More accurate 1JCH coupling measurement in the presence of 3JHH strong coupling in natural abundance

Abstract

J couplings are essential for measuring RDCs (residual dipolar couplings), now routinely used to deduce molecular structure and dynamics of glycans and proteins. Accurate measurement of 1JCH is critical for RDCs to reflect the true structure and dynamics in the molecule of interest. We report noticeable discrepancies between 1JCH values measured with HSQC type pulse sequences in the 1H dimension from those measured in the 13C dimension for 17 sugars and show that these discrepancies arise from strong scalar coupling. In order to determine how to minimize errors in measuring 1JCH, we analyze the strong coupling effects in detail using the product operator-formalism and spectral simulations based on the solution of the Liouville equation (not considering relaxation effects) in the presence of strong coupling. We report that the apparent 1JCH measured with 2D HSQC-based sequences in either dimension can be in error by up to 4Hz and that the values measured in the 1H dimension can disagree with those in the 13C dimension by up to 7Hz. We demonstrate that spectral simulations can reproduce the errors induced by strong coupling and that these can be used to extract true 1JCH values. We find that the 1JCH values measured using a modified Z-filtered coupled HSQC are still affected by strong coupling. We conclude that spectral simulation yields accurate 1JCH with errors as low as 1% in the presence of strong coupling.