Relaxation, cross many-spin problem

There are several potentially serious problems involved in the interpretation of NMR data in terms of distance constraints. First, a phenomenon called spin diffusion may result in spurious NOESY cross-peaks between protons that share a common neighboring proton, but which themselves are greater than 5 A apart. Second, biological macromolecules are always flexible to at least some degree, and NOESY cross-peak intensities are expected to reflect the inverse average sixth root of the interproton distances. As a result, it is entirely possible for a proton to appear to be adjacent to two other protons simultaneously, when the other two protons are nevertheless always greater than 10 A apart. Finally, NOESY cross-peak intensities are affected by numerous spectral artefacts and well as by other sources of relaxation, which may cause many cross-peaks to be missing. 

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