Thymine 2,3 -imino-1 -

The tautomerisation of the purine bases adenine and guanine and of the pyrimidine bases thymine, cytosine, and uracil has important implications in molecular biology, and the occurrence of rare tautomeric forms of these bases has been suggested as a possible cause of spontaneous mutagenesis (Lowdin, 1965 Pullman and Pullman, 1971 Kwiatowski and Pullman, 1975). Three of the most likely tautomers for cytosine are shown in [87]—[89], together with the less likely imino forms [90] and [91] (Scanlan and Hillier,... 

Fig. 3. Stereoviews of the central trinucleotide d(CA G ) d(CTG) of the A G -platinated duplex d(CTCA G CCTC)d(GAGGCTGAG) (2). Top energy-minimized model in which the thymine conserves its Watson-Crick imino hydrogen bond with A, whereas the amino hydrogen bond is disrupted. Bottom energy-minimized model in which the thymine retains its Watson-Crick amino hydrogen bond with A, whereas the imino hydrogen bond is disrupted. The H(2 ) proton of the cytidine of the platinated strand lies in the shielding cone of the five-membered ring of A in both models, which accounts for the strong upfield shift observed for its NMR signal (reproduced from [70] with permission).

The pyrimidine nucleobases have the highest quantum yields for photoreactivity, with thymine uracil > cytosine. The purine nucleobases have much lower quantum yields for photochemistry, but can be quite reactive in the presence of oxygen. As can be seen from Figure 9-3, thymine forms primarily cyclobutyl photodimers (ToT) via a [2ir + 2tt cycloaddition, with the cis-syn photodimer most prevalent in DNA. This is the lesion which is found most often in DNA and has been directly-linked to the suntan response in humans [65]. A [2Tr + 2Tr] cycloaddition reaction between the double bond in thymine and the carbonyl or the imino of an adjacent pyrimidine nucleobase can eventually yield the pyrimidine pyrimidinone [6 1]-photoproduct via spontaneous rearrangement of the initially formed oxetane or azetidine. This photoproduct has a much lower quantum yield than the photodimer in both dinucleoside monophosphates and in DNA. Finally, thymine can also form the photohydrate via photocatalytic addition of water across the C5 = C6 bond. 

All of the bases commonly fonnd in DNA [adenine (A), cytosine (C), gnanine (G), and thymine (T)] can undergo spontaneous, pH-dependent chemical changes called tautomeric shifts (2). The base T normally exists in the keto form (C=0) in the C-4 position, bnt it can occasionally exist in the rare enol form (C-OH) (Fig. 1). Under snch circnmstances it can anomalously pair with G in the nsnal keto state. Conversely, the C-6 position of G in the rare enol form can pair with T in the usual keto form (Fig. 1). The same is true of the N-6 position of A, usually in the amino form (NH2), switching to the imino form (NH) tautomer in which case, it can mispair with cytosine in the amino form (Fig. 1). Reciprocally C can mispair with A when the C-4 position switches to the rare imino form (Fig. 1). 

Tautomerization of bases in DNA. The most stable forms of adenine and cytosine are the amino conformations. With low probability these bases can tautomerize into the imino form if this occurs during replication, an incorrect base pair (a point mutation) may result. The stable forms of guanine and thymine are the keto conformations the enol conformations also can result in mistakes in base pairing during replication. 

Recent studies on DNA-M adducts have also revealed possible semiconductor properties of these structures. In this context, of particular mention are the experiments of Lee et al. (1993), who demonstrated M-DNA formation by coordinative binding of the Zn ion to the N3 imino proton of thymine in the nucelobase pair A-T, and to the Ni proton of guanine in the C-G nucleo-base pair (Figure 7.9). The so-called engineered DNA - which is referred to as M-DNA - is able to conduct electricity. 

Fig. 7.9 Specific bindings for M-DNA formation (coordinative bond of Zn ion to the imino protons in N3 position of thymine and position of guanine).

Ag-acetate added to a soln. of 5 -deoxy-5 -iodo-3 -0-mesylthymidine in methanol, stirred and refluxed 0.5 hr. 2,5 -anhydro-l-(2-deoxy-3-0-mesyl-y -D- ry /zro-pentofuranosyl)thymine (Y 67%) allowed to react 5 days at room temp, with liq. NHg in a glass-lined steel bomb -> 2,3 -imino-l-(2-deoxy-y -D-r/zr o-pento-furanosyl)thymine (Y 84%). I. L. Doerr, R. J. Cushley, and J. J. Fox, J. Org. Chem. 33, 1592 (1968) pyrimidine nucleoside transformations via anhydronucleosides, review, s. Pure Appl. Chem. 28, 223 (1969). 

The calculated values of Table II confirm our previous statement [6] that H2 of adenine and the Imino protons of uracil (or thymine) and guanine are those for which the dimerization shift Is, to a rather large extend, not Influenced by the "rearrangement effect" but unfortunately Table II shows also that the shifts of these protons are not the most sensitive to the exact conformation of the polynucleotide chain. 

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