Fig. 1.
Schematic drawing of the magnetization transfer pathways used for the analysis of the EXSY data for 1, 2 (left) and 8 (right). In the symmetric conformation of 1 and 2, one of the two chemically equivalent amide bonds can flip into a cis-configuration to reach the asymmetric conformation (amide bond between Lac15 and Mle26, see Scheme 1). In this process, magnetization is transferred via two different site-to-site pathways (A ↔ B and A ↔ C with kAB = kAC = k1 and kBA = kCA = k2), each leading to a separate set of EXSY cross-peaks. During a transition from the symmetric to the asymmetric conformation, each nucleus in a symmetric pair undergoes either pathway equally likely. In the reverse process from the asymmetric to the symmetric conformation a nucleus at site B will always follow A ↔ B whereas a nucleus at site C will always follow A ↔ C. As a consequence, the site-to-site exchange rates k1 and k2 for 1 and 2 differ from the mechanistic exchange rates: k’1 = 2 × k1 and k’2 = k2 where K = k’1/k’2. In 8, the C2 symmetry is broken by the two additional nitrogen atoms in the backbone and only a single magnetization transfer pathway has to be considered. Therefore for 8 k1 = k’1 and k2 = k’2.