Alkyl phosphotriester modified oligodeoxyribonucleotides. VI. NMR and UV spectroscopic studies of ethyl phosphotriester (Et) modifiedRp−RpandSp−Spduplexes, (d[GGAA(Et)TTCC])2

Abstract

¹ H NMR chemical shift assignments for the title compounds were made for all but a few H ₅ ′ and H ₅ ” signals using two-dimensional nuclear Overhauser effect (2D-NOE) data, which was also used for the first time to assign absolute configuration at phosphorus. The chemical shifts were, in general, similar to those reported [Broido, M.S., et al . (1985) Eur. J. Biochem. 150 , 117–128] for the B-like conformation of the unmodified, parent duplex, [d(GGAATTCC)] ₂ . Differences in chemical shifts for corresponding protons were mostly localized to the AA(Et)TT region, and showed some stereochemical dependence. Unambiguous assignment of the phosphotriester ³¹ p signals was achieved in a novel way using selective insensitive nucleus enhancement by polarization transfer (selective INEPT) NMR. The R_p − R_p duplex melted ca . 11 °C lower than either the S _p −S _p or parent duplexes, as evidenced by T _m and variable temperature ¹ H/ ³¹ P NMR measurements. The 2D-N0E data for the R _p −R _p duplex suggested possible steric interactions between the ethyl group and the H ₃ ′ of the flanking A residue. At low ionic strength, the S _p −S _p and parent duplexes had similar stability but at high ionic strength the S _p −S _p duplex was less stable.