Cytosine, proton magnetic resonance

There are seven possible tautomeric forms for cytosine (XLIX, Lllla-f). That the actual predominant tautomeric structure is (XLIX) has been adequately shown from ultraviolet spectral comparisons with N- and 0-methyl derivatives in aqueous solution [214-216], from proton magnetic resonance studies in dimethyl sulphoxide and other solvents [214, 217], from infrared spectra in the solid state [218], and from X-ray crystallography [219]. Cytosine has pX values of 4-45 and 12-2 for the dissociation of the cation and base, respectively [215] therefore, in neutral medium, the molecule is uncharged. The corresponding 1- and 3-methylcytosines have pK values of 4-55 [215] and 7-49 [214] respectively for the dissociation of the cations, from which the equilibrium constant for the tautomeric forms (XLIX) and (Lllla) of cytosine may be calculated as c. 10 in favour of (XLIX) [214] pro tonation of cytosine occurs at N3 [214],... 

Remin M, Shugar D (1973) Crmfoimational analysis of cytidine l-p-D-(arabinofuranosyl) cytosine and their O -methyl derivatives by proton magnetic resonance spectroscopy. J Am Chem Soc 95 8146-8156... 

As in the case of cytosine, several NMR and NQR studies were performed in search of the predominating tautomeric structures of uracil and thymine and their nucleotides and nucleosides. Investigation of PMR spectra of these compounds in nonaqueous solvents, such as dimethyl sulfoxide, localized the mobile protons in a number of 5- and 6-substituted uracils.59,61,328 These and similar studies63,85,329,330 indicated that dilactam structure 32 predominates in uracil compounds in aqueous and nonaqueous solutions as well as in the solid state. Proton and N-15 magnetic resonance spectra of several pyrimidines85 confirmed the diketo structure usually ascribed to uracil. 

Cytosine, protonated at N3 Proton nuclear magnetic resonance Metal complex of rare cytosine tautomer 9 -Methy ladenine... 

The magnetic resmance studies of the pyrimidine bases or their derivatives are scarce. Roberts el aZ. have measured the and magnetic resonance spectra of a number of pyrimidines including uracil and 1-methylcytosine. The most important result of this study was the elucidation of the dominant tautomeric structures of uracil and proto-nated 1-methylcytosine as the diketo, 32, and keto-amine form, 7, respectively (cf. Sections II and IV and the spectrum of 1-methyl-cytosine hydrochloride labeled only in the amino group ). In the case of uracil, the two N-bonded protons gave two doublets centered at 10.78 and 10.96 ppm (measured downfield from internal tetramethyl-... 

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