P-32 POSTLABELING ANALYSIS OF ISOMERIC 7-ALKYLGUANINE ADDUCTS OF STYRENE OXIDE

Styrene 7,8-oxide, which reacts preferentially at the N-7 position of guanine, yielded two pairs of diastereomers 7-(1'-hydroxy-2'-phenyleth yl)-dGMP (the alpha-isomer) and 7-(2-hydroxy-2-phenylethyl)-dGMP (the beta-isomer) on reaction with deoxyguanosine-3'-monophosphate (3'-dGMP ), The alpha- and beta-isomers were formed in the ratio 32:68, T4 poly nucleotide kinase preferentially mediated labelling of diastereomers c orresponding to the beta-isomer, The beta-diastereomers showed a label ling efficiency of 52%, whereas the a-isomers showed a labelling effic iency of 4%, Molecular modelling experiments showed intrinsic differen ces between the two isomers, The torsion angles of C8-N7-2'-Ar and C8- N7-2'-1' for the alpha-isomers were 149.9 degrees and -26.4 degrees, w hereas the torsion angles of C-8-N7-1'-2' and N-7-1'-2'-Ar for the bet a-isomers were 105.3 degrees and 179.4 degrees, The consequent interat omic distance between one of the hydrogens on the alpha-carbon and the 3'-phosphate group on the sugar residue was 5.3 Angstrom in the alpha -isomer whereas the closest distance between the hydrogens attached to the alpha-carbon and 3'-phosphate group in the beta-isomer was 6.2 An gstrom. This arrangement probably leads to steric overcrowding at the 3'-phosphate group in alpha-isomers and these are less efficiently pho sphorylated than beta-isomers, In irt vitro styrene oxide-modified sal mon testis DNA alpha- and beta-isomers of 7-alkylguanines were formed in the ratio 37:63, The recovery of two diastereomeric beta-isomers in a P-32-postlabelling assay was 14%, but one of the diastereomers was obtained in 3-fold greater yield than the second isomer.