Off-diagonal intensity

Fig. 41 (top)], which are straightforwardly related to the jump angles [11,72]. In contrast, the dynamics involved in the a-process results in complete randomization of the molecular orientation so that the NMR frequencies at tm — 0 and tm xa> respectively, are uncorrelated and a box-like 2D spectrum with off-diagonal intensity spread all over the frequency plane is observed [cf. Fig. 41 (bottom)]. Information about the course of the reorientation process is available when 2D spectra are measured for different mixing times tm [72,94]. 

The application of proton-driven CSA correlation spectroscopy to amino-acid specifically carboxylic-labeled spider silk [63] is shown in Fig. 4.11. Spider silk is known to consist of alanine- and glycine-rich domains [64, 65] and is known to be semicrystalline. The assignment of alanine to the (crystalline) /3-sheet domains [66] is clearly supported by the chemical-shift correlation spectrum of Fig. 4.11. Because the tensors in a j8-sheet structure are almost parallel, or antiparallel, with the tensors in spatial proximity, a diagonal spin-diffusion spectrum is expected for that structure and is indeed found. In contrast, the glycine spectrum shows considerable off-diagonal intensity. Simulations have shown that the spectrum is compatible with a local 3i-helical structure [63]. 

Among two-dimensional experiments, wideline exchange spectroscopy plays a prominent role in H NMR [1, 4, 61-64]. By correlating the frequency distributions at two different times, any changes in the resonance frequency due to reorientation can be detected in the off-diagonal intensity pattern. With the aid of lineshape simulation and by comparing different mixing times, detailed conclusions about the type and rate of motion can be drawn, as illustrated in Section 6.2.5 and Fig. 6.2.2. 

Figure 5. The data from the 2D experiment is displayed in contours of intensity, each axis represents a proton shift axis. The off-diagonal intensities show homonuclear couplings. For instance, the proton at 950 Hz (as indicated by an arrow) has off-diagonal intensity showing couplings to the four other indicated protons. Figure courtesy of Varian Associates.

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