Proton NMR relaxation study of the binding of pyridine and 1-methylimidazole to some ferriporphyrins and to metmyoglobin

Proton spin-lattice relaxation times of pyridine and 1-methylimidazole complexed on Fe(III)protoporphyrin IX dimethyl ester, Fe(III)tetraphenylporphyrin in chloroform and on metmyoglobin in 2H2O have been measured. Caused by chemical exchange of the ligand molecules into the bulk solvent phase, the decay of their MZ-magnetization is given by a superposition of two exponentials from which the mean lifetime of the complexed species can be determined. This method offers the possibility to study exchange kinetics of ligand molecules from a well defined molecular configuration. The present data are compared with the results from linewidth measurements of the bulk phase molecules. From both sets of parameters a detailed picture of the ligand exchange can be gained, particularly if spin transition of the paramagnetic organic metal complex occurs, as is the case for some ferriporphyrins. For metmyoglobin, the NMR result is compared with relaxation times extracted from temperature jump experiments under similar conditions.