What Is the Crucial Factor for Vibrational Circular Dichroism in Hemoprotein Ligands?

Intense vibrational circular dichroism (VCD) bands for the azide antisymmetric stretching vibration in the azide complex of a C 2 -chiral strapped iron porphyrin have been observed with anisotropy values of ca. 2 × 10 - 3 . This is the first case in which the extremely strong VCD band has been measured for a ligand vibration of iron porphyrin without an apo-protein. The VCD spectrum of myoglobin azide shortly after reconstitution with its own heme exhibited a weak band but over time the intensity with negative sign gradually recovered. The combination of these two results suggests that the chiral environment produced by the peripheral substituents on the porphyrin ring, which give rise to a diastereotopic plane, is responsible for the generation of the intense VCD in hemoproteins whereas specific interactions of distal residues with the porphyrin ring and the ligands in halo-proteins are not always necessary.