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Molecular structure of α, β, [delta] γ-tetraphenylporphinatoindium(III) chloride, and perturbed angular correlation study on reconstituted myoglobin Lee, Kai Mon


The first crystal and molecular structure of an indium porphyrin, α,β,δ,γ-tetraphenylporphinatoindium(III) chloride (InTPP:Cl) has been determined by X-ray diffraction. The indium is five-coordinate in a square-pyramidal complex with chloride as the axial ligand. The average In-N bond is 2.156(6) Å, with an In-Cl distance of 2.369(2) Å. The porphinato core is somewhat expanded with an average radius of 2.067(3) Å, and the macrocycle is non-planar with a net doming (0.1Å) toward the indium. The indium atom is located 0.61 Å above the mean plane of the four pyrrole nitrogen atoms. Based on the high degree of analogy between the principal structural features of InTPP:Cl, Fe(III) porphyrins, and Fe(II) deoxyhemoglobin (e.g., displacement of metal atom above the mean porphyrin plane, distance from the metal to pyrrole nitrogen, porphyrin core expansion, doming of porphyrin skeletal core atoms), the present study suggests that reconstitution of an indium porphyrin into apohemoglobin and apomyoglobin should produce the quaternary "tense" conformation. This provides an unique opportunity to study the porphyrin-apoprotein interactions in the "T" state of hemoglobin and myoglobin. The present reconstitution of indium-Ill meso-protoporphyrin IX (¹¹¹InMPP IX) into myoglobin represents the first location of a motional probe at the central metal of the active site of the protein. The rotational correlation time (T[sub c]) of the reconstituted myoglobin has been determined by the Perturbed Angular Correlations (P.A.C.) technique. Since. ¹¹¹InMPP IX is structurally similar to ferro-protoporphyrin IX (heme), there should be very little internal rotation of the ¹¹¹InMPP IX at the active site of the reconstituted myoglobin. Furthermore, the indium-labelled myoglobin is very likely to retain its native conformation, in contrast to other labelling techniques in which there is always uncertainty as how much the label distorts the structure of the protein being studied. Finally, since no additional flexibility is introduced at the labelled site of the protein, the present determination of T[sub c](P.A.C.) for myoglobin is more direct than fluorescence depolarization or ESR determinations.

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