Figure 2.

Possible fast exchange pathways between E-I and E-II. In pathway A, the interconversion between E-II and E-I occurs inside the complex by tilting, switching, and tilting via the intermediate E-III.2/E-III.1. In contrast, in pathway B (n = 0; H-bond switching off), the exchange occurs via rotation only. In pathway C (n > 0; H-bond switching on), rotation and switching are coupled, yielding either the same conformation (n = odd) or different conformation (n = even). n is the number of occurring switching events prior to tilting.