NMR studies of an exocyclic 1,N2-propanodeoxyguanosine adduct (X) located opposite deoxyadenosine (A) in DNA duplexes at basic pH.: simultaneous partial intercalation of X and A between stacked bases

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Abstract
The NMR parameters for the 1,N2-propanodeoxyguanosine (X) opposite deoxyadenosine positioned in the center of the complementary d(C1-A2-T3-C5-G6-T-A8-C9).cntdot.d(G10-T11-A12-C13-A14-C15-A16-T17-G18) X.cntdot.A 9-mer duplex are pH dependent. A previous paper established protonated X5(syn).cntdot.A14(anti) pairing in the Z.cntdot.A9-mer duplex at pH 5.8 [Kouchakdjian, M., Marinelli, E., Gao, X., Johnson, F., Grollman, A., .ANG. Patel, D. J. (1989) biochemistry 28, 5647-5657]; this paper focuses on the pairing alignment at the lesion site at pH 8.9. The observed NOEs between specific exocyclic CH2 protons and both the imino proton of G6 and the sugar H1'' protons of C13 and A14 establish that X5 is positioned toward the G6.cntdot.C13 base pair with the exocyclic ring directed between C13 and A14 on the partner strand. The observed NOE between the H2 proton of A14 and the imino proton of G4, but not G6, establishes that A14 at the lesion site is directed toward the G4.cntdot.C15 base pair. NOEs are detected between all exocyclic CH2 protons of X5 and the H2 proton of A14, conforming that both X5 and A14 are directed toward the interior of the helix. The X5(anti).cntdot.A14(anti) (alignment at pH 8.9 is accommodated within the helix with retention of Watson-Crick pairing at flanking G4.cntdot.C15 and G6.cntdot.C13 base pairs. The energy-minimized conformation of the (G4-X5-G6).cntdot.(C13-A14-C15) segment at pH 8.9 establishes that X5 and A14 are directed into the helix, partially stack on each other, and are not stabilized by intermolecular hydrogen bonds. The C5 base is partially intercalated between C13 and A14 on the unmodified strand, while A14 is partially intercalated between G4 and X5 on the modified strand. This results in a larger separation between the G4.cntdot.C15 and G6.cntdot.C13 base paris flanking the lesion site in the basic pH conformation of the X.cntdot.A 9-mer duplex. The midpoint of the transition between the the protonated X5(syn).cntdot.A14(anti) and X5(anti.cntdot.A14(anti) conformations occurs at pH 7.6, establishing an unusually high pKa for protonation of the A14 ring opposite the X5 exocyclic adduct site. Thus, the interplay between hydrophobic and hydrogen-bonding contributions modulated by pH defines the alignment of 1,N2-propanodexoxyguanosine opposite deoxyadenosine in the interior of DNA helices.

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