Electron spin resonance spectra of penta- and hexacoordinated nitrosyl iron protoporphyrin IX complexes |
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Authors: | M Christahl H Twilfer K Gersonde |
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Institution: | (1) Abteilung Physiologische Chemie, Rheinisch-WestfÄlische Technische Hochschule Aachen, Schneebergweg 213, D-5100 Aachen, Germany |
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Abstract: | Electron spin resonance (ESR) spectra of frozen aqueous solutions of NO · haem · base complexes and NO · haem intercalated into dodecyl sulfate micelles have been measured at 77 K and analyzed for the hyperfine components of 15NO,14N-base, 14N-pyrroles and 57Fe which coincide with the principal directions of the g tensor. The influence of the basicity of the nitrogen base on the spin distribution and geometry of the Fe-N-O grouping has been demonstrated by replacing imidazole for pyridine and by comparing the ESR spectra with those obtained for the monomeric insect haemoglobin CTT IV.The comparison of the hyperfine parameters described for the so-called pentacoordinated nitrosyl complex of CTT IV with those of the NO · haem intercalated into detergent micelles has furnished evidence that the ESR spectrum of this conformation state of haemoglobin has to be definitely assigned to a pentacoordinated nitrosyl complex.The azz values increase with the following orders: CTT IV (2.98 mT) < imidazole complex (3.04mT) < pyridine complex (3.15mT) for 15NO, and pyridine complex (0.59 mT) < imidazole complex (0.67 mT) < CTT IV (0.70 mT) for the 14N-base. This result is in conformity with an increase of the donor and the acceptor strengths of the nitrogen base in trans-position to 15NO. The ayy and axx components of 15NO and the 14N-base are strongly nonequivalent in the nitrosyl haemoglobin CTT IV, and less nonequivalent in the NO · haem · pyridine complex, indicating bending of the Fe-N-O grouping. The hyperfine components of the axial ligands coinciding with the x and y component of the g tensor are nearly equal for the NO · haem · imidazole complex. |
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Keywords: | Electron spin resonance Nitrosyl-haem complexes Coordination state Hyperfine constants |
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