Thymine atom

Fig. 38 The hydrogen-bonded 2-D sheet architecture of [CuL2m] [BF4]2-thymine (atom identification as for Fig. 37) [67]...

Figure 7.7 Color codes for the recognition patterns at the edges of the base pairs in the major (a) and minor (b) grooves of B-DNA. Hydrogen-bond acceptors are red hydrogen-bond donors are blue. The methyl group of thymine is yellow, while the corresponding H atom of cytosine is white.

Attached by a covalent bond to carbon atom 1 of the deoxyribose ring is an amine (and therefore a base), which may be adenine, A (22) guanine, G (23) cytosine, C (24) or thymine, T (25). In RNA, uracil, U (26), replaces thymine. The base bonds to carbon atom 1 of deoxyribose through the nitrogen of the —NH— group (printed in red) and the compound so formed is called a nucleoside. All nucleosides have a similar structure, which we can summarize as the shape shown in (27) the lens-shaped object represents the attached amine. 

Hydrogen Atom Abstraction from the C5-Methyl Group of Thymine 356... 

There are four bases in DNA guanine, thymine, cytosine and adenine. Each has a ketone C=0 group in which the oxygen is quite electronegative and bears an excess negative charge <5 , and an amine in which the electropositive hydrogen atoms bear an excess... 

Hg2 + selectively binds the AT region, apparently due to its great affinity for thymine. Thus, Hg2+ interacts with DNA at nitrogen atoms, replacing hydrogen between the thymine and adenine bases [114]. This interaction results in increased helix stability and an increased diameter of the double helix, approximating the difference between the atomic radius of hydrogen and mercury [116]. 

One possible solution of this problem is to differentiate a radical first as electrophilic or nucleophilic with respect to its partner, depending upon its tendency to gain or lose electron. Then the relevant atomic Fukui function (/+ or / ) or softness f.v+ or s ) should be used. Using this approach, regiochemistry of radical addition to heteratom C=X double bond (aldehydes, nitrones, imines, etc.) and heteronuclear ring compounds (such as uracil, thymine, furan, pyridine, etc.) could be explained [34], A more rigorous approach will be to define the Fukui function for radical attack in such a way that it takes care of the inherent nature of a radical and thus differentiates one radical from the other. 

An almost complete description of both OH radical-mediated and one-electron oxidation reactions of the thymine moiety (3) of DNA and related model compounds is now possible on the basis of detailed studies of the final oxidation products and their radical precursors. Relevant information on the structure and redox properties of transient pyrimidine radicals is available from pulse radiolysis measurements that in most cases have involved the use of the redox titration technique. It may be noted that most of the rate constants implicating the formation and the fate of the latter radicals have been also assessed. This has been completed by the isolation and characterization of the main thymine and thymidine hydroperoxides that arise from the fate of the pyrimidine radicals in aerated aqueous solutions. Information is also available on the formation of thymine hydroperoxides as the result of initial addition of radiation-induced reductive species including H" atom and solvated electron. 

Various isomers of 2,3-difluoro-2,3-dideoxynucleosides have been synthesized (Figure 6.7). The configuration of the fluorine atoms plays an essential role in the antiviral activity. ° In the case of the D-lyxo compound, coupling with thymine has failed, due to the participation of the protecting group. 

---