Investigations of the conformation of DNA, native or alterated by irradiation, with ultraviolet radiation

An extension of the range of scattering angles in the direction of smaller angles (up to delta = 120) made it possible to successfully use the light scattering methods for the determination of DNA molecular weights >= 3 x 106. In order to determine the conformation of native DNA in solution, different molecular weights were prepared by ultrasonic degradation. According to their hyperchromicity, these preparations are practically native. When native DNA in solution is irradiated with UV light of the wavelength lambda = 313 nm, two different photoreactions may occur: a) double and single strand breaks leading to degradation of the DNA molecule, and b) dimerisation of neighbouring thymine bases. The two reactions are independent of each other. In the presence of acetophenone as photosensitizer, the reaction type a) is greater by a factor 4 (in terms of single-strand breaks), while the reaction type b) is greater by a factor 16. The number of thymidine dimers per single strand break amounts to 100 for photosensitized reactions and to 25 for non-photosensitized reactions. The number of single strand breaks in terms of the quantum flux of 1 μ Einstein absorbed by the DNA is greater by a factor 3 during irradiation with UV light lambda = 254 nm as compared to the wavelength lambda = 313 nm. At this wavelength, DNA degradation starts at absorption energies as low as >= 2 x 107 erg/cm3. Light scattering and measurements with DNA containing thymidine dimers indicated neither a change in the total conformation nor a noticeable change in the microstructure. The hyperchromicity of the DNA was also unchanged. From these experimental results, it is concluded that the double helix of DNA is essentially stable to thymidine dimerisation. (orig./MG)