Low-level psoralen--deoxyribonucleic acid cross-links induced by single laser pulses.

While many intercalated psoralens require a 1.3-mus relaxation time between absorption of the first and second 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 oxygen independent. Structural, kinetic, and energetic considerations suggest that the 1.3-mus delay may be due to a conformational change in the deoxyribonucleic acid (DNA) at the intercalation site which could be required for proper alignment of the double bonds which react in the second 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 second 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-mus delay. Unlike "ordinary" cross-links, these rapidly forming cross-links appear to be monophotonic; i.e., they increase linearly with laser pulse energy. This suggests that the second photostep for these adducts effectively saturates at much lower laser intensities than is the case for ordinary cross-links.