Effective dipolar coupling

A very important aspect of solid-state NMR studies of uniaxiaUy oriented peptides is the possibility to directly determine the conformation of the protein in the bilayer. The PISA wheels 2 for regular secondary structures, which may be considered a direct NMR mapping of the so-called helical wheels, help interpreting the experimental spectra. More recently, Opella and co-workers have suggested to exclusively use the effective dipolar couplings in the analysis of such spectra to alleviate uncertainties from small residue/structure-specific variations in the chemical shifts. 

Fig. 17. The ll DQ sideband patterns arising from DQ SQ correlation experiments on the OCH2 group of ethanol bound to amorphous silica surfaces.24 The sideband patterns are shown for two different DQ excitation times, as indicated. The solid lines are the experimental results the dotted lines are simulations assuming the motion indicated at the top of the figure and a distribution of effective dipolar coupling constants for the dipolar coupling within the OCH2 group.

The effective dipolar-coupling frequency d depends on the dipolar-coupling constant d,y and, therefore, inversely on the third power of the internuclear... 

Equation (23) is rewritten in terms ofgenerafized fictitious ZQ spin-1/2 operators, X = (T — Sx)/2, Y = ( IzSy — lySz), and Z = [izSz + lySy), to emphasize the relevant rotation subspace. Please note that the effective dipolar coupling only depends on the y pR Euler angle as fpR appears as a phase. Using the effective Hamiltonian in Eq. (23), the initial magnetization on spin I is transferred to spin S as... 

Bq = magnetic flux density D = dipolar coupling constant D = effective dipolar coupling constant h = Planck s constant I = spin- nucleus / = coupling constant = field gradient tensor Q = nuclear quadrupole moment s = doublet splitting ... 

Finally, it is important to recognize that the dipolar coupling constant measured in any NMR experiment also has, in principle, a contribution from the anisotropy in the indirect spin-spin coupling. A/. That is, only an effective dipolar coupling constant, Rgff can be measured where... 

Figure 3-14 shows simulated SC-2D NMR spectra for the a methyl protons and the 2,3,5,6 ring protons, both of which were calculated as a 7 spin system consisting of the a methyl protons and the 2,3,5,6 ring protons, with effective dipolar coupling parameters of = -3410, = 400, = 0, A32 = 3250, A35 = -400, and... 

---