Magnetic circular dichroism spectra of soybean leghaemoglocin a at room temperature and 4.2 K

Abstract

Magnetic circular dichroism (MCD) and electron paramagnetic resonance (EPR) measurements on soybean leghaemoglobin α have shown that at room temperature leghaemoglobin a is a mixture of a high-spin compound with the proximal histidine and water as fifth and sixth ligands of haem iron, and of a low-spin derivative which is a bishistidine compound with proximal and distal histidines as axial ligands. Addition of imidazole gives a histidine-imidazole compound with pH-dependent MCD and EPR spectra. At acid pH the compound is similar to other bisimidazole derivatives with MCD maximum at 1610 nm and EPR signals at 3.03, 2.29, and approx. 1.50. At alkaline pH the spectrum has an MCD maximum at 1350 nm and g factors 2.82, 2.29, 1.69. The spectra interconvert with a pK a between 6.5–7.0. It is suggested that at alkaline pH the proton at N-1 of the exogenous imidazole dissociates, resulting in an imidazolate ion bound to the iron. Leghaemogiobin can also bind phenol. This derivative is high-spin at room temperature, but mainly low-spin at 4.2 K. The leghaemoglobin-phenol compound may serve as a model for haemoprotein with histidine-phenolate as fifth and sixth axial ligands.