1H resonances of proximal histidine in CO complexes of hemoglobins provide a sensitive probe of coordination geometry.

A straightforward strategy for assignment of the C epsilon H, C delta H and N delta H proton resonances of the proximal histidine ligand in diamagnetic complexes of monomeric hemoglobins and myoglobins is reported. These resonances are subject to large ring current shifts and are highly sensitive to coordination geometry. There are no significant differences between the CO complexes of myoglobin, leghemoglobin or hemoglobin alpha-subunits in proximal His coordination geometry or hydrogen bonding to the backbone at Leu F4. Ring current calculations show that the His F8 coordination geometry in the CO complexes of myoglobin and hemoglobin alpha-subunits is very similar in crystal and solution.