HALOGENATED NUCLEIC-ACIDS - STRUCTURE AND CONFORMATIONAL STUDIES OF 5-FLUOROCYTIDINE BY X-RAY CRYSTALLOGRAPHY AND NMR-SPECTROSCOPY

The stereochemistry and the conformational equilibria of 5-fluorocytid ine (5FCyd) have been determined by X-ray crystallography and correlat ed with NMR spectroscopy in the solution state. Crystals of SFCyd have unit cell dimensions a=9.854(1), b=15.012(2), c=15.290(2)Angstrom, al pha=beta=gamma=90 degrees with two molecules in the asymmetric unit. B oth molecules are in the anti-conformation, C3'-endo sugar pucker and a <Upsilon> (C4'-C5') of g(+). The two molecules in the asymmetric uni t show slight variation in their bond distances and bond angles but th eir overall solid state conformation is similar. The NMR results indic ate the SFCyd has an anti-conformation, a mixed sugar pucker of 36% C2 '-endo and 64% C3'-endo and an exocyclic furanose conformation (Upsilo n) of 74%(g(+)), 19%(t) and 7%(g(-)). Certain purine and pyrimidine an alogues readily replace the natural bases in nucleic acid if they are present during replication. Halogenated nucleic acids have been known for the past thirty years when for the first time it was found that 5- iodouracil could be incorporated into the nucleic acid of E . coli(1). This provided the basis for the synthesis of 5-fluoro-nucleic acid ba ses and their use as a chemotherapeutic agent. The biological properti es of 5-halogenated uracil derivatives have been examined in detail. T he incorporation of the halogenated base into mRNA could lead to error s in the reading frame which would alter the phenotype without permane nt change in DNA genotype(2-4), also it can substitute for uracil into mRNA and may exert a mutagenic effect(5) or can occasionally lead to the production of altered protein(6-7). A substitution of uracil by fl uorouracil in the mRNA and a preferential pairing of the 5FUra with a guanosine in the tRNA anti-codons can account for the first substituti on. This is because 5FU assumes properties similar to cytosine when th e base ionizes. This occurs readily for 5-FU (pKa = 7.8) due to the el ectronegative fluorine atom at C5 instead of hydrogen. 5FCyd shows ant ifungal activity against Saccharomyces cerevisiae and Candida species. It is the drug of choice for systemic infections. In order to obtain a more basic understanding of the potential of such processes, the pre sent study was undertaken as the first step using solid phase study by X-ray crystallography and its structure in solution by NMR spectrosco py(8).