Nitrosative guanosine deamination: pyrimidine ring opening implications of effects in homogeneous solution as well as anisotropic environments
Metadata[+] Show full item record
[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Nitrosative DNA base deamination represents one of the most abundant sources of endogenous DNA damage and it constitutes a significant portion of damage in various disease states via mutagenesis and cytotoxicity. A complete understanding of deamination chemistry is important because of the considerable amount of dietary and environmental exposure of humans to nitrogen oxides. Theoretical studies of the isolated guaninediazonium ion reveals that it is very unstable and its unimolecular dediazoniation is accompanied by concomitant amide bondcleavage leads to the pyrimidine ring-opened intermediate 5-cyanoimino-4- oxomethylene-4,5-dihydroimidazole. ¹⁸O and ¹⁷O labeling studies were employed to gain insights into the mechanism of guanine deamination. [6-¹⁸O]-guanosine had been synthesized and the ¹⁸O-iotopic shift observed in the ¹³C NMR of the products of these experiments suggested the possible involvement of two different pathways (Path A: major, Path B: minor). ¹⁷O studies totally ruled out the Pathway B, which suggest oxanosine was formed via Path A, whereas xanthosine was formed by direct hydrolysis of guaninediazonium ion. Oxanine formation by ring closure was also studied computationally according to the mechanistic hypothesis proposed. As part of studies of the mechanism of oxanosine formation, the synthesis of [7- ¹⁸O]-oxanosine was attempted by enzymatic ¹⁶O/¹⁸O-exchange with adenosine deaminase (ADA) in analogy to the synthesis of [6-¹⁸O]-guanosine from 2-amino-6-chloropurine. Interestingly, it was discovered that the incubation of oxanosine 3r with ADA in sodium phosphate buffer (pH = 7.4) results in 1-[beta]-(D-ribofuranosyl)-5-ureido-1H-imidazole-4- carboxylic acid 4r. The reaction of the 2'-deoxyribose derivative 3d forms 4d in analogy. Implications are discussed for studies of nitrosative deamination of oligonucleotides. In the pH variation study of dG nitrosation in homogenous and anisotropic environments the similarities between the product ratios in homogeneous as well as ss-DNA environment proved that in "free nucleobase" chemistry (e.g., nucleobase, nucleoside, nucleotide and ss-oligonucleotide) the formation of [X+dX] occurs via SN2Ar chemistry, whereas formation of [dO] occurs via SN1Ar chemistry. Fasicularin is a tricyclic derivative of 2-hexyl-1-(2-thiocyanatoethyl)piperidine and it is believed that its DNA alkylation involves an aziridinium ion intermediate. This mechanistic hypothesis has been explored with electronic structure theoretical methods. The low energy barrier of the formation of aziridinium ion from boat-conformation of fasicularin in the presence of the continuum solvent model proves fasicularin as the first natural product, which has the ability to form DNA-alkylating aziridinium ion by displacing thiocyanate moiety.
Access is limited to the campus of the University of Missouri-Columbia.