Kinetics of formation of deoxyribonucleic acid cross-links by 4'-(aminomethyl)-4,5',8-trimethylpsoralen.

If a mixture of T4 deoxyribonucleic acid (DNA) and 4'-(aminomethyl)-4,5',8-trimethylpsoralen is irradiated with two closely spaced pulses of long-wave UV laser light, the resulting cross-linking is dependent on the time delay between the pulses. As the delay lengthens to 1 mus, a rise in the number of cross-links is observed which follows first-order kinetics. This delay, the time required for most monoadducts to be able to absorb a second photon and thereupon form a cross-link, is interpreted in terms of a conformational change in the DNA at the psoralen intercalation site which may occur upon monoadduct formation.