Fig. 1.

The combined effect of conformational averaging and the non-linear and multiple-valued relationships linking NMR observables to structural properties. Upper row four different distributions of inter-nuclear distances r _ij and (dashed lines) the corresponding (red) linearly, (blue) r ⁻³ and (green) r ⁻⁶ averaged distances. All four distributions have the same r ⁻⁶ average (0.227 nm) upon which PREs and many NOEs depend, and are therefore indistinguishable at the level of the NMR observable. Lower row three different distributions of values of a dihedral angle ϕ and (rightmost graph) the Karplus relation for the ϕ angle of the protein backbone (Pardi et al. 1984). All three distributions give rise to the same average ³ J-value (5.45 Hz, red dashed line in rightmost graph) and thus are indistinguishable at the level of the NMR observable. (Rightmost graph, green dashed line) the four dihedral angle values that would be predicted from a ³ J-value of 5.45 Hz without allowing for conformational averaging