13C and 15N NMR studies of iron-bound cyanides of heme proteins and related model complexes:: Sensitive probe for detecting hydrogen-bonding interactions at the proximal and distal sides

Studies of the C-13 and N-15 NMR paramagnetic shifts of the iron-bound cyanides in the ferric cyanide forms of various heme proteins containing the proximal histidine and related model complexes are reported. The paramagnetic shifts of the C-13 and N-15 NMR signals of the iron-bound cyanide are not significantly affected by the substitution of the porphyrin side chains. On the other hand, the paramagnetic shifts of both the C-13 and N-15 NMR signals decrease with an increase in the donor effect of the proximal ligand, and the C-13 NMR signal is more sensitive to a modification of the donor effect of the proximal ligand than the N-15 NMR signal. With the tilt of the iron-imidazole bond, the paramagnetic shift of the C-13 NMR signal increases, whereas that of the N-15 NMR signal decreases. The hydrogen-bonding interaction of the iron-bound cyanide with a solvent decreases the paramagnetic shift of both C-13 and N-15 NMR signals, and the effect is more pronounced for the N-15 NMR signal. Data on the C-13 and N-15 NMR signals of iron-bound cyanide for various heme proteins are also reported and analyzed in detail. Substantial differences in the C-13 and N-15 NMR shifts for the heme proteins can be explained on the basis of the results for the model complexes and structures around the heme in the heme proteins. The findings herein show that the paramagnetic shift of the C-13 NMR signal of the iron-bound cyanide is a good probe to estimate the donor effect of the proximal imidazole and that the ratio of N-15/C-13 NMR shifts allows the hydrogen-bonding interaction on the distal side to be estimated.