1H-NMR studies of ferric soybean leghemoglobin Assignment of hyperfine shifted resonances of complexes with cyanide, nicotinate, pyridine and azide

Abstract

The 1H-NMR spectra of complexes of soybean ferric leghemoglobin a with cyanide, nicotinate, pyridine and azide have been recorded. Assignments of many of the hyperfine shifted resonances to specific groups on the periphery of the heme have been made on the basis of their intensities and chemical shifts, pH dependence, nuclear Overhauser effects, spin decoupling and the use of Gd 3+ as a relaxation probe. The resonances of the protons at positions 3 and 5 of pyridine and nicotinate ligands have also been assigned. The iron(III) atom in the cyanide, nicotinate and azide complexes is almost completely low spin. In the pyridine complex, which is predominantly low spin, a high-spin state is thermally populated at room temperature. Information on the conformation of the heme propionate and vinyl side-chains is obtained. The average rotational position of one of the heme vinyl groups appears to differ between the nicotinate and cyanide complexes. In both of these complexes conformational rearrangement of a heme propionic acid side-chain occurs upon deprotonation of its carboxylic acid group (p K approx. 5.0). A further change in the conformation of this group occurs in leghemoglobin nicotinate upon deprotonation of the distal histidine. The p K of the heme propionic acid side-chain in leghemoglobin pyridine is 5.6. Its conformation and environment appears to differ from that in the nicotinate and cyanide complexes. In leghemoglobin cyanide, evidence for an interaction between the protonated distal histidine and the cyanide ligand is obtained. In each of the complexes studied the unpaired electron spin distribution over the porphyrin ring is highly anisotropic. Considerable differences exist between the unpaired spin distribution in the cyanide complexes of leghemoglobin and myoglobin. The axial ligand field is stronger in leghemoglobin and this may be of significance in the reaction with oxygen.