Low-level psoralen-deoxyribonucleic acid crosslinks induced by single laser pulses

Abstract

While many intercalated psoralens [skin, photosensitizing compounds] require a 1.3 .mu.s relaxation time between absorption of the 1st and 2nd photons for cross-link formation to occur, some psoralens can form cross-links within the lifetime of a 10 ns laser pulse. This effect is largely or completely O2 independent. Structural, kinetic, and energetic considerations suggest that the 1.3 .mu.s delay may be due to a conformational change in the T4 phage DNA at the intercalation site which could be required for proper alignment of the double bonds which react in the 2nd photoreaction. The cross-links which can form with single pulses of light may result from intercalation complexes which are already in a conformation such that, within 20 ns after absorption of an initial photon, a monoadduct is formed which can absorb a 2nd photon and thence result in a cross-link. These intercalation sites may be distinguished by the type and sequence of base pairs at the site or, alternatively, at the moment of the pulse, random motions of the DNA may have brought those sites into a conformation which allows cross-linking without the 1.3 .mu.s delay. Unlike ordinary cross-links, these rapidly forming cross-links appear to be monophotonic; i.e., they increase linearly with laser pulse energy. The 2nd photostep for these adducts effectively saturates at much lower laster intensities than is the case for ordinary cross-links.