The present work has several implications for the chemistry of heme proteins. (1) The existence of the PTM in GlbN [40] and its engineered presence in GlbN [22] and CtrHb cautions that the PTM may occur in additional heme proteins, not only wild-type but also unwittingly in His-tagged versions, (histidine) variants, or designed proteins. (2) In preparing the histidine Nε2–heme vinyl Cα crosslink, care must be exercised in the choice of a reducing agent to avoid heme and protein damage, as occasionally caused by the oxidative by-products of aerobic DT reduction. Anaerobic DT treatment or reduction with alternative agents such as a ferredoxin system [56] if sufficiently powerful may be preferable. (3) It is possible to engineer the crosslink in a b heme protein using relatively mild reaction conditions. Although detailed kinetic analysis was not performed on CtrHb variants, the products are consistent with the mechanism described previously [21], in which case the pKa of the reactive histidine, in addition to its ability to adopt the geometry of the product, is an important factor.