segments between two single strand breaks on alternate strands forming a double strand break. Above 40° C a third mode of LSI decay withτ 1/2 of 5–10 s arises, indicating detachment of relatively long segments. The total relative decrease of LSI after irradiation A, which can be taken as a measure of the degree of degradationα, follows the square of the absorbed dose in the case of native DNA, whereas on irradiating denatured DNA A rises linearly with dose. The decay of LSI due to the degradation of denatured DNA is much faster than that of native DNA withτ 1/2 down to 150 µs, depending on the absorbed dose. The half lives are interpreted in terms of the separation of fragments by diffusion and of the melting of double strand segments between two single strand breaks.