The conformations of the adenosine moiety of MgADP and MgATP bound to rabbit muscle creatine kinase were investigated by two-dimensional transferred nuclear Overhauser effect spectroscopy (TRNOESY). The effects arising from adventitious binding of the ligands to the enzyme on the measurements were delineated. It was shown that, with sample protocols typically used thus far with the TRNOE method (enzyme, approximately 1 mM; ligand, approximately 10 mM), the TRNOESY pattern for the nucleotides with creatine kinase is similar to that with gamma-globulin and bovine serum albumin, which do not have specific nucleotide binding site(s). Measurements of NOE between the H1'-H2' proton pair as a function of ligand concentrations with the enzyme-ligand ratio kept constant at 1:10 showed that, for ligand concentrations over about 3-4 mM, weak nonspecific binding makes a significant contribution to the observed NOE. Thus the NOE values relevant for the determination of the nucleotide conformation at the active site were measured at nucleotide concentrations of about 1.5 mM. The TRNOE buildup curves for all the ligand-proton pairs were analyzed using a complete relaxation matrix approach. The interproton distances derived from the NOE's were then used as constraints in elucidating the ligand structure by using the program CHARMm. The NOE-determined structures of both MgADP and MgATP bound to creatine kinase correspond to an anti conformation with the glycosidic angle (O'4-C'1-N9-C8) chi = 51 +/- 5 degrees. The ribose pucker nominally representative of these data is a O4'T with a phase angle of pseudorotation (p) of 70.5 degrees.