1-methyl thymine

Folic acid derivatives are essential for DNA synthesis, in that they are cofactors for certain reactions in purine and pyrimidine biosynthesis, including the uracil-thymine methylation just described. They are also cofactors for several reactions relating to amino acid metabolism. The folic acid system thus offers considerable scope for drug action. 

Fig. 24.6. Stereo diagram showing the four fused water pentagons in the major groove of the A-DNA-type octanucleotide d(GCBrUABrUACC). Bromine atoms [or thymine methyl groups in the isomorphous d(GGTATACC)] are indicated by concentric circles. Note water molecules and phosphate oxygens surrounding the methyl groups (or bromine atoms) (see also Fig. 24.9a, [865])...

Another feature associated with B-DNA, but not with A- and Z-DNA, is the extra water molecule located between free phosphate oxygen and thymine methyl groups. Apparently, this water molecule is loosely bound and disappears upon dehydration, allowing the sugar conformation to change from C(2>endo to C(3 )-endo. 

Conformational transitions of DNA are cooperative. The phosphate and thymine methyl group hydration is intimately connected with the sugar puckering... 

NMR studies of water dynamics in the major groove of B-DNA are sparse. Although NOE cross-peaks between water and thymine methyl protons and guanine 8H and adenine 8H protons in the major groove of the Dickerson-Drew dodecamer have been identified, the estimated lifetimes on the order of 200 to 500 ps are short compared with that of water in the minor groove (26, 28). MRD measurements at 4 ° C have... 

DNA, the 1 1 complex and the 2 1 complex, would have produced four times as many observable peaks relative to the free DNA species. At intermediate ligand concentration, however, only two sets of peaks arising from ENA molecules were detected. In the 2 1 complex only four thymine methyl resonances were detected (1.0-1.5 ppm), as expected for a symmetrical DNA duplex. These are all overlapped in the free DNA spectrum. In the 1 1 mixture, only signals from free DNA and from the 2 1 complex were detected. 

It is used to maximize the maintenance of the integrity of the information encoded by DNA. Cytosine can spontaneously deaminate to form uracil this damage is repaired by base excision repair. If uracil rather than thymine were used in DNA, then correctly positioned uracil would be indistinguishable from that arising from cytosine deamination. Use of thymine (methylated uracil) in DNA avoids this problem. 

Umezawa, Y. Nishio. M. Thymine-methyl/7c interaction implicated in the sequence-dependent deformability of DNA. Nucleic Acids Res. 2002, 30, 2183-2192. 

The modem method of choice (manual or automated) is a solid phase system, using phosphoramidite chemistry (Fig. 4). The most advanced automated systems employ P-cyanoethylamidites, rather than me-thylamidites. Cyanoethyl protection avoids the poten-tud hazard of thymine methylation by intemucleotide methyl phosphate, which is present when methylami-dites are used the cyanoethyl group is also more easily removed than methyl, so that deprotection of the phosphate, deprotection of the bases, and release from the solid support can be performed in a single stage. 

Studies on the mechanism of the bacterial thymidylate synthetase reaction employing tritiated H4-folate showed that a hydrogen atom of 5,10-methylene H4-folate was transferred to the thymine methyl group and suggested the formation of a methylene bridge between folate and deoxyuridylate (9). 

Thymine, methylation of, 256 Triacetic acid lactone, methylation of, 277-278... 

More recently it has been observed that 2-amino-4-hydroxy-6,7-dimethyltetra-hydropteridine, which lacks N °, is unable to substitute for FH as a coenzyme in the utilization of formaldehyde for the enzymic formation of serine and of thymine-methyl (48a). The corresponding dimethyldihydropteridine can serve as a substrate for dihydrofolic acid reductase. 

The (9-carbon of serine as well as the a-carbon of glycine were extensively incorporated into the methyl group of thymine in the adult rat (334). In addition, formate-C served as a precursor of the methyl group of thymine both in vivo (29) and in vitro (335) formaldehyde was also utilized for thymine methyl group synthesis (336). Isotopic data indicated that the hydroxymethyl group of serine, doubly labeled with and D, was transferred to position 5 of thymine with a minimum of 1.5 atoms of D per carbon atom this demonstrated that the /3-carbon of serine was not metabolized all the way to formate (337),... 

The known role of vitamin B12 and folic acid in the formation of labile methyl groups (339) helps to explain the replaceability of folic acid (330) or vitamin B12 (331) by thymidine in certain deficient oi anisms. Also of interest is the role of folic acid compounds in the pathway leading to formation of the thymine methyl group. Low concentrations of a folic acid antimetabolite, aminopterin, blocked the utilization of deoxyiuidine for thymidine synthesis (333). The antimetabolite effects of aminopterin on the utilization of one-carbon donors have been known for some time (333, 334). The details of thymine biosynthetis will be discassed elsewhere in this volume (Chapter 24). 

Many animal tissues synthesized nucleic acids de novo from elementary precursors as well as from preformed purines in the diet. Other tissues, such as bone marrow, had a limited capacity for purine synthesis by the de novo route and appeared to utilize preformed purine compounds that were manufactured in the liver (450). In this experiment, the amount of formate-C incorporated into the bases of DNA of marrow cells was measured in control and hepatectomized rabbits. In control animals, the ratio of the specific activity of thymine to adenine was approximately 1, indicating a similar degree of incorporation of formate-C into the purine ureide carbons and into the thymine methyl group of DNA in bone marrow cells in vivo. In hepatectomized rabbits, the total incorporation of formate-C was substantially depressed but the thymine adenine ratio was increased to 27. The conclusion that the liver supplied preformed purines to the marrow cells was supported by the earlier observation that marrow cells in vitro utilized preformed adenine quite efficiently for nucleic acid synthesis (451). 

Pyrimidine nucleosides of cytosine, uracil, thymine, methyl cytosine are obtained as products of nucleic acid hydrolysis. Vicine, a D-glucoside of 2, 5-diamino-4, 6-dihydroxy pyrimidine occurs in plants. 

Having a 5-methyl group, thymine is not nitrated or halogenated normally, but with aqueous bromine it does give the dihydropyrimidine (948) (25JBC(64)233) its other reactions parallel those of uracil although its behavior on irradiation is somewhat different (Section 2.13.2.1.4). 

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