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Radiosensitizing properties of substituted uridines are of great importance for radiotherapy. Very recently, we confirmed 5-iodo-4-thio-20-deoxyuridine (ISdU) as an efficient agent, increasing the extent of tumor cell killing with ionizing radiation. To our surprise, a similar derivative of 4-thio-2’-deoxyuridine, 5-bromo-4-thio-20-deoxyuridine (BrSdU), does not show radiosensitizing properties at all. In order to explain this remarkable difference, we carried out a radiolytic (stationary and pulse) and quantum chemical studies, which allowed the pathways to all radioproducts to be rationalized. In contrast to ISdU solutions, where radiolysis leads to 4-thio-2’-deoxyuridine and its dimer, no dissociative electron attachment (DEA) products were observed for BrSdU. This observation seems to explain the lack of radiosensitizing properties of BrSdU since the efficient formation of the uridine-5-yl radical, induced by electron attachment to the modified nucleoside, is suggested to be an indispensable attribute of radiosensitizing uridines. A larger activation barrier for DEA in BrSdU, as compared to ISdU, is probably responsible for the closure of DEA channel in the former system. Indeed, besides DEA, the XSdU anions may undergo competitive protonation, which makes the release of X? kinetically forbidden.

The rational design of fluorescent nucleoside analogues is greatly hampered by the lack of a general method to predict their photophysics, a problem that is especially acute when base pairing and stacking change fluorescence. To better understand these effects, a series of tricyclic cytidine (tC and tCO) analogues ranging from electron-rich to electron-deficient was designed and synthesized. They were then incorporated into oligonucleotides, and photophysical responses to base pairing and stacking were studied. When inserted into double-stranded DNA oligonucleotides, electron-rich analogues exhibit a fluorescence turn-on effect, in contrast with the electron-deficient compounds, which show diminished fluorescence. The magnitude of these fluorescence changes is correlated with the oxidation potential of nearest neighbor nucleobases. Moreover, matched base pairing enhances fluorescence turn-on for the electron-rich compounds, and it causes a fluorescence decrease for the electron-deficient compounds. For the tCO compounds, the emergence of vibrational fine structure in the fluorescence spectra in response to base pairing and stacking was observed, offering a potential new tool for studying nucleic acid structure and dynamics. These results, supported by DFT calculations, help to rationalize fluorescence changes in the base stack and will be useful for selecting the best fluorescent nucleoside analogues for a desired application.

Tricyclic cytosines (tC and tCO frameworks) have emerged as a unique class of fluorescent nucleobase analogues that minimally perturb the structure of B-form DNA and that are not quenched in duplex nucleic acids. Systematic derivatization of th

Unambiguous characterization of 5-substituted-4-thiopyrimidine nucleosides (ribonucleosides and 2'-deoxynucleosides) was performed using NMR spectroscopy. Assignments of all proton and carbon signals of 5-bromo-4-thiouridine and related nucleosides were systematically carried out and firmly established by COSY and HMQC techniques. The NMR data of various 4-thiopyrimidine nucleosides are compared, and the key contributing factors discussed. The approach presented here is applicable to other modified nucleosides and nucleotides, as well as nucleobases. Copyright

Treatment of the protected and unprotected nucleosides with 1,3-dibromo-5,5-dimethylhydantoin in aprotic solvents such as CH 2Cl2, CH3CN, or DMF effected smooth bromination of uridine and cytidine derivatives at C-5 of pyrimidine rings as well as adenosine and guanosine derivatives at C-8 of purine rings. Addition of Lewis acids such as trimethylsilyl trifluoromethanesulfonate enhanced the efficiency of bromination.

Synthesis of antiviral 5-halouracil nucleosides, also used as key precursors for the synthesis of other potential antiviral drugs, has been demonstrated using ionic liquids as convenient and efficient reaction medium.

(Chemical Equation Presented) Melting temperatures of DNA duplexes containing the phenoxazine (P) and G-clamp (X) cytosine analogues exhibited a strong and unusual dependence on the nucleoside flanking the modified nucleobase, and the same trend was obser

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A mild and efficient methodology for the synthesis of 5-halogeno (iodo, bromo, or chloro) uracil nucleosides has been developed. 5-Halo-2'-deoxyuridines 4a-c (84-95%), 5-halouridines 7a-c (45-95%), and 5-haloarabinouridines 8a-c (65-95%) were synthesized in good to excellent yields by the reaction of 2'-deoxyuridine (2), uridine (5) and arabinouridine (6), respectively with iodine monochloride, or N-bromo (or chloro)succinimide, and sodium azide at 25-45°C. These C-5 halogenation reactions proceed via a 5-halo-6-azido-5,6-dihydro intermediate (3), from which HN3 is eliminated, to yield the 5-halogeno uracil nucleoside. The 5-halo-6-azido-5,6-dihydro intermediate products (10a, 10b) could be isolated from the reaction of 3',5'-di-O-acetyl-2'-deoxyuridine (9) with iodine monochloride or N-bromosuccinimide and sodium azide at 0°C. The isolation of 10a, 10b indicates that the C-5 halogenation reaction proceeds via a 5-halo-6-azido-5,6-dihydro intermediate.

Reaction of anodically generated "halonium" species (LiX or Bu4NX, LiClO4, MeCN, Pt/Pt; I2, LiClO4, MeCN) with pyrimidine bases and their nucleosides leads to 5-halo compounds in good yields.

Treatment of protected uracil nucleosides 1 or 2 with elemental iodine or metal halogenides and ceric ammonium nitrate (CAN) at 80 deg C gave the corresponding protected 5-halouracil nucleosides 3a-f in excellent yields.Treatment of the resulting crude 3a-f with 0.1 M NaOMe/MeOH at ambient temperature gave the corresponding 5-halouridines 4a-f in high overall yields from 1 or 2.Further, 5-halouraciles 9a-f were prepared in good yields by treatment of 1,3-dimethyluracil (7) or uracil (8) with elemental iodine, metal halogenides, or hydrochloric acid and CAN.Halouridines 4a-e also were obtained in good yields by treatment of unprotected uracil nucleosides 5 or 6 with halogen sources as above and CAN.