Constant-time dipolar dephasing

To illustrate the procedure of background signal correction, let us consider a hypothetical fibril system formed by polypeptides containing 21 residues, where a 

In a more general case, some signals from natural abundance 13C nuclei will dephase (5,na d) because of the finite probability that they are in close proximity of other 13C nuclei. Furthermore, some labeled peptide molecules are unfibrillized and we assume that their signals will not dephase (+Lu). Consequently, we can write 

An estimation of the 5nad signal can be obtained by measurements of the unlabeled fibril sample. For convenience, here we assume that 10% of the natural abundance signal will dephase, i.e., Sna d = VliLU x 0.1/0.9, so that we can write 

Consequently, the signal fractions of 51 f(r)/51 f(0) can be calculated and compared with simulations. For samples prepared by bacterial expression, correction of the background signals can be carried out in a similar fashion [48, 53, 54], It is noteworthy that spin geometries deviated from linearity, which may occur in the side-chain 13C nuclei such as C , would produce more rapid signal decays and smaller residual signal fraction [55], 

The novelty of (9) is that the net homonuclear dipolar dephasing can be controlled by a systematic variation of the number of the Ho, Hi, and Hi blocks. This technique has the acronym of PITHIRDS-CT [55] and has the virtues that the effect of T2 is identical for all data points and that the rf field of all the pulses is only 1.67 times the spinning frequency. The only experimental concern is that very stable spinning or active rotor synchronization may be required for the implementation of PITHIRDS-CT. 

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As shown in the preceding two sections, the constant time version of REDOR, CT-REDOR, may be applied as an expedient alternative to the existing REDOR versions in the presence of strong heteronuclear dipolar couplings. In these cases, only few data points are available for the data analysis, which especially in the case of multiple-spin systems renders an evaluation of the second moments impossible. The efficiency of the dipolar recoupling may be intentionally reduced either via a dislocation of the dephasing Ti-pulses from the centre of the rotor period or via an application of non-Ti-dephasing pulses. A variation of the pulse position fpp... 

Reff = observed dipolar coupling constant t = time T20 = spin term in the spherical tensor representation of the dipolar Hamiltonian = zero-quantum relaxation time constant U = propagator = magne-togyric ratio of spin / A/ = anisotropy of the indirect spin-spin interaction 0 = angle between the applied field and the internuclear vector A = dephasing parameter /Uq = permeability of free space Vj. = rotor frequency in Hz 1/, = isotropic resonant frequen-... 

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