3- Methylcytosine, tautomerism

In 1 JMSO-z4, or DMF-(/, the 1 1 condensation products of cytosine or iV-methylcytosine with triformylmethane, compounds 39a and 39b, showed ring-chain tautomerism with the ring-closed products 40a and 40b (see Equation (2), Section 12.04.2.4) <1996ACS1137>. 

For instance Shugar and Fox106 have shown that the spectrum of cytosine resembles that of 1-methylcytosine (form 2), not that of 2-methoxycytosine (form 1). Similarly, Kenner et al.100 have established that lactam-imine forms 6 were unimportant both for 1-methylcytosine (thus also for cytosine itself) and for its aminoacetyl derivatives. Katritzky and Waring60 have extended these studies and in particular showed that tautomers 3 and zwitterionic structures 10 were unimportant in the tautomeric equilibrium in both aqueous and dimethyl sulfoxide solutions. Although these last comparisons were made with only two partially methylated cytosines still capable of tautomerism, the reported conclusions agree with those reached later by Brown and Lyall96 on much wider evidence. 

Experimental evidence122 shows that 5-azacytosine and 5-azacytidine exist in the lactam-amine form, type 13. Unfortunately, the tautomeric properties of 3-methyl-5-azacytosine have not been studies, so it is difficult to decide whether 5-aza-substitution in 3-methylcytosine causes a tautomeric shift toward the imine form. 

The 15N magnetic resonance studies of the pyrimidine bases or their derivatives are scarce. Roberts et al.BB have measured the 1H and 15N 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-methylcytosine hydrochloride labeled only in the amino group62). In the case of uracil,85 the two 15N-bonded protons gave two doublets centered at 10.78 and 10.96 ppm (measured downfield from internal tetramethyl-... 

Dowideit P, Mertens R, von Sonntag C (1996) The non-hydrolytic decay of formyl chloride into CO and HCI in aqueous solution. J Am Chem Soc 118 11288-11292 Dreyfus M, Bensaude 0, Dodin G, Dubois JE (1976) Tautomerism in cytosine and 3-methylcytosine. 

Compounds capable of a solvent-dependent amino/imino tautomerism are 3-methylcytosine (14a,b) and 1-alkyladenines [69b]. It has been shown by IR and UV/Vis spectroscopy that, in all cases, the imino forms such as (14b) predominate in nonpolar media e.g. 1,4-dioxane). However, the content of the amino form (14a) increases with increasing solvent polarity, and in aqueous solution the amino form predominates [69b]. Further interesting examples of solvent-dependent tautomeric amino/imino equilibria... 

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],... 

Before analysis of the interactions of the nucleic acid bases with the clay minerals in the presence of water and cation one needs to understand the individual interactions of NAs with isolated water and with a cation. Such theoretical study was performed for 1 -methylcytosine (MeC) [139]. The study revealed influence of water and cation in the proton transfer for this compound. This leads to the formation of imino-oxo (MeC ) tautomer. Topology of the proton transfer potential surface and thermodynamics of stepwise hydration of MeCNa+ and MeC Na+ complexes is further discussed. The one dimensional potential energy profile for this process followed by the proton transfer with the formation of hydrated MeC Na+ is presented in Fig. 21.2. One-dimensional potential energy profile for amino-imino proton transfer in monohydrated N1-methylcytosine (this represents the situation when tautomerization is promoted by a single water molecule without the influence of Na+ cation) and for the case of pure intramolecular proton transfer (tautomerization is not assisted by any internal interactions) is also included. The most important features of this profile do not depend on the presence or absence of Na+ cation. All the potential energy curves have local minima corresponding to MeC and MeC. However, the significant difference is observed in the relative position of local minima and transition state, which results in a different thermodynamic and kinetic behavior for all presented cases (see Fig. 21.2). 

The thermodynamic and kinetic parameters of the stepwise hydration of 1-methylcytosine and its imino-oxo tautomer in the presence of the Na" " cation have been investigated [139]. Hydrationof 1-methylcytosineby one water molecule leads to an increase of the concentration of its imino-oxo tautomer in the equilibrium mixture and decrease of the barrier of the tatutomer formation (to 15.6 kcal/mol). If the sodium cation is present the tautomeric form is much less favored and tautomerization barrier increases to 25.2 kcal/mol. The computationally predicted values of the rate constants suggest that the tautomerization of 1-methylcytosine to its imino-oxo form proceeds mainly due to a presence of the hydrated (MeCW) species. Based on the kinetic analysis of the tautomerization process in hydrated MeC in the presence of sodium ions it was concluded that complexes of hydrated MeC with Na+ are unlikely to contribute to the frequency of DNA point mutations caused by the tautomers. This is due to the fact that the interactions with Na" " lead to a decrease of both the rate and the equilibrium constants of the tautomerization reactions in hydrated 1-methylcytosine. 

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