Relaxation electron paramagnetic resonance

A. Wennmalm, B. Lanne, and A.S. Petersson, Detection of endothelial-derived relaxing factor in human plasma in die basal state and following ischemia using electron-paramagnetic resonance spectrometry. Anal. Biochem. 187, 359-363 (1990). 

The FeMo cofactor (or M center) in the MoFe-proteinis in the native paramagnetic M state. Reduction of the MoFe-protein by the Fe-protein results in the reduction of FeMo-co from the M state to the M state at a potential estimated to be less than —0.465 V (NHE). The electron paramagnetic resonance (EPR) silent M state is only transiently produced during catalysis, and relaxes to the M state when catalysis stops. The intimate consequences of the M state reduction are not precisely known. A more oxidized diamagnetic state may also be generated (M ) at —0.042 V but its biological relevance is unclear [9]. 

The substitution of a different metal into an enzyme provides a very useful method for studying the immediate environment of the metal site. In addition to the use of Co2 for spectral studies, appropriate substitution allows the use of physical methods such as electron paramagnetic resonance (Co . Cu2 ). the Mdssbauer effect tFe2 ). proton magnetic resonance relaxation techniques (Mir ), or X-ray crystallography (with a heavy metal atom to aid in the structure solution). ... 

It is important to realize that no one method of spectroscopy is clairvoyant. Electron paramagnetic resonance spectroscopy cannot sense low-spin Fe(II) as this state is diamagnetic, nor reliably the high-spin Fe(II) state because of rapid spin-lattice relaxation, large zero-field splittings or both Mossbauer spectroscopy cannot distinguish Fe(III) spin states... 

Electron paramagnetic resonance has been used to investigate the MnnMn" and mixed valence (MnnMnm and MnmMnIV) states but not the MnniMnni state, which is EPR-silent as a result of the fast relaxing, S = 2, ground state. 

The triplet state of the unpaired electrons of oxygen play a key role in both the photon excitation and the potential relaxation mode of the excited chromophores of vision. The paramagnetic properties of oxygen provide a definitive method of determining whether oxygen is present in the chromophores of vision, a condition that would eliminate the Shiff-base theory of retinol reaction with opsin to form rhodopsin. The evaluation of the electron paramagnetic resonance of the chromophores of vision is discussed in Chapter 7. 

BH4 = Tetrahydrobiopterin CAM = Cytotoxic activated macrophage cNOS = Constitutive nitric oxide synthase CPR = Cytochrome P450 reductase EDRF = Endothelial-derived relaxation factor EPR = Electron paramagnetic resonance spectroscopy IL-1 = Interleukin-1 iNOS = Inducible nitric oxide synthase EPS = Lipopolysaccharide, or endotoxin NMMA = ISp-monomethyl-L-arginine NOS = Nitric oxide synthase ROS = Reactive oxygen species SOD = Superoxide dismutase TNF = Tumor necrosis factor. 

Electron paramagnetic resonance (EPR) Quadrupole tensor, nuclear Zeeman splitting, g values, coupling constants, relaxation times Usually for odd electron metal sites probes ground-state wave function at high resolution... 

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