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Pulsed Third-Spin-Assisted Recoupling NMR for Obtaining Long-Range 13C–13C and 15N–13C Distance Restraints
journal contribution
posted on 2020-08-06, 22:01 authored by Martin
D. Gelenter, Aurelio J. Dregni, Mei HongWe
present a class of pulsed third-spin-assisted recoupling (P-TSAR)
magic-angle-spinning solid-state NMR techniques that achieve efficient
polarization transfer over long distances to provide important restraints
for structure determination. These experiments utilize second-order
cross terms between strong 1H–13C and 1H–15N dipolar couplings to achieve 13C–13C and 15N–13C polarization transfer, similar to the principle of continuous-wave
(CW) TSAR experiments. However, in contrast to the CW-TSAR experiments,
these P-TSAR experiments require much less radiofrequency (rf) energy
and allow a much simpler routine for optimizing the rf field strength.
We call the technique PULSAR (pulsed proton-assisted recoupling) for
homonuclear spin pairs. For heteronuclear spin pairs, we improve the
recently introduced PERSPIRATIONCP (proton-enhanced rotor-echo
short pulse irradiation cross-polarization) experiment by shifting
the pulse positions and removing the z-filters, which significantly
broaden the bandwidth and increase the efficiency of polarization
transfer. We demonstrate the PULSAR and PERSPIRATIONCP
techniques on the model protein GB1 and found cross peaks for distances
as long as 10 and 8 Å for 13C–13C and 15N–13C spin pairs, respectively.
We then apply these methods to the amyloid fibrils formed by the peptide
hormone glucagon and show that long-range correlation peaks are readily
observed to constrain intermolecular packing in this cross-β
fibril. We provide an analytical model for the PULSAR and PERSPIRATIONCP experiments to explain the measured and simulated chemical shift
dependence and pulse flip angle dependence of polarization transfer.
These two techniques are useful for measuring long-range distance
restraints to determine the three-dimensional structures of proteins
and other biological macromolecules.