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

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 τ_rot(Hb) =23 ns. For ¹¹¹In:mesoprotoporphyrin IX reconstituted similarly into apomyoglobin, PA measurements yield τ_rot(Mb)=16 ns. The PAC ratio τ_rot(Hb)/τ_rot(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 τ_rot(Hb)/ τ_rot(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, PA 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.