2-DIMENSIONAL H-1-NMR SPECTROSCOPY OF PARAMAGNETIC HEME MODELS OF THECYTOCHROMES - SUCCESSES, FAILURES AND UNANSWERED QUESTIONS

As part of an effort to assign completely the H-1 resonances of the py rrole protons of model haems, the NOESY spectra of a series of unsymme trically phenyl-substituted derivatives of a low-spin paramagnetic hae m model complex phenylporphyrinatoiron(III)bis(N-methylimidazole), [TP PFe(N-MeIm)(2)](+), for which the COSY spectra have been reported prev iously, were recorded in CD2Cl2 or CDCl3 at temperatures of 30 to - 35 degrees C using a variety of mixing times. Four phenomena of importan ce to the understanding of the cross-correlation patterns obtained in the NOESY spectra of these paramagnetic complexes were observed: (1) N OE cross peaks can be detected for these intermediate molecular weight complexes, even in non-viscous solvents; (2) for some complexes only one set of NOE cross peaks different from those previously observed in the COSY maps were obtained, whereas for other complexes two sets wer e observed; (3) cross correlations between protons in the same pyrrole ring (i.e. cross peaks previously observed in the COSY maps) were obs erved in the NOESY spectra of some complexes, but not of others; and ( 4) at ambient temperature, where axial ligand exchange is detectable t hrough cross peaks between the methyl resonances of free and coordinat ed N-methylimidazole, additional sets of cross correlations between pa irs of pyrrole protons, not observed at -35 degrees C, were detected. Each of these phenomena is discussed and questions are raised concerni ng the cross-relaxation pathways leading to the observed NOESY maps of these paramagnetic complexes. At the present state of development of two-dimensional NMR studies of these paramagnetic model haem complexes , the electron density distribution in the orbital utilized for spin d elocalization can be delineated (with some cautionary notes), but ther e still remains some ambiguity in the assignment of the four pyrrole p roton resonances of these mono-ortho-substituted tetraphenylporphyrina toiron(III) complexes.