CRYSTAL-STRUCTURE OF A DNA DUPLEX CONTAINING 8-HYDROXYDEOXYGUANINE-ADENINE BASE-PAIRS

The crystal structure of the oligonucleotide d(CGCAAATTO8GGCG), contai ning the chemically modified base 8-hydroxydeoxyguanine (08G), has bee n determined at 2.5-Angstrom resolution and refined to a crystallograp hic R-factor of 16.8%. The B-type DNA helix contains standard Watson-C rick base pairs except at the mismatch sites, where O8G adopts a syn c onformation and forms hydrogen bonds to adenine in the anti conformati on. The thermodynamic stability of the duplex was found by UV melting techniques to be intermediate between the native oligonucleotide d(CGC AAATTTGCG) and an oligonucleotide containing A.G mispairs d(CGCAAATTGG CG). Comparison of the structure of the O8G(syn).A(anti) base pair wit h those of Watson-Crick base pairs has given a reason why O8G.A base p airs are not well repaired by DNA proofreading enzymes.