Proton-spin diffusion observations

Fig. 6.1.9. 200 MHz spectra of H pertaining to spin diffusion experiments on a nylon 6,6 PBZT blend (VanderHart [46]). Left, magnetization gradient created by dipolar echo sequence with spacing 30 [xs, thus, initially favoring the PBZT portion of the blend. Right, results of spin diffusion observed using CRAMPS to obtain high resolution proton NMR of the blend, and observation of magnetization transfer between the phenyl protons of the PBZT, and the methylene protons of the nylon 6,6.

Experiments for the Direct Observation of Proton Spin-Diffusion... 

Depending on experimental parameters, NOE intensities will be affected by spin diffusion (Eig. 8). Magnetization can be transferred between two protons via third protons such that the NOE between the two protons is increased and may be observed even when the distance between the two protons is above the usual experimental limit. This is a consequence of the distance dependence of the NOE. Depending on the conformation, it can be more efficient to move magnetization over intennediate protons than directly. The treatment of spin diffusion during structure refinement is reviewed in more detail in Refs. 31, and 71-73. 

These intermolecular correlation peaks mean that intermolecular cross polarization (CP) occurs between the carbon and proton of intermolecular -interacting polypeptides in blend. There may be two pathways for the observed intermolecular CP. One is that a direct transfer from proton to carbon exists, and another is that a change in the magnetization by spin diffusion (homonuclear Hartmann Hahn transfer) exists. It is thought that the former is much more efficient than the latter because the former comes from only one magnetization transfer process, but the latter comes from two... 

The main source of conformational information for biopolymers are the easy-to-obtain chemical shifts that can be translated into dihedral restraints. In addition, for fully 13C labeled compounds, proton-driven spin diffusion between carbons [72] can be used to measure quantitatively distances between carbons. The CHHC experiment is the equivalent of the NOESY in solution that measures distances between protons by detecting the resonances of the attached carbons. While both techniques, proton-driven spin diffusion and CHHC experiment [73], allow for some variation in the distance as determined from cross-peak integrals, REDOR [74] experiments in selective labeled compounds measure very accurate distances by direct observation of the oscillation of a signal by the dipolar coupling. While the latter technique provides very accurate distances, it provides only one piece of information per sample. Therefore, the more powerful techniques proton-driven spin diffusion and CHHC have taken over when it comes to structure determination by ss-NMR of fully labeled ligands. 

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