HYDROGEN-BONDING NETWORKS IN PROTEINS AS REVEALED BY THE AMIDE (1)J(NC') COUPLING-CONSTANT

A regular dependence of the (1)J(NC') coupling constant on the nature of amide group hydrogen bonding is observed in the model compound, N-a cetylglycine. Hydrogen bonding of the amide oxygen increases the coupl ing constant, whereas hydrogen bonding of the amide hydrogen decreases it. This establishes the (1)J(NC') coupling constant as a useful prob e for amide group hydrogen bonding in proteins. From measured (1)J(NC' ) coupling constants in human ubiquitin, characteristic sequences of t he coupling constant are observed which correlate with the protein sec ondary structure. In the alpha-helix, the coupling constant does not v ary much (15.3 +/- 0.5 Hz). In beta-sheets, variations are larger (15. 5 +/- 1.5 Hz), except in the central region of the parallel and antipa rallel P-sheets, where the coupling constant exhibits rather regular v alues. In this region, significant correlation of the coupling constan ts within the lines of interstrand N-H (...) O=C hydrogen bonding is o bserved, indicating a cooperative polarization of peptide bonds in the H-bonding network. The largest change of the coupling constant is fou nd within reverse turns. At position 2 of reverse turns the coupling c onstant has the lowest values (13.9 +/- 0.8 Hz), whereas at position 4 the values are highest (16.6 +/- 0.6 Hz). This change of the coupling constant within the three residues of reverse turns is caused by spec ific hydrogen bonding of amide groups in the reverse turns. The result indicates that the intraprotein N-H (...) O=C hydrogen bonds of the m ain-chain amide groups are weaker than the hydrogen bonds; of these gr oups to water.