extends the range of heme chemistry questions 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.