Hexacyanochromate ion as a paramagnetic anion probe for active sites of enzymes. Application to ribonuclease A

The 270 MHZ proton MNR spectra of an aqueous solution of ribonuclease A in the presence of hexacyanochromate ions were measured. The C2 and C4 proton signals of the His 12 and His 119 residues were selectively broadened, indicating that this ion was bound to the active site. From analyses of concentration dependences of diamagnetic shifts, one hexacyanocobaltate ion was found to bind to the active site, with the dissociation constant of 8 mM. The paramagnetic hexacyanochromate ion is useful for mapping the active sites of enzymes for anionic substrates.