A picosecond UV laser was used to cross-link proteins to DNA in nuclei, whole cells and reconstituted nucleohistone. Irradiation of the nucleohistone resulted in crosslinking 15-20% of bound histones to DNA in a very short time (one or several picosecond pulses), the efficiency of crosslinking to single stranded DNA being higher than to double stranded DNA. All histones as well as high mobility group 1 proteins were identified in the covalently linked protein-DNA complexes upon irradiation of isolated nuclei and whole cells. A method is suggested for isolation of crosslinked material from cells and nuclei in amounts sufficient for further analysis. Experiments with reconstituted nucleohistones showed that upon irradiation at a constant dose the efficiency of crosslinking depended on the intensity of the light, thus suggesting a two-quantum process is involved in the reaction.

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A picosecond flash-photolysis study of the ionization of psoralen derivatives and ethidium bromide, either free or intercalated in DNA, was carried out. A single short high-intensity 355 nm laser pulse with a few tens of J/cm2 dose, was found to be sufficient for achieving close to the ultimate 100% yield of ionization of either free or DNAbound psoralens. Interestingly, the photoionization efficiency of ethidium bromide is considerably reduced upon intercalation.