Samir Mansy

Raman spectra of methyl derivatives of 5′-adenosine monophosphate, tubercidin, inosine, uridine and cytidine. Perturbation of nucleoside vibrations by electrophilic attack at different sites

Abstract Raman spectra have been obtained for 1-methyladenosine-5′-phosphate in H2O and D2O over a wide pH range, and its rearrangement in mild alkali to N6-methyladenosine-5′-phosphate has been followed. Similarly, the spectra of tubercidin (7-deazaadenosine) and 1-methyltubercidin were measured and the latter molecule also was observed to rearrange to the N6-derivative in base. The spectra of 7-methylinosine and of 1,7-dimethylinosine were obtained over a wide pH range in H2O and D2O as were the spectra of 3-methyluridine and 3-methylcytidine. The effect of methylation at different sites on the acidity and the spectra of nucleosides is discussed and compared with the effects of electrophilic attack by the proton, mercury(II) and platinum(II). The ring modes above 1500 cm−1 were observed to vary systematically depending upon the strength of the electrophile. The application of vibrational spectra to the assignment of the sites of electrophilic attack on nucleotides is discussed.

Heavy metal-nucleotide interactions. IX. Raman difference spectroscopic studies on the binding of CH3Hg(II) to 1-methylthymine, thymidine-5'-monophosphate, DNA models and native DNA.

Raman spectra have been obtained for dTMP and its complex with CH3Hg (II) in aqueous solution as a function of pH. Difference spectroscopy is employed to increase the sensitivity of the Raman technique. The binding reaction is essentially quantitative from pH 3 to 9, and the value of the equilibrium constant for CH3HgOH2+ + dThd in equilibrium CH3Hg(dThdH--1) + H30+ is estimated from intensity measurements to be 0.6 in reasonable agreement with an earlier value based upon uv spectrophotometric data. Binding is to N(3) with substitution of CH3Hg+ for the proton. A similar reaction occurs with 1-MeThy. Raman spectra for aqueous and crystalline 1-MeThy and for the complex CH3Hg(1-MeThyH--1) are reported. The spectrum of crystalline Hg(1-MeThyH--1)2, for which the crystal structure is known, also was obtained for comparison. Raman difference spectroscopy was used to confirm that CH3Hg (II) binds to N(3) of dTMP and N(1) of GMP at r = 0.2 (MeHg+: phosphate) ratios with mixtures of GMP + CMP + AMP + dTMP. In contrast, native calf thymus DNA does not appear to bind CH3Hg(II) at these sites at r = 0.15, although no significant amount of free CH3HgOH is present. With r = 0.3, extensive binding occurs both to the Thy and Gua bases. Raman difference spectroscopy is a valuable technique for studying the binding of ions and molecules to polynucleotides in moderately dilute aqueous solution.

Metal ion catalysis of proton exchange between purine nucleosides or nucleotides and solvent water

Co-ordination of the methylmercury(II) cation at N-7 of inosine and guanosine-5′-phosphate catalyses proton exchange between the proton bound to C-8 and the solvent water leading to disappearance of the 1H n.m.r. singnal in D2O.