Time differential perturbed angular correlation measurements in metal ion complexed ATP

The perturbed angular correlation technique is being increasingly used to study the motion, conformation, or binding properties of biologically relevant molecules. It does, however, suffer because, except in a few cases, the P.A.C. nuclides are not directly involved in biological processes and the results therefore must be treated with caution. Nevertheless, it has been argued, for example, that a study of the binding of In ATP to 3-phosphoglycerate kinase is not adversely affected by the replacement of Fe/sup 3 +/ by In/sup 3 +/. In this work we wish to point out that whether or not a metal ion replacement is valid from a biological viewpoint, the interpretation of the results can be frustrated by the particular radiochemical properties of the probe nuclide. Our work has used /sup 181/Hf and /sup 57/Co bound to ATP in an attempt to separate effects due on the one hand to the rotational diffusion of the metal ion-ATP unit as a whole, and on the other to the aftereffects of the nuclear decay and any possible static quadrupole effects at the metal ion binding site. To do this successfully, complementary data also has to be obtained. Because of the difficulties associated with the interpretation of viscosity data on large molecules, i.e., the use of Debye theory to give the rotational correlation time and the measurement of viscosity itself in nonnewtonian fluids, we have used dielectric relaxation measurements to provide additional information on molecular rotation.