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Solid-state NMR methods are particularly applicable to proteins associated with phospholipid membranes, as even the smallest lipid particles that can be made are generally too large for solution-state NMR, and the presence of lipid makes crystallization difficult. Samples of protein associated with lipid for solid-state NMR can be very close to native conditions, allowing control of factors such as lipid composition, ionic conditions, and hydration. Additionally, samples can be made which are macroscopically ordered with the membrane normal aligned along a specific direction. This makes it possible to obtain more structural and orientational information than with randomly oriented samples. Switched-angle spinning (SAS) NMR, an extension of MAS in which the spinning axis is varied in two correlated Fourier dimensions, allows isotropic spectra and anisotropic coupling values to be collected in the same experiment.
SAS has been used measure scaled dipolar couplings in a liquid crystalline system. Although this technique is not yet widely used because it is experimentally challenging, it has the potential to be a powerful analogue to existing solution-state measurements of residual dipolar couplings in oriented media. In conventional residual dipolar coupling experiments, scalar couplings are measured from a protein in isotropic solution. A separate measurement in then performed in a weakly orienting environment, yielding different splittings due to the presence of residual dipolar couplings. Spinning an oriented sample at angles other than the magic angle directly produces scaled dipolar couplings. SAS experiments therefore have an advantage over the solution-state method; the isotropic and dipolar coupled spectra can be measured using the same sample, an approach that will save time in sample preparation and will also minimize artifacts from using different solvents. Additionally, they will allow measurements to be made on new samples, such as membrane-associated proteins, which are too large or insoluble for the traditional methods.
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