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 15Nepsilon of arginine 71, which serves as the hydrogen donor, and the acceptor carboxylate carbon 13CO2gamma of aspartate 100 in a 12 kDa protein, human FKBP12, is detected via the trans-hydrogen bond 3h JNepsilonCO2gamma coupling by employing a novel HNCO-type experiment, soft CPD-HNCO. The 3h JNepsilonCO2gamma coupling constant appears to be even smaller than the average value of backbone 3h JNC' 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.