NMR SOLUTION STRUCTURE OF A NONANUCLEOTIDE DUPLEX WITH A DG MISMATCH OPPOSITE A 10S ADDUCT DERIVED FROM TRANS ADDITION OF A DEOXYADENOSINE N-6-AMINO GROUP TO DROXY-9,10-EPOXY-7,8,9,10-TETRAHYDROBENZO[A]PYRENE - AN UNUSUAL SYN GLYCOSIDIC TORSION ANGLE AT THE MODIFIED DA())

A nonanucleotide, d(G(1)G(2)T(3)C(4)[BaP]A(5)C(6)G(7)A(8)G(9)), in whi ch droxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (7-hydroxyl grou p and epoxide oxygen are trans) is covalently bonded to the exocyclic N-6-amino group of deoxyadenosine (dA(5)) through trans addition at C1 0 of the epoxide (to give a 10S adduct) has been synthesized. The solu tion structure of the duplex, d(G(1)G(2)T(3)C(4)[BaP]A(5)C(6)G(7)A(8)G (9)). d(C(10)T(11)C(12)G(13)G(14)G(15)A(16)C(17)C(18)), containing a d G mismatch opposite the modified dA (designated 10S-[BaP]dA . dG 9-mer duplex) has been investigated using a combination of 1D and 2D (inclu ding COSY, PECOSY, TOCSY, NOESY, and indirect detection of H-1-P-31 HE TCOR) NMR spectroscopies. The NMR results together with restrained mol ecular dynamics/energy minimization calculations show that the modifie d dA(5) adopts a syn glycosidic torsion angle whereas all other nucleo tide residues adopt anti glycosidic torsion angles. The sugar ring of dA(5) is in the C3'-endo conformation, and the sugar rings of the othe r residues are in the C2'-endo conformation. The hydrocarbon attached at dA(5) orients toward the 3' end of the modified strand (i.e., dC(6) direction) and intercalates between and parallel to bases of dG(13) a nd dG(14) Of the complementary strand directly opposite dC(6) and dA(5 ), respectively. The edge of the hydrocarbon bearing H11 and H12 is po sitioned between the imino protons of dG(13) and dG(14) in the interio r of the duplex, whereas H4 and H5 at the opposite edge are positioned near the sugar H1' and H2 '' protons of dG(13) and facing the exterio r of the duplex. The mismatched AG base pair is stabilized by dA(sym)- dG(anti) base pairing in which the imino proton and the O-6 Of dG(14) are hydrogen bonded to N7- and the single N-6-amino proton, respective ly, of the modified dA(5). The modified DNA duplex remains in a right- handed helix, which bends at the site of intercalation about 20 to 30 degrees away from the helical axis and toward the direction of the mod ified strand.