Effects of Cyclization on Peptide Backbone Dynamics.

Despite the widespread use of cyclization as a structure optimization tool in peptide chemistry, little is known about the effect of cyclization on peptide internal dynamics. In this work, we used a combination of multifield NMR relaxation and molecular dynamics techniques to study both monocyclic and polycyclic peptides that have promising biopharmaceutical properties, namely, VH, SFTI-1, and cVc1.1, and their less constrained analogues to study the effects of backbone cyclization (which forms a macrocycle) and disulfide-bond cyclization (which forms internal cycles). We confirmed that backbone cyclization contributes to the rigidity of the monocyclic VH. Interestingly, however, backbone cyclization of the bicyclic SFTI-1 had a limited effect on rigidity, with changes in internal dynamics localized around the ligation site. This suggests that the disulfide bond, which creates an internal cycle, has an insulating effect, protecting the internal cycle from external motional effects. An insulating effect was also observed for the polycyclic cVc1.1: The rigidity of the core was not enhanced by macrocyclization. Additionally, we found that disulfide bonds provide a greater contribution to overall rigidity than macrocyclization. Overall, our results suggest that, although backbone cyclization can improve rigidity, there is a complex interplay between dynamics and cyclization, particularly for polycyclic systems.