NMR detection of side chain-side chain hydrogen bonding interactions in ¹³C/¹⁵N-labeled proteins

We describe the direct observation of side chain-side chain hydrogen bonding interactions in proteins with sensitivity-enhanced NMR spectroscopy. Specifically, the remote correlation between the guanidinium nitrogen ₁₅N(ε) of arginine 71, which serves as the hydrogen donor, and the acceptor carboxylate carbon ¹³CO₂(γ) of aspartate 100 in a 12 kDa protein, human FKBP12, is detected via the trans-hydrogen bond (3h)J(NεCO2γ) coupling by employing a novel HNCO-type experiment, soft CPD-HNCO. The (3h)J(NεCO2γ) coupling constant appears to be even smaller than the average value of backbone (3h)J(NC') couplings, consistent with more extensive local dynamics in protein side chains. The identification of trans-hydrogen bond J-couplings between protein side chains should provide useful markers for monitoring hydrogen bonding interactions that contribute to the stability of protein folds, to alignments within enzyme active sites and to recognition events at macromolecular interfaces.