SOLUTION STRUCTURE OF OLIGONUCLEOTIDES COVALENTLY-LINKED TO A PSORALEN DERIVATIVE

Psoralen (pso) was attached via its C-5 position to the 5'-phosphate g roup of an oligodeoxynucleotide d(TAAGCCG) by a hexamethylene linker ( ms). Complex formation between pso-m(6)-d(TAAGCCG) and the complementa ry strands d(CGGCTTA)[7-7mer] or d(CGGCTTAT)[7-8mer] was investigated by nuclear magnetic resonance in aqueous solution. Structural informat ions derived from DQF-COSY and NOESY maps, revealed that the mini doub le helix adopts a B-form conformation and that the deoxyriboses prefer entially adopt a C2'-endo conformation. The nOe connectivities observe d between the protons of the bases or the sugars in each duplex, and t he protons of the psoralen and the hexamethylene chain, led us to prop ose a model involving an equilibrium between two conformations due to different locations of the psoralen. Upon UV-irradiation, the psoralen moiety crosslinked the two DNA strands at the level of 5'TpA3' sequen ces. NMR studies of the single major photocross-linked duplex pso-m(6) -d(TAAGCCG) and d(CGGCTTA) were performed. The stereochemistry of the diadduct is indeed cis-syn at both cyclobutane rings. In addition, the effects of this diadduct on the helical structure are analyzed in det ail.