THE EFFECT OF O-17 ON THE RELAXATION OF AN AMIDE PROTON WITHIN A HYDROGEN-BOND

The relaxation rates of the multiple-quantum coherence for the amide h ydrogen of Gly(13) in ras p21 . GDP were determined in the presence an d absence of O-17 labeling in the beta-phosphate of GDP. No significan t difference could be observed between labeled and unlabeled samples, in spite of the fact that the hydrogen bond from the amide group of Gl y(13) to the beta-phosphate is shorter than is typical, based on its c hemical shift. For macromolecules in which an oxygen atom is the accep tor of a hydrogen bond, dipolar coupling between O-17 and hydrogen is unlikely to be observable, except for extremely short H-bonds.