The Effect of Polyamines on the Photochemical Reactivity of DNA

The effects of diaminopentane (cadaverine), diaminoethane and the polyamine, spermine, on the photoreactions of 4,5′, 8‐trimethylpsoralen with DNA were studied. Near ultraviolet (UV) light (360 nm) irradiation of psoralen with DNA results in the formation of psoralen monoadducts with pyrimidine bases and DNA interstrand crosslinks. The polyamines studied reduced the rate of both photoreactions to the same extent by a factor of 1.5–2.0. The magnitude of the effect increased with polyamine concentration. Effectiveness was in the order spermine > cadaverine > diaminoethane. This is also the same order for stabilization of the DNA double‐helix. Under conditions of higher DNA stability (higher ionic strength), higher polyamine concentrations were required to obtain an effect which was smaller than at low ionic strength. It is concluded that the photoreactions of psoralen with pyrimidines in DNA require some distortion of the double‐helix. This distortion is made more difficult in the presence of stabilizing agents like polyamines and therefore the rate of the reaction is reduced. If crosslinking of DNA by psoralen is a two‐step reaction then it must be concluded that the second step does not require further distortion. Consistently with the above, polyamines also reduced the UV‐induced dimerization of thymine in DNA, although to a lesser extent.