Abstract
The effects of spin diffusion, which is the process of indirect magnetization transfer mediated by dipole-dipole interaction among neighboring spins ( 1)) often plague the interpretation of NOESY spectra of macromolecules. Problems arise when, during the mixing period, magnetization exchange proceeds by more than one step and results in cross peaks between hydrogens that are not nearest neighbors. Macromolecules contain many spin-diffusion pathways that lead to the production of excessive numbers of cross peaks and affect the volumes of nearest-neighbor cross peaks. Spin diffusion can sometimes include other mechanisms of magnetization transfer such as chemical exchange. All sources of the effect confuse the quantitative interpretation of crossrelaxation data. Any particular mechanism through which magnetization can exchange much faster than through cross relaxation acts as a “short circuit” that connects otherwise unrelated cross-relaxation pathways. In macromolecules, chemical-exchange processes can provide efficient magnetization-transfer mechanisms: ( 1) chemical-exchange rates, which are unrelated to cross-relaxation rates, can exceed the latter by one or more orders of magnitude, and (2) chemical-exchange processes can bring distant atoms close together transiently. One of the simplest examples of chemical exchange in a protein is the flipping of aromatic rings by 180” about the CB-Cy bond. For symmetrical rings (tyrosine or phenylalanine), such ring flips do not lead to a structural change. However, depending on the resonance frequency and the chemical-shift difference between atoms in the two orientations, separate resonances for the two orientations may be observed even at flip rates as fast as 100 SK’. By comparison, cross-relaxation rates occur between 0.1 and 10 s-l. Thus, ring-rotation rates may be an order of magnitude faster than the fastest cross-relaxation rate and can influence unrelated cross-relaxation pathways across the ring, over a distance of 4 A or more. We demonstrate here how flipping of
Published Version
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