Correlation between 3hJNC‘ and Hydrogen Bond Length in Proteins

Establishing a quantitative relationship between backbone−backbone hydrogen-bond (H-bond) length observed in protein crystal structures and recently observed 3hJNC' couplings across such bonds is limited by the coordinate precision of the X-ray structure. For an immunoglobulin binding domain of streptococcal protein G, very high-resolution X-ray structures are available. It is demonstrated that over the small range of N−O H-bond lengths (2.8−3.3 Å) for which 3hJNC' couplings are observable, the 32 measured 3hJNC' values can be fit to: 3hJNC' = −59000 exp(−4RNO) ± 0.09 Hz, or RNO = 2.75 − 0.25 ln(−3hJNC') ± 0.06 Å. Backbone amide to side-chain carboxyl hydrogen bonds were also investigated, and the measured 3hJNC' values tend to be smaller than expected from their crystallographically determined H-bond lengths. The sign of 3hJNC', determined from a zero-quantum/double-quantum experiment, is found to be the same as that of the 1JNH coupling, i.e., negative.