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As after irradiation with 254-nm UV light, exposure of thymidine and three isomeric pyridopsoralen derivatives to UVA radiation, in the dry state, leads to the formation of the six diastereomers of cyclobutadithymidine as the predominant reaction. This unexpected photosensitized reaction, which also gives rise to both 5R* and 5S* diastereomers of 5,6-dihydro-5-(alpha-thymidylyl)thymidine (or "spore" photoproduct), is selective since [2 + 2] dimerization of 2'-deoxycytidine was not detected under the same experimental conditions. The cis-syn isomer of cyclobutadithymine was also found to be produced within isolated DNA following UVA irradiation in aqueous solutions containing 7-methylpyrido[3,4-c]psoralen. Quantitatively, this photoproduct represents about one-fifth of the overall yield of the furan-side pyridopsoralen [2 + 2] photocycloadducts to thymine. DNA sequencing methodology was used to demonstrate that pyridopsoralen-photosensitized DNA is a substrate for T4 endonuclease V and Escherichia coli photoreactivating enzyme, two enzymes acting specifically on cyclobutane pyrimidine dimers. Furthermore, the dimerization reaction of thymine is sequence dependent, with a different specificity from that mediated by far-UV irradiation as inferred from gel sequencing experiments. Interestingly, adjacent thymine residues are excellent targets for 7-methylpyrido[3,4-c]psoralen-mediated formation of cyclobutadithymine in TTTTA and TTAAT sites, which are also the strongest sites for photoaddition. The formation of cyclobutane thymine dimers concomitant to that of thymine-furocoumarin photoadducts and their eventual implication in the photobiological effects of the pyridopsoralens are discussed.

JEAN CADET, LUCIENNE VOITURIEZ, FRANK E. HRUSKA, LOU-SING KAN, FRANK A. A. M. DE LEEUW, and CORNELIS ALTONA. Can. J. Chem. 63, 2861Chem. 63, (1985.We describe the preparation of the six configurationally distinct cyclobutane-type photodimers generated by the acetonesensitized uv irradiation of thymidine in aqueous solution. Also prepared as minor photoproducts are the 5R and 5 S diastereoisomers of 5,6-dihydrothymidine, and two novel molecules, the 5R and 5.3 diastereoisomers of 5-acetonyl-5,6-dihydrothymidine. The purification of the ten molecules by chromatographic techniques (hplc, tlc) is described, along with their extensive characterization by uv, ir, cd, FAB-ms and 'H nmr. The proton chemical shifts and coupling constants are discussed in terms of the geometry of the pyrimidine, cyclobutane, and sugar rings. Les six diastCrCoisomkres possibles de la cyclobutidithymidine ont CtC prCparCs par photosensibilisation de la thymidine B la lumikre du proche ultra-violet en utilisant I'acetone. Les diastCrCoisomkres 5R et 5.3 de la dihydro-5,6 thymidine et deux nouveaux photoproduits, les formes 5R et 5 S de I'acCtonyl-5 dihydro-5,6 thymidine ont aussi CtC isolts. L'identification de ces photoproduits a fait appel B diverses techniques spectroscopiques: uv, ir, dichroi'sme circulaire, spectromktrie de masse par bombardement avec des atomes rapides et rmn 'H. Les valeurs des paramktres de rmn (deplacements chimiques, constantes de couplage) sont discutCes en termes de stCrCochimie des cycles pyrimidiniques, cyclobutyles et osidiques. IntroductionThe cyclobutane-type of photoproduct formed from adjacent thymine bases is the most thoroughly studied of the lesions induced by uv irradiation of DNA (1). The biological significance of these thymine photodimers is evidenced by the existence of mechanisms for their removal (2). Several studies reveal that the presence of the dimers leads to distortions in nucleic acid structure (3-7) that could play a role in the recognition by the enzymes of the repair system and may account for the altered specificity of, for example, restriction enzymes which employ DNA as a substrate (8). The numerous studies of the photodimerized thymine bases, Thy()Thy (R = H; Fig. I), have provided valuable photochemical and structural information (I), but relatively fewer data are available for the corresponding thymidine derivatives, dThd()dThd (R = 2-deoxy-P-D-ribose), which are the molecules of main interest in the present work. Wulff and Fraenkel (9) originally listed four isomeric forms of Thy()Thy. Actually, six configurationalIy-distinct forms are possible, since the trans-syn (ts) and cis-anti (ca) isomers exist as enantiomeric pairs ts 1, ts2 and c a l , ca2 whereas the cis-syn (cs) and trans-anti (ta) isomers exist as single (rneso) molecules. Similarly, six dThd()dThd are possible and are denoted CS, TS1, etc. according to the geometry of the Thy()Thy fragment (Fig. 1).

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