Photochemical halogen-exchange reaction of 5-iodouracil-containing oligonucleotides

Modifications at the C(5) position of pyrimidine nucleosides

This review summarizes the state of knowledge on the chemical methods of C(5)-modifications of uridine and cytidine derivatives and may serve as a useful tool for synthetic chemists to choose an appropriate reaction protocol. The synthesis of 5-substituted uracil derivatives is gaining an increasing interest because of their possible applications in medicine and pharmacy. Modifications at the C(5) position of pyrimidine nucleosides can enhance their biostability, bioavailability or(and) biological activity. Among the C(5)-modified nucleosides, 5-halopyrimidines exhibit anticancer, antiviral, radio- and photosensitizing properties. Besides 5-halo-substituted derivatives, there are other examples of nucleosides with confirmed biological activity containing a C–C bond at the C(5) position in the pyrimidine ring. In recent decades, scientists have achieved great progress in the field of cross-coupling reactions. Among them, nickel-catalyzed processes provide a broad spectrum of synthetic methods that are based on less toxic and cheaper starting materials. This review summarizes the synthetic approaches based on the coupling or halogenation reactions, which enable 5-substituted pyrimidine nucleosides to be obtained. Moreover, the importance of the systems considered for medicine and pharmacy is briefly discussed.

The bibliography includes 197 references.

Photoaddition of 5-bromouracil to uracil in oligonucleotides leading to 5,5'-bipyrimidine-type adducts: mechanism of the photoreaction

5-Bromouracil ((Br)U) modified di- and hexanucleotides having (Br)U flanked on the 5' or the 3' side by uracil (U) have been synthesized, and their photochemical reactivity was examined under the conditions of irradiation with near UV light. The results indicate that the primary photochemical process in all of these compounds involves the formation of an intermediate cyclobutane phodoadduct composed of (Br)U and U, which undergoes further photochemically and thermally induced transformations to 5,5'-bipyrimidine type adducts.

Efficient C2'alpha-hydroxylation of deoxyribose in protein-induced Z-form DNA

DNA local conformations are thought to play an important biological role in processes such as gene expression by altering DNA-protein interactions. Although left-handed Z-form DNA is one of the best-characterized and significant local structures of DNA, having been extensively investigated for more than two decades, the biological relevance of Z-form DNA remains unclear. This is presumably due to the lack of a versatile detection method in a living cell. Previously, we demonstrated that the incorporation of a methyl group at the guanine C8 position (m(8)G) dramatically stabilizes the Z-form of short oligonucleotides in a variety of sequences. To develop a photochemical method to detect Z-form DNA, we examined the photoreaction of 5-iodouracil-containing Z-form d(CGCG(I)UGCG)(ODN 1)/d(Cm(8)GCAm(8)GCG)(ODN 2) in 2 M NaCl and found stereospecific C2'alpha-hydroxylation occurred at G(4) to provide d(CGCrGUGCG), 5. Recently, Rich and co-workers [Schwartz et al. Science 1999, 284, 1841. Schwartz et al. Nat. Struct. Biol. 2001, 8, 761] found that an ubiquitous RNA editing enzyme, adenosine deaminase 1 (ADAR1), and tumor-associated protein DML-1 specifically bind to Z-form DNA. In the present study, we investigate the photoreactivity of octanucleotide ODN 1-2 in Z-form induced by Zalpha, which is the NH(2)-terminal domain of ADAR1 responsible for tight binding of ADAR1. Detailed product analysis revealed that the C2'alpha-hydroxylated products 5 and 6 produced significantly higher yields in Z-form ODN 1-2 induced by Zalpha compared with that in 2 M NaCl. Upon treatment with ribonuclease T1, 5 and 6 were quantitatively hydrolyzed at the 3'-phosphodiester bond of the rG residue to provide d(UGCG) as a common hydrolyzed fragment on the 3' side. Quantitative analysis demonstrated that the amount of photochemically formed 5 and 6 from ODN 1-2 directly correlated with the proportion of Z-form induced by Zalpha or NaCl. These results suggest that this photochemical and enzymatic procedure can be used as a specific probe for the existence of local Z-form structure in cellular DNA.