CONFORMATIONAL STUDIES OF DITHYMIDINE BORANOMONOPHOSPHATE DIASTEREOISOMERS

The boranophosphate ester nucleotides are a new class of nucleic acid analogues that are isoelectronic and isostructural to normal phosphodi ester nucleic acids and that maintain the anionic charge of the nuclei c acid backbone. The two P-diastereoisomers of dithymidine boranomonop hosphates were separated using reverse phase HPLC; the faster and slow er eluting isomers are designated as d(Tp(B)T)-1 and d(Tp(B)T)-2, resp ectively. Conformations of the isomers were studied using circular dic hroism (CD) and NMR, and compared to the analogous phosphate diester, d(TpT). This comparison allowed the effects of the borane group and ch irality of the boranophosphate linkage on sugar and base conformations to be assessed. The CD spectra of the diastereoisomers are consistent with both having a B-type conformation. Analysis of the H-1-H-1 and H -1-P-31 coupling constants showed that these conformations are similar to those of the unmodified parent dimer; specifically, the 2'-deoxyri bose rings prefer the S (C2'-endo) conformation, and the C4'-C5' and C 5'-O5' rotamers are primarily in the gamma(+) and beta(+) conformation s, respectively. Conformational differences between the diastereoisome rs and between the modified and unmodified dimers are manifested by di fferences in the preferences of the 3'-residues to adopt S sugar pucke r and beta(+) conformations. There is reduced preference for the S sug ar pucker of the 3'-residue in d(Tp(B)T)-1 relative to d(Tp(B)T)-2, wh ich is similar to d(TpT). There is less preference for the beta(+) con formation of the 3'-residue in d(Tp(B)T)-2 relative to d(Tp(B)T)-1 and d(TpT). Based on the CD results, the temperature dependences of the t hymidine H6 chemical shifts, and the derived sugar ring and backbone c onformational parameters, we conclude that the borane group exerts a m inimal influence on the sugar conformations and base stacking interact ions. Preliminary assignment of the absolute configuration of the pair of S-p and R-p diastereoisomers to d(Tp(B)T)-1 and d(Tp(B)T)-2, respe ctively, is made on the basis of enzyme selectivity and NOE difference experiments. (C) 1997 Elsevier Science Ltd.