EMR spectroscopy

This chapter reviews the work ofthe last five to six years on paramagnetic states of carotenoids using electron magnetic resonance. Mainly radical cation and neutral molecular triplet states are treated. Part of this ch te deals with paramagnetic states of carotenoids in model systems. These have been synthesized in order to mimic both electron and energy transfer processes in the natural photosynthetic systems. Consequently, the electron magnetic resonance (EMR) spectroscopy of carotenoid triplet and radical states yields important information about their photochemistry. Finally, the EMR spectroscopy on carotenoid radicals is reviewed. It serves to establish the database on their intrinsic properties which is necessary for the analysis of carotenoid radicals in vivo. 

The photochemistry of a molecular triad consisting of a porphyrin covalently linked to a carotenoid polyene and a fullerene derivative has been studied at 20 K by time-resolved EMR spectroscopy following laser excitation (Carbonera et al., 1998). Excitation of the porphyrin yields a coupled radical pair with a carotenoid cation and a C o anion. The exchange interation in the pair has been determined to approx. 

Electron spin resonance (ESR) spectroscopy is also known as electron paramagnetic resonance (EPR). spectroscopy or electron magnetic resonance (EMR) spectroscopy. The main requirement for observation of an ESR response is the presence of unpaired electrons. Organic and inorganic free radicals and many transition metal compounds fulfil this condition, as do electronic triplet state molecules and biradicals, semicon-ductor impurities, electrons in unfilled conduction bands, and electrons trapped in radiation-damaged sites and crystal defect sites. 

The difficulty of this definition (or detection) of interactions is one of scale the perturbations that one is trying to observe are orders of magnitude smaller than actual measurements. Computationally, a better approach is to attempt evaluation of the individual perturbations directly, and then define the total interaction energy as a sum of the individual perturbation energies. In the case of EMR spectroscopy, this is exactly what we are doing by using ENDOR or ESEEM. We know that the effects that we expect to see will become manifest in the nuclear hyperfine terms, so rather than try to measure these from differences in the EMR spectrum, which includes the electron Zeeman term, we turn instead to the ENDOR and ESEEM, which detect the nuclear hyperfine interactions. 

EMR (electron magnetic resonance) See electron paramagnetic resonance spectroscopy. 

Spin-label substituted short polyenes have also been synthesized and their molecular structure in frozen glass determined by ENDOR spectroscopy (Mustafi et al., 1993). Nitric oxide has been shown to react with )3-carotene to yield stable nitroxide radicals that can be detected and analyzed by EMR (Gabr et al., 1995). 

The spectroscopy of solids is defined as the qualitative or quantitative measurement of the interaction of electromagnetic radiation (emr) with atoms or molecules in the solid state. The emr interacts as scattering, absorption, reflectance, or emission with solid matter. A variety of spectrometer configurations are used to optimize the measurements of electromagnetic radiation as it interacts with solid matter. This chapter provides an overview of... 

The principal advantage of ENDOR spectroscopy is the much finer energy scale upon which the state-to-state transitions are recorded. As discussed in 3, conventional cw-EMR spectroseopy detects the weak hyperfine interactions of the spin Hamiltonian as a perturbation of the electronic Zeeman effect in many practical situations, inhomogeneous broadening will wash out the hyperfine structure of the speetrum. In such cases of inhomogeneously broadened EMR spectra, interaction can only be deconvoluted for practical analysis via simulations (Hyde Fron-cisz, 1982). The ENDOR method, however, records a spectram that represents the... 

Oriented solid-state NMR spectroscopy and biochemical cross-linking experiments were used to show that the ligand-free membrane protein transporter EmrE forms anti-parallel dimers with different monomer tilt angles relative to the lipid bilayer. In addition, subtle conformational changes were detected upon drug binding that emphasize the need for an atomic-resolution structure. ... 

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