A theoretical model of the intensity of the near-infrared porphyrin-to-iron charge-transfer transitions in low-spin iron(III) haemoproteins. A correlation between the intensity of the magnetic circular dichroism bands and the rhombic distortion parameter of iron

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A theoretical model is presented of the source of the intensity of the porphyrin-to-iron charge-transfer (c.t. ) band of low-spin iron(III) haems in proteins and model complexes. The intensity mechanism involves intensity borrowing via the ‘transfer term’ from the intense visible and Soret bands of the haem spectrum. The experimental evidence suggests that about 5% of the intensity is borrowed. The intensity of the magnetic circular dichroism (m.c.d) spectrum of the c.t. bands at 4.2 K depends upon the ground-state gz value and upon the product ab where a and b are the coefficients of the iron (III) ion d-orbitals 3dxz and 3dyz′ respectively, in the ground-state Kramers doublet. This leads to a dependence of the m.c.d. C-term intensity upon the rhombic distortion, V, experienced by the iron (III) ion. IfV is zero then a=b, and the hole in the t2g5d subshell is equally divided between the two orbitals. The c.t. band is x,y- polarized and the m.c.d, intensity a maximum value. As V tends to a large value either a or b drops to zero, and the hole becomes localized in one of the dxz,yz orbitals. This results in a c.t. transition which is predominantly x- or y-polarized. The m.c.d. intensity drops to zero since a pair of perpendicularly polarized transitions is needed for the absorption of circularly polarized light. Experimental evidence drawn from measurements on a wide range of low-spin iron (III) haemoproteins has been used to substantiate the model. This study shows that the intensity of the near-i.r m.c.d. transition can be a useful qualitative indicator of the magnitude of the rhombic crystal-field component at the iron(III) ion. This in turn allows some conclusions to be drawn about the relative orientation of the axial ligands to iron(III) in bis (histidine) co-ordinated haems.