Heisenberg spin exchange rate

Fig. 8. Two-dimensional exchange spectroscopy (often called 2D-ELDOR) of the spin-labeled 3K-8 peptide with mixing time T = 296 nsec. Both the 2D surface and the contour map are shown. The peaks along the diagonal are related to the absorption spectrum of the spin label. The high-held M,= - line is weak because of experimental dead time artifacts. The cross-peaks, especially those between the outermost hyperfine lines, provide direct evidence of Heisenberg spin exchange. The cross-peak intensity can be used to determine the second-order rate constant for collisions between peptides.

Secondary stmcture can be obtained by studying the accessibility of the nitroxide label to paramagnetic colliders. The collision rate (more precisely the Heisenberg exchange frequency) with the spin label influences the relaxation time of the latter which can be measured and be used to estimate the local concentration of a paramagnetic quencher near a nitroxide spin label. 

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