Rotational-echo, double-resonance (REDOR) NMR measurements of 3 1 P- 1 5 N dipolar couplings have been made on a complex of Mg guanosine diphosphate (MgGDP) with uniformly 1 5 N-labeled elongation factor Tu. The complex was embedded in a lyophilized buffer glass. The observed 1 5 N REDOR dephasing by 3 1 P was accounted for quantitatively by distances from 1 5 N of Gly23 and Lys24 to P α and P β of MgGDP as determined by X-ray crystallography of a MgGDP complex formed using an elongation factor Tu that is missing a 15 residue loop in the vicinity of the binding site.