In vitro crosslinking of DNA by 8-methoxypsoralen visualized by electron microscopy

By electron microscopic visualisation of totally denatured DNA, we have detected photochemically induced 8-methoxypsoralen crosslinks in vitro after irradiation at 360 nm. The amount of crosslinks was expressed as the percentage of DNA length which was kept in double-stranded appearance by closely situated crosslinks. This percentage correlated well with irradiation time, irradiation intensity, and the concentration of 8-methoxypsoralen. These parameters have also been correlated with the mean size and the size distribution of non-crosslinked regions of DNA, so called bubbles. For a comparison with another psoralen type, we have carried out a similar set of experiments using 4,5,8-trimethylpsoralen.