We report the solution structure of two heptanucleotides each containing a central N4-methoxycytosine, in one case paired with adenine on the opposite strand and the other with guanine. For the N4-methoxycytosine . adenine pair, only the imino form of the N4-methoxycytosine residue is observed and base pairing is in Watson-Crick geometry. However, rotation of the methoxy group about the N-OCH, bond is not constrained to a particular orientation although it must be anti to the N3 of N4-methoxycytosine. The slow exchange on a proton NMR time scale between the single strand and double strand forms is attributed to the strong preference of the cis conformation of the OCH, group in the single strand, which inhibits base pair formation. For the N4-methoxycytosine that is base paired with guanine, we observe an amino form in Watson-Crick geometry in slow exchange with a base paired imino form in wobble geometry. The amino form is predominant at low temperature whereas the imino form predominates above 313 K. We have measured the exchange rate between the two forms at 303 K and observed a value of approximately 1 s-'. The relative ratio of amino and imino forms of N4-methoxycytosine is influenced by both the base that is in front and the temperature. Our results explain the preferential replacement of dTTP by N4-methoxycytosine in primer elongation.Keywords: DNA; mutagenesis; tautomerism; NMR The reaction of methoxyamine with nucleic acids results in conversion of cytosine bases to N4-methoxycytosine (mo4C). Substitution of one hydrogen atom of the cytosine amino group by an electron-withdrawing methoxy group can induce formation of the imino tautomer form with tautomeric equilibrium constant orders of magnitude lower than those for the standard DNA bases.Studies of the stability of oligonucleotide duplexes containing mo4C [I, 21 revealed that mo4C, which is ambivalent in its hydrogen bonding potential, forms stable base pairs with both adenine and guanine. It was predicted that mo4C would base pair with G in the amino form and with A in the imino form. An NMR study [3] proposed that mo4C when paired with A exists in a rapid equilibrium between Watson-Crick and wobble conformations.An X-ray crystal structure [4] of an oligonucleotide containing a mo4C. G base pair, found that the base pair adopts a wobble conformation with the mo4C base in the imino form. The oligonucleotide was observed to be a Z DNA form. Recent NMR studies [5, 61 proposed wobble and Watson-Crick structures in equilibrium. In one of these studies [6], a further equilibrium between syn and anti conformations of the methoxy group Abbreviations. ROESY, rotating-frame Overhauser spectroscopy; lD, one dimensional; 2D, two dimensional; 2QF, double quantum filtered correlated spectroscopy ; mo4C, N4-methoxycytosine. of mo4C . G in the imino form was invoked to explain the complexity of the NMR spectra.We recently reported [5] preliminary NMR studies on two heptamer duplexes with mo4C incorporated into the central position with G or A on the opposite strand...
En solution aqueuse les polyvinyle pyridines quaternisees par des bromures d'alkyles normaux A longue chaine sont susceptibles de former des micelles. Cette propriete a CtC mise en evidence par solubilisation d'hydrocarbures et de colorants' , 2 a 7 3 ) , v i~c o s i m~t r i e ' "~~*~" ) et diffusion de la l~m i k r e '~,~' . La relaxation nucleaire convenant particulikrement A 1'Ctude de la dynamique de ces macromolecules en solution a ete appliquee aux polyvinyle-4 pyridines, (PV4P), quaternisees par l'acide bromhydrique (PV4PH') ainsi que par des bromures d'alkyles normaux comportant 2 A 10 carbones (1 I x I 9), de formule generale (n = degre de polymerisation, q = taux de quaternisation): -CH -CH 4 Les mesures du temps de relaxation TI ont et6 effectuees en transformee de Fourier A 25MHz pour I3C, 100 et 250MHz pour 'H, sur des PV4P atactiques (avec les triads mm = 5 %, mr = 35 %, rr = 60%) de masse molkculaire 1601
The adducts of tungsten(V1) WF6.F-py and WOF4.F-py have been prepared from the reaction of WF6 or WOF4 with 2-fluoropyridine (F-py) in dichloromethane. The two adducts were characterized by X-ray powder data and vibrational spectroscopy. The adduct of WOF4 is only slightly volatile at room temperature, whereas that of WF6 has a dissociation pressure of 26.5 mmHg at 21 OC. Solutions of these adducts in CD2C12 were studied by I9F, 'H, and I3C NMR spectroscopy. In WOF,.F-py, the fluorine atoms bonded to the tungsten atom are magnetically equivalent, and their coupling with the fluorine atom of F-py was observed at 25 OC (b(Fw) = 66.8 ppm/CFCl,, JFF = 14.3 Hz). The labile character of F-py in WF6.F-py did not permit such observations. The crystal structures of wF6.F-p~ and WOF4.F-py have been determined by X-ray diffraction methods. In WF6.F-py, the tungsten atom is surrounded by a capped trigonal prism of ligands with the nitrogen atom of the organic ligand occupying the capping site. In WOF4.F-py, the tungsten atom is surrounded by a distorted octahedron of ligands with the nitrogen atom of the organic ligand trans to the oxygen atom and the equatorial plane made up of four fluorine atoms located at the corners of a square. The space groups, unit cell parameters, and R factors are as follows. WF6.F-py: monoclinic, P2'/c (No. 14), a = 7.008 (9) A, b = 15.373(3) A, c = 8.023(3) A, B = 101.62(4)O, V = 846(1) A3, Z = 4, R = 0.028. WOF4.F-py: monoclinic, P2'/c (No. 14), a = 8.742(3) A, b = 9.349(8) A, c = 10.737(4) A, / 3 = 106.85(3)', V = 840(1) A3, Z = 4, R = 0.033. Supplementary Material Available: Tables of crystal data, bond distances, bond angles, atomic positional parameters, and anisotropic thermal parameters (6 pages). Ordering information is given on any current masthead page.
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