Protein Structure Determination by Solid-State NMR - NMR of Proteins and Small Biomolecules - page 186

influenza M2 proton channel structure, function and ligand binding [ 27 , 28 ]; the
human prion protein [ 29 ]; the structural conversion of neurotoxic amyloid beta
oligomers to fibrils [ 30 ]; and the structure and dynamics of the retinylidene proteins
from bacteria, including bacteriorhodopsin, sensory rhodopsin, halorhodopsin, and
proteorhodopsin [ 31 - 41 ], and the structure of the HET-s(218-289) fibril [ 42 , 43 ].
Some other membrane proteins, such as membrane-embedded enzymes [ 44 - 46 ],
histidine kinases [ 47 ], ABC transporters [ 48 ], and bacterial outer membrane
proteins [ 49 , 50 ], have also been investigated through multidimensional correlation
experiments in SSNMR.
In this chapter we will briefly review some of the recent progress in studying
membrane proteins by using magic-angle spinning solid-state NMR from biological
structure point of view; for a complete overview of the achievements in this field,
please refer to the following excellent reviews [ 19 , 51 - 67 ].
2 Basic Experimental Techniques Used in Solid-State NMR
Unlike solution-state NMR, the resolution and sensitivity of SSNMR are affected
heavily by orientation dependent anisotropic spin interactions such as chemical
shift anisotropy, homonuclear and heteronuclear dipolar couplings, quadrupole
coupling, etc. These interactions generally cannot be averaged out by the molec-
ular tumbling motions in solids, presenting a very broad line shape with poor
sensitivity of SSNMR spectra. Combining MAS with cross-polarization (CP),
high power proton decoupling, recoupling, and isotopic labeling can achieve
high resolution and signal-to-noise ratio in SSNMR for protein structure
determination.
2.1 Magic-Angle Spinning
MAS is an essential technique in SSNMR for obtaining a high resolution spectrum
[ 68 - 70 ]. The basic idea is to spin the sample container (rotor) about an axis, which
subtends an angle of 54.74 o , the magic-angle, with respect to the static field B 0 .
The spatial rotation of the sample introduces time-dependence to anisotropic spin
interactions, such as chemical shift anisotropy, homonuclear dipole-dipole
couplings, and heteronuclear dipole-dipole couplings, which are averaged out
more efficiently as the sample spinning frequency increases. Due to the periodic
time-dependence of the CSA and dipolar spin interactions, the broad static line
shape breaks up into a center band at the isotropic position and a set of spinning
sidebands separated by the spinning frequency. As the spinning frequency
increases, the time averaging is more effective, which leads to a decrease in
the sideband intensities and an increase in the center band intensity. The advan-
tage of MAS is that both resolution and sensitivity are greatly increased.