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Nucleic Acids Research, 1984, Vol. 12, No. 20 7929-7947
© 1984

Articles

The photochemistry of d(T-A) in aqueous solution and in ice

Samarendra N. Bose+,1, Shiv Kumar1, R.Jeremy H. Davies*,1, Satinder K. Sethi§,2 and James A. McCloskey2

1Biochemistiy Department, Medical Biology Centre, Queen's University Belfast BT9 7BL, UK 2Departments of Medicinal Chemistry and Biochemistry, University of Utah Salt Lake City, UT 84112, USA

+To whom reprint requests should be addressed

Received July 6, 1984. Revised September 24, 1984. Accepted September 24, 1984.

When d(T-A) is irradiated at 254 nm in aqueous solution an internal photoadduct is formed between its constituent adenine and thymine bases. The resultant photoproduct, designated TA*, arises from a singlet excited state precursor; a similar photoreaction is not observed with d(C-A) or d(T-G). In contradistinction, irradiation of d(T-A) in frozen aqueous solution yields a dimeric photoproduct in which two d(T-A) molecules are coupled together by a (6-4) photoadduct linkage between their respective thymine bases. Both photoproducts have been extensively characterised by a combination of electron impact and fast atom bombardment mass spectrometry, UV, CD, 1H NMR and fluorescence spectroscopy. Acid treatment of TA* gives 6-methylimidazo[4,5-b]pyridin-5-one whose identity was established by an independent chemical synthesis involving photorearrangoment of 6-methylimidazo N(4)-oxide. A tentative mechanism is presented to account for the acid degradation of TA*. The structure of the dimeric ice photoproduct follows from its cleavage, by snake venom phosphodiesterase, to 5'-dAMP and the (6-4) bimolecular photoadduct of thymidine; on acid hydrolysis it gives adenine and 6-(5'-methyl-2'-oxopyrlmidin-4'-yl)thymine.

+Present address: Chemistry Department, North Bengal University. Dt. Darjeeling, W. Bengal, India.

§Present address: Travenol Laboratories, Morton Grove, Illinois, U.S.A.


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