ISSN:
0749-1581
Keywords:
1H NMR
;
NOESY
;
COSY
;
Paramagnetic
;
Model haems
;
Cytochromes
;
Chemistry
;
Analytical Chemistry and Spectroscopy
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
As part of an effort to assign completely the1H resonances of the pyrrole protons of model haems, the NOESY spectra of a series of unsymmetrically phenyl-substituted derivatives of a low-spin paramagnetic haem model complex tetraphenylporphyrinatoiron (III)bis(N-methylimidazole), [TPPFe(N-Meim)2]+, for which the COSY spectra have been reported previously, were recorded in CD2Cl2 or CDCl3 at temperatures of 30 to -35°C using a variety of mixing times. Fourphenomena of importance to the understanding of the cross-correlation patterns obtained in the NOESY spectra of these paramagnetic complexes were observed: (1) NOE 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 were observed; (3) cross correlations between protons in the same pyrrole ring (i.e. cross peaks previously observed in the COSY maps) were observed in the NOESY spectra of some complexes, but not of others; and (4) at ambient temperature, where axial ligand exchange is detectable through cross peaks between the methyl resonances of free and coordinated N-methylimidazole, additional sets of cross correlations between pairs of pyrrole protons, not observed at -35°C, were detecte d. Each of these phenomena is discussed and questions are raised concerning 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 delocalization can be delineated (with some cautionary notes), but there still remains some ambiguity in the assignment of the fourpyrrole proton resonances of these mono-ortho-substituted tetraphenylporphyrinatoiron(III)complexes.
Additional Material:
7 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/mrc.1260311325
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