The angular correlation of the 150-247 kev gamma-ray cascade of 111mCd is strongly perturbed when this nucleus is bound to the enzyme carbonic anhydrase. A comparison of the perturbed angular correlation for the apoenzyme with that for native carbonic anhydrase confirms that the 111mCd binds at the active region of the enzyme. These results provide good evidence that the perturbed angular correlation reflects the effective molecular rotational correlation time at the metal binding site, and that this radioactive nucleus can be used as a rotational tracer to label biological macromolecules. The qualitative dependence of the perturbed angular correlation of the 111mCd cascade on the molecular rotational correlation time at the metal binding site is illustrated using a cadmium-complex solution at various temperatures.
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