Rapid characterization of cross-links, mono-adducts, and non-covalent binding of psoralens to deoxyoligonucleotides by LC-UV/ESI-MS and IRMPD mass spectrometryElectronic supplementary information (ESI) available: Impact of irradiation time on the formation of DNA/psoralen complexes; a comparison of dissociation trends for CID and IRMPD of the D1/8-MOP complexes. See DOI: 10.1039/b924023c

Upon UV photoactivation, psoralen analogs form covalent mono-adducts and cross-links with DNA at thymine residues. Electrospray ionization mass spectrometric analysis allowed rapid and efficient determination of the reaction percentages of each psoralen analog with DNA duplexes containing different binding sites after exposure to UV irradiation. The distribution of cross-linked products and mono-adducts was monitored by both LC-UV and IRMPD-MS methods with the highest ratio of cross-linked products to mono-adducts obtained for 8-methoxypsoralen (8-MOP), psoralen (P), and 5-methoxypsoralen (5-MOP). Reactions at 5′-TA sites were favored over 5′-AT sites, and duplexes containing two and three binding sites showed extensive binding by the psoralens. 4′-Aminomethyl-4,5′,8-trimethylpsoralen (AMP) bound non-selectively via non-covalent interactions and was the only psoralen analog to show significant binding in the absence of UV irradiation. 8-MOP binding displayed the greatest sequence selectivity among the psoralen analogs. The sites of interstrand cross-linking were determined by fragmentation of the duplex/psoralen complexes by infrared multiphoton dissociation (IRMPD), which produced cross-linked product ions containing an intact single strand, the psoralen analog, and either a w n or a n -B portion of the complementary strand. IRMPD of DNA/AMP complexes after UV irradiation also produced high abundances of the intact single strands with the AMP ligand attached, products indicative of a significant population of mono-adducts. The formation of non-covalent complexes and both mono-adducts and cross-links between DNA and a series of psoralen analogs was evaluated by infrared multiphoton dissociation (IRMPD) and LC-UV-MS.