Hydroxylamine and methoxyamine mutagenesis: displacement of the tautomeric equilibrium of the promutagen N6-methoxyadenosine by complementary base pairing

Abstract

The imino-amino tautomeric equilibrium of the promutagenic adenosine analogue N6-methoxy-2',3',5'-tri-O-methyladenosine [OMe6A(Me)3], in solvents of various polarities, has been studied with the aid of 1H and 13C NMR spectroscopy. The high energy barrier (free enthalpy delta G = 80 +/- 5 kJ X mol-1) between the two tautomeric species renders possible direct observation of the independent sets of all 1H and 13C signals from each of them. The equilibrium ranges from 10% imino in CCl4 to 90% in aqueous medium. Thermodynamic parameters, including energy barriers and lifetimes, were calculated from the temperature dependence of the equilibrium. Essentially similar results prevail for the promutagenic N6-hydroxy analogue. The conformations of the sugar moieties, and of the base about the glycosidic bond, for both tautomers are similar to those for adenosine. The conformation of the exocyclic N6-OCH3 group, which determines the ability of each species to form planar associates (hydrogen-bonded base pairs), has also been evaluated. Formation of autoassociates of OMe6A(Me)3 and of heteroassociates with the potentially complementary 2',3',5'-tri-O-methyluridine and -cytidine, in chloroform solution, was also investigated. The amino form base pairs with uridine and the imino form with cytidine. Formation of a complementary base pair by a given tautomeric species was accompanied by an increase of up to 10% in the population of this species and a concomitant decrease in population of the other species.(ABSTRACT TRUNCATED AT 250 WORDS)

Crystallographic studies on damaged DNAs. I. an N6-methoxyadenine residue forms a watson-crick pair with a cytosine residue in a B-DNA duplex

“…Two crystal structures of N 6 -methoxyadenine derivatives, N 9 -benzyl-N 6 -methoxyadenine (Fujii et al, 1990) and N 6 -methoxy-2 H ,3 H ,5 H -tri-O-methyl-adenosine (Birnbaum et al, 1984), indicate that the N 6 -methoxylated base prefers the imino form. This fact is consistent with the results from proton nuclear magnetic resonance (Stolarski et al, 1984(Stolarski et al, , 1987, and ultraviolet and infrared spectroscopy studies (Fujii et al, 1987). In the crystals, the methoxy groups take a syn conformation around the N 6 -C 6 bond to the N 1 atom, so that the two purine moieties are associated through three hydrogen bonds including a C-HÁ Á ÁN interaction.…”

“…To change the conformation from syn to anti or vice versa, the adenine moiety must pass through the amino form by tautomerization. Since it is thought that N 6 -methoxyadenine derivatives are in equilibrium between the amino and the imino forms in solution (Stolarski et al, 1984), the imino form with the anti confor-mation may be induced to stabilize the base-pairing with cytosine.…”

Aminyl Radical Generation via Tandem Norrish Type I Photocleavage, β-Fragmentation: Independent Generation and Reactivity of the 2′-Deoxyadenosin- N6-yl Radical

“…The protons in the aminyl tautomer appear downfield in the spectrum relative to the corresponding protons in 7b -im, as expected based upon the loss in aromaticity in the latter, and consistent with trends reported in the literature. 47 The relevance of the tautomeric equilibrium to the photochemistry of 7b is discussed below.…”

Crystallization and preliminary analysis of a DNA dodecamer of d(CGCGmo6AATCCGCG) containing 2′-deoxy-N 6-methoxyadenosine: change in crystal packing with different humidity

“…In the case of an adenine derivative methoxylated at N(6), it has been shown that this modi®cation allows the adenine base to tautomerize between amino and imino forms with the latter more stabilized under hydrophilic conditions (Stolarski et al, 1984;Fujii et al, 1987). Matsuda and his colleagues showed by an in vitro polymerase incorporation assay that when a DNA fragment containing the modi®ed adenosine was used as a template, not only complementary thymidine residues but also cytidine residues were incorporated into the newly synthesized opposite DNA strand (Nishio et al, 1992).…”