The Photoreactivity of Pyrimidine‐purine Sequences in Some Deoxydinucleoside Monophosphates and Alternating Dna Copolymers

Abstract

Abstract— A reversed‐phase HPLC system has been developed which separates the common nucleo‐bases from the 6‐methylimidazo[4,5‐b]pyridin‐5‐one (6‐MIP) produced on acid hydrolysis of a thymine‐adenine photoadduct (TA*) that is formed between adjacent thymine and adenine bases in UV‐irradiated polydeoxyribonucleotides. By measuring the relative amounts of adenine and 6‐MIP in acid hydrolysates, this system has been used to investigate how polynucleotide conformation affects the yield of TA* in poly(dA‐dT) irradiated at 254 nm. The photoreactivity of other pyrimidine‐purine sequences has been examined with the deoxydinucleoside monophosphates d(TpI) and d(m5CpA) and with the alternating DNA copolymers poly(dA‐dU), poly(dI‐dC), poly(dG‐dC) and poly(dA‐dC).poly(dG‐dT). Samples were irradiated at 254 nm in aqueous solution and in ice, and at wavelengths >290 nm with acetone as photosensitizer. A photoproduct resembling TA*, and giving 6‐MIP on acid hydrolysis, was isolated from d(TpI) irradiated at 254 nm in solution or in ice; d(m5CpA) was comparatively unreactive. Acid hydrolysates of the irradiated DNA copolymers were screened by HPLC and by TLC and paper electrophoresis, for the presence of imidazo[4,5‐b]pyridin‐5‐one, 6‐MIP, or other species attributable to specific photoproduct formation. By this criterion, however, none of the copolymers showed evidence of significant photoreactivity in either their single‐ or double‐stranded conformational states. The formation of mixed pyrimidine‐purine photoadducts in DNA is therefore probably restricted to T‐A doublets.

Role of Sequence and Conformation on the Photochemistry and the Photophysics of A−T DNA Dimers: An Experimental and Computational Approach

“…Nonetheless, our quantum chemical calculations (Tables −4 and Figures −6) and recent femtosecond transient absorption experiments in AT-oligonucleotides , provide support to our hypothesis. Further support of the intermediacy of a charge separated or radical pair state in the formation of TA* comes from photochemical experiments in TpdI and 5-methyl-CpdA dinucleotides . The TpdI dinucleotide, where adenine has been replaced by inosine, also form an eight-member ring TI* photoproduct (an analogue of TA*), whereas 5-methylCpdA does not .…”

“…Long-range DNA-mediated charge transfer currently attracts considerable interest because of the key role played by oxidation in aging, mutagenesis, and carcinogenesis. − However, the influence that base sequence and conformation has on the photochemical and photophysical properties of DNA and its constituents has received much less attention. − In 1983, Davies and co-workers presented a classic example of the significant role of base sequence and conformation on the photochemistry of adjacent purine and pyrimidine bases in DNA. − The photoreaction of deoxyribo-dinucleoside monophosphate thymidylyl (3‘−5‘)-2‘-deoxyadenosine sodium salt (TpdA) at 254 nm, produced the thymine−adenine adduct as major photoproduct (TA*). Significantly, this product was not detected in the photochemistry of its sequence isomer 2‘-deoxyadenylyl (3‘−5‘)-thymidine (dApT).…”

The structure of d(TpA), the major photoproduct of thymidylyl-(3'5')- deoxyadenosine

“…Because of the unique fluorescence properties of the acid hydrolysis product of TA*, compound 2, (1,3) it was possible to demonstrate the formation of TA* in duplex poly(dA-dT) and calf thymus DNA irradiated with 254 nm light (4). The TA* photoproduct is of interest because of its implications for gene expression, which depends on a highly conserved TATA site in the promoter region.…”

“…In addition to d(TpA), direct irradiation of thymidylyl-(3′,5′)-2′-deoxyinosine, d(TpI), with 254 nm has also been reported to give the analogous d(TpI)* photoproduct with a similar quantum yield (4). The far-UV irradiation of various other adenine-containing substrates, however, have been reported to lead to other ring fusions (Fig.…”

Preparation and Characterization of a Deoxyoligonucleotide 49-mer Containing a Site-Specific Thymidylyl-3',5'-adenosine Photoproduct

“…Further structural proof and assignment of the trans-syn stereochemistry of the dinucleotide photoproduct were based on the analysis of 2D NMR data and molecular dynamics simulations (Koning et al, 1990). Because of the unique fluorescence properties of the acid hydrolysis product of TA* (Bose et al, 1983(Bose et al, , 1984, it was possible to demonstrate the formation of TA* in duplex poly(dA-dT) and calf thymus DNA irradiated with 254 nm light (Kumar & Davies, 1987). The biological properties of this interesting photoproduct are as yet unknown, primarily because of a lack of substrates containing site-specific TA* photoproducts for precise structure-activity studies.…”