The prosthetic group of low-spin haem proteins is an iron porphyrin with two axial ligands, typically histidine, methionine or lysine. Determining the geometry of the axial ligands is an important step in structural characterisation, particularly in the paramagnetic oxidised forms. This work extends earlier studies of the hyperfine nuclear magnetic resonance (NMR) shifts of haem substituents in bis-His and His^Met cytochromes to His^Lys co-ordination in the M100K mutant of Paracoccus versutus cytochrome c 550 . The electronic structure of the His^Lys haem is shown to be similar to that produced by His^cyanide co-ordination, such that NMR can be used to determine the geometry of the His ligand. ß
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.