Figure 1.

A peptide plane in an external magnetic field, B_o, showing the anisotropic effect of an 180° rotation about the N-NH bond axis upon the N-NH and C′-Cα dipolar interactions. The N-NH and C′-Cα dipolar interactions are represented schematically by magnetic dipoles centered on N, H, C′, and Cα. The N-H dipolar interactions are seen to be invariant on this rotation, whereas the C′-Cα dipolar interactions vary significantly, even changing sign. The figure illustrates a fast local motion superimposed on a slower overall isotropic motion.