The effect of 17O on the relaxation of an amide proton within a hydrogen bond

The relaxation rates of the multiple-quantum coherence for the amide hydrogen of Gly13 in ras p21·GDP were determined in the presence and absence of 17O labeling in the β-phosphate of GDP. No significant difference could be observed between labeled and unlabeled samples, in spite of the fact that the hydrogen bond from the amide group of Gly13 to the β-phosphate is shorter than is typical, based on its chemical shift. For macromolecules in which an oxygen atom is the acceptor of a hydrogen bond, dipolar coupling between 17O and hydrogen is unlikely to be observable, except for extremely short H-bonds.