Polarization inversion spin exchange

Polarization inversion spin exchange at the magic angle... 

Polarization Inversion Spin Exchange at the Magic Angle (PISEMA) Experiment... 

Polarization Inversion Spin Exchange at the Magic Angle (PISEMA) high-resolu-tion solid-state NMR techniques can be used to investigate the influence of local chain dynamics on cooperative chain motions between the OCH3 protons of PMMA and the aromatic protons of PVPh [45,47]. The 2D experimental... 

Z. Gan, Spin dynamics of polarization inversion spin exchange at the magic angle in multiple spin systems, J. Magn. Reson. 143 136-143 (2000). 

D Correlation Spectroscopy. A simple, quahtative approach has been described for the determination of membrane protein secondary structure and topology in lipid bilayer membranes." The new approach is based on the observation of wheel-like resonance patterns in the NMR H- N/ N polarization inversion with spin exchange at the magic angle (PISEMA) and H/ N HETCOR spectra of membrane proteins in oriented lipid bilayers. These patterns, named Pisa wheels, have been previously shown to reflect helical wheel projections of residues that are characteristic of a-helices associated with membranes. This study extends the analysis of these patterns to P-strands associated with membranes and demonstrates that, as for the case of a-helices, Pisa wheels are extremely sensitive to the tilt, rotation, and twist of P-strands in the membrane and provide a sensitive, visually accessible, qualitative index of membrane protein secondary structure and topology. 

When the primary acceptor is prereduced, electron spin polarization can transfer by exchange interaction from BPh to Q, leading to an inversion of the EPR line of in RCs where was magnetically uncoupled from Fe [112] (Fig. 8). From a phenomenological treatment [112-114] it was concluded that the exchange interaction /(BPh QA) was 3 - 5 G, whereas /(P BPh ) was between 1 and 5 G. A more sophisticated treatment of the three-spin system P BPh QX [115] led to 7(P BPh ) between 0 and +8 G. (Note that for / = 0 polarization may develop if D = 0.) A positive value of / for a biradical state is unusual it might be explained by some form of superexchange via an intermediate (possibly one of the accessory bacteriochlorophylls). 

The spectra were obtained at a frequency of 100 MHz and with a MAS frequency of 12.487 kHz corresponding to the n = 1 rotational resonance that is the spinning speed was equal to the difference in isotropic shifts between the labeled sites. The time shown denotes the interval between inversion of the labeled methylene resonance, which occurs immediately after cross polarization, and application of the observation pulse. Thus, 0 ms corresponds to the initial nonequilibrium state established by the selective inversion. The rate of this decrease may be modeled to determine the distance between the labeled sites (Fig. 23.24 (bottom)). Comparison of magnetization exchange data (filled circle) for a37-38 along with the calculated curves for four distances 4.74 A (dotted line), 4.0 A (dashed line), 3.9 A (solid line) and 3.8 A (dot-dashed line). The 4.74 A distance would be expected for an idealized antiparallel j8-strand. The best fit to the data is 3.9 A. This result is for the undiluted a37-38 sample. To eliminate possible effects due to intermolecular interactions, measurements were performed on isotopically pure samples and on samples in which the doubly labeled peptide was diluted 1 5 and 1 10 in unlabeled peptide. This produced a corrected distance for a37-38 of 4.0 A and in all cases, the distance could be defined to within... 

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