Recent Developments in Biomolecular Solid-State NMR - Recent Developments in Biomolecular NMR - page 325

Similar measurements can also be made using proton detection experiments
with perdeuterated proteins. 85-89
13.7 Static Solid-State NMR
Spectral anisotropy can be exploited in oriented samples, with 15 N, 31 P, 19 F
and 2 H being the most widely used. Since the anisotropy is much larger (in
some cases up to chemical shift y200 ppm for 15 N; 10 6 Hz for quadrupolar
nuclei) than chemical shift dispersion, the dominant spectral features give
information about orientation within the applied field. Thus, helical
orientation has been determined both for uniformly and specifically labelled
peptides, with kink position being determined in specifically labelled ( 2 H, 15 N)
membrane-embedded peptides. 2 Combining solid-state NMR data, either from
direct orientational constraints (for example from 2 H- or 15 N-labelled
peptides) or through the well-established PISEMA 2D spectral representation
(of 15 N- 1 H dipolar couplings in one dimension and 15 N chemical shift in the
other) approach, is well described 2,90,91 and gives an insight into helix tilt for
uniformly labelled transmembrane samples, but when combined with specific
labelling, increases the structural resolution. Additionally, REDOR ( 13 C- 2 H)
distance measurements of the 2 H-labelled drug, amantadine, combined with
other NMR information, enabled a model of the drug-channel interactions
and drug-binding site. 4
Extending these approaches, dynamic models that lead to functional
descriptions have been proposed. Most recently, 5,92 the influenza virus A
M2 pentamer has been described from oriented and MAS solid-state NMR
studies in bilayers, with descriptions of the pH-dependent (low pH is open,
high pH is closed) ion-translocation pathway, suggesting dynamic shuttling of
protons into the virion mediated by a specific (His37) histidine of the channel
when in bilayers, which is not resolved in solution NMR approaches using
detergent—a general case is therefore made for studying membrane proteinsin
bilayers as a method of choice. 3,19
13.8 Proton Detection
Currently, most solid-state NMR experiments are based upon 13 C detection
which results in severe loss of sensitivity compared to 1 H-detection because 13 C
has a lower gyromagnetic ratio than 1 H. The use of 1 H-detection in solid-state
NMR would not only be advantageous in order to increase sensitivity, but also
in order to provide an additional nucleus with which to resolve spectra, as is
possible in solution NMR. The main limitation to using 1 H detection is the
high number of strong 1 H- 1 H dipolar couplings which results in extreme line
broadening. The extensive network of dipolar-coupled protons can be removed
by high levels of deuteration, thus making proton-detected experiments
possible. 59,93 High-resolution spectra have been obtained at spinning speeds of
around 20 kHz when back-exchanging only around 10-40% of exchangeable