KINETICS OF THYMINE PHOTODIMERIZATION IN DNA.

The kinetics of thymine photodimerization in E. coli DNA have been measured at various wavelengths of ultraviolet light. The initial quantum yield is not strongly dependent on wavelength. The ratio of thymine dimer to thymine in the photostationary state is much more dependent on wavelength. At the 235 mmu photosteady state 1.7 per cent of the thymine is present as dimer. This shifts to 6.5 per cent at 254 mmu and to 20 per cent of 275 mmu. While the change in position of the photosteady state with wavelength fails to fit a simple model, the data do indicate that not all thymines are capable of participation in dimer formation.