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 1 J CH is critical for RDCs to reflect the true structure and dynamics in the molecule of interest. We report noticeable discrepancies between 1 J CH values measured with HSQC type pulse sequences in the 1 H dimension from those measured in the 13 C dimension for 17 sugars and show that these discrepancies arise from strong scalar coupling. In order to determine how to minimize errors in measuring 1 J CH , 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 1 J CH measured with 2D HSQC-based sequences in either dimension can be in error by up to 4 Hz and that the values measured in the 1 H dimension can disagree with those in the 13 C dimension by up to 7 Hz. We demonstrate that spectral simulations can reproduce the errors induced by strong coupling and that these can be used to extract true 1 J CH values. We find that the 1 J CH values measured using a modified Z-filtered coupled HSQC are still affected by strong coupling. We conclude that spectral simulation yields accurate 1 J CH with errors as low as 1% in the presence of strong coupling.