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.

The cyclobutane dimers of 2'-deoxyuridine, 2'-deoxycytidine, 5-methyl-2'-deoxycytidine and 5-bromo-2'-deoxyuridine

Irradiation of DNA and RNA pyrimidine nucleosides with UV light in frozen aqueous solution or in solution with acetone often results in the formation of cyclobutane dimers (CBDs). Many of these photodimers have not been characterized. We present here the results of work designed to achieve the isolation, spectroscopic characterization and determination of the stereochemical nature of a number of little studied or previously unstudied CBDs of four 2'-deoxyribonuclesides. These nucleosides are 2'-deoxyuridine (dUrd), 2'-deoxycytidine (dCyd), 5-methyl-2'-deoxycytidine (5-MedCyd) and 5-bromo-2'-deoxyuridine (5-BrdUrd). In particular, we have isolated and characterized six dUrd CBDs, five dCyd CBDs, five 5-MedCyd CBDs and four 5-BrdUrd CBDs. Photoproducts were studied by UV spectroscopy, mass spectrometry, proton NMR spectroscopy and via chemical approaches. Also presented are results from less definitive studies of a number of (6-4) (or 5-4) photoadducts of these nucleosides. In addition, results from exploratory photochemical studies of other 2'-deoxyribonucleosides in frozen solution, as well as some mixtures of two nucleosides, are given. The latter results indicate that 5-iodo-2'-deoxyuridine (5-IdUrd), 5-bromo-2'-deoxycytidine and 5-iodo-2'-deoxycytidine each form putative CBDs and that 5-BrdUrd is capable of forming putative mixed CBDs and (6-4) and/or (5-4) adducts with thymidine (Thd); 5-IdUrd similarly forms a (6-4) (or (5-4)) adduct with Thd.

The photochemistry of 5-BrdCpdC, dCp5-BrdCpdA, and dCp5-BrdCdT in aqueous solution

We isolated an intrastrand crosslink product from the Pyrex-filtered UV light irradiation of 5-BrdCpdC in aqueous solution. ESI-MS, MS/MS, and 2-D nuclear Overhauser effect spectroscopy (NOESY) results showed that the C5 carbon atoms of the two cytosines are covalently bonded. The same cross-link product can also be induced from the similar UV irradiation of dCp5-BrdCpdA and dCp5-BrdCpdT.

Ice (photo)chemistry. Ice as a medium for long-term (photo)chemical transformations--environmental implications

This review accounts for the current knowledge about the distribution, accumulation, and chemical/photochemical transformations of persistent, bioaccumulative, and toxic compounds (PBTs) in water ice, especially in the connection with polar regions and atmospheric cloud particles. (Photo)reactions on/in ice are discussed in terms of photochemistry, photobiology, paleochemistry, as well as astrophysics. Authors propose a model, in which a significant amount of some PBTs are generated by (photo)chemistry of primary pollutants in ice, which may subsequently be released to the environment. It is argued that ice photochemistry might play an important role in the chemical transformations in cold ecosystems and in the upper atmosphere, particularly now when the ozone layer is partially depleted.

Photochemical coupling of 5-bromo-1,3-dimethyluracil and its 6-alkyl derivatives to 3-methylindole and N alpha-acetyl-L-tryptophan methyl ester

Photochemical reactions between 5-bromo-1,3-dimethyluracils and 3-substituted indoles in acetone solution were studied. Irradiation (lambda greater than 290 nm) of 5-bromo-1,3-dimethyluracil (1) and N alpha-acetyl-L-tryptophan methyl ester (2) yields, in addition to 5-(2-indolyl)uracil (3), a new photoadduct 5-(7-indolyl)uracil (4). 5-Bromo-1,3-dimethyluracils with 6-alkyl substituents irradiated in the presence of 2 give the 5-(2-indolyl)uracil-type photoadducts exclusively.