For POTEntial=3DPO, a restraining function for 3D NOE-NOE experiments is used. The intensity of a 3D NOE-NOE crosspeak, , between spins i,j, and k, is proportional to the product of the individual NOE transfer efficiencies of each mixing time:
where is the first and the second mixing time, R the cross-relaxation matrix, the equilibrium magnetization of spins k, and c a constant.
The 3D NOE-NOE intensity can be approximated by
where and are the distances between spins i, j and j, k, respectively, and the proportionality constant is set equal to one. The 3D NOE-NOE function is given by
where S is the energy constant specified by the SCALe statement, is the observed NOE-NOE intensities, and are error bounds, and K is a scale factor specified by the RSWITch statement. The NOE-NOE connectivity and the intensity and error-bound information are provided by a pair of ASSIgn statements:
The first ASSIgn statement specifies spins i and j, along with the observed intensity and the error bounds and . The second ASSIgn statement specifies spins j and k; the three real numbers following the second selection are irrelevant and can be set to arbitrary values. Note: the 3D NOE-NOE function can be used in combination with R-6 or center averaging. For further details, the reader is referred to Habazettl et al. (1992a, 1992b) and Bernstein et al. (1992).