that can be addressed by biophysical methods. For example, to what extent does protonation of the alkylated histidine influence the heme reduction potential? How do the histidine-heme linkages differ from the cysteine-heme thioether linkages of c-type cytochromes? What is the electronic influence of the distal ligand? Practical usage of the crosslinking reaction includes the formulation of artificial enzymes or oxygen transporters with non-dissociable heme; such systems are expected to have an elongated lifetime, have increased resistance to proteolytic cleavage, and may be capable of functioning over a wide range of conditions (e.g. high temperature, low pH) for industrial or biomedical applications.