Selective Inverse-Detected Long-Range HeteronuclearJ-Resolved NMR Spectroscopy and Its Application to the Measurement of3JCH

Abstract

A new method for measuring and assigning long-range1H–13C spin coupling constants is presented. This is based on the HSQC pulse sequence with substitution of the 180°1H pulse by a selective 180°1H pulse and a 180°13C pulse in the middle of the evolution period. The result is an inverse-detected heteronuclear two-dimensionalJ-resolved spectrum from which the long-range heteronuclear coupling can be determined with reasonable precision and which has the added advantage of allowing the assignment of all of the long-range coupling constants from the inverted proton to specific heteronuclei. A one-dimensional version of the method is also given which can be represented as a difference spectrum. The approach has been applied to the measurement of three-bond1H–13C couplings from nucleoside base protons to the anomeric carbons of the carbohydrate moiety of nucleosides, as these couplings are used to determine nucleoside conformations. For the measurement of couplings to quaternary carbons, a modification of the pulse sequence for direct heteronuclear detection is presented. The effect of replacing the selective pulse with a hard pulse on alternate scans is shown to give information useful for assigning spin systems, and the performance of the new pulse sequence is assessed by spectral simulation.