Motional freedom of the central metal atom in apohemoglobin reconstituted with 111In: protoporphyrin IX. Time-differential perturbed gamma-ray angular correlations

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Abstract
Based on time‐differential perturbed gamma–gamma angular correlations (PAC), the rotational correlation time for indium‐111 in apohemoglobin reconstituted with 111In:protoporphyrin IX is τrot(Hb)=23 ns. For 111In:mesoprotoporphyrin IX reconstituted similarly into apomyoglobin, PAC measurements yield τrot(Mb)=16 ns. The PAC ratio τrot(Hb)/τrot(Mb)=1.4 is the same (within experimental error) as that from fluorescence polarization (FP) measurements for apo‐Hb and apo‐Mb reconstituted with an aminonaphthalene sulfonate in place of the natural heme. Since the rigid‐sphere ratio τrot(Hb)/τrot (Mb)=4.0 is larger than the experimental PAC and FP ratios, the metalloporphyrin exhibits significantly more internal motional freedom in hemoglobin than in myoglobin. Finally, PAC quadrupolar asymmetry parameter values show that the electron distribution about the central metal atom is axially symmetric in reconstituted In–Mb, but not in reconstituted In–Hb.

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