title: The photochemistry of d(T-A) in aqueous solution and in ice.
abstract: 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 photorearrangement of 6-methyl-imidazo[4,5-b]pyridine 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'-oxopyrimidin-4'-yl) thymine.
mesh_terms: D002942:Circular Dichroism; D005615:Freezing; D007202:Indicators and Reagents; D009690:Nucleic Acid Conformation; D009838:Oligodeoxyribonucleotides / Q000138:chemical synthesis / Q000528:radiation effects*; D009841:Oligonucleotides / Q000528:radiation effects*; D010777:Photochemistry; D013329:Structure-Activity Relationship; D014466:Ultraviolet Rays; D014867:Water
chemical_list: D007202:Indicators and Reagents; D009838:Oligodeoxyribonucleotides; D009841:Oligonucleotides; D014867:Water; C042899:deoxythymidylyl-3'-5'-deoxyadenylate