Polar/apolar interfaces modulate the conformational behavior of cyclic peptides with impact on their passive membrane permeability.

Cyclic peptides have the potential to vastly extend the scope of druggable proteins and lead to new therapeutics for currently untreatable diseases. However, cyclic peptides often suffer from poor bioavailability. To uncover design principles for permeable cyclic peptides, a promising strategy is to analyze the conformational dynamics of the peptides using molecular dynamics (MD) and Markov state models (MSMs). Previous MD studies have focused on the conformational dynamics in pure aqueous or apolar environments to rationalize membrane permeability. However, during the key steps of the permeation through the membrane, cyclic peptides are exposed to interfaces between polar and apolar regions. Recent studies revealed that these interfaces constitute the free energy minima of the permeation process. Thus, a deeper understanding of the behavior of cyclic peptides at polar/apolar interfaces is desired. Here, we investigate the conformational and kinetic behavior of cyclic decapeptides at a water/chloroform interface using unbiased MD simulations and MSMs. The distinct environments at the interface alter the conformational equilibrium as well as the interconversion kinetics of cyclic peptide conformations. For peptides with low population of the permeable conformation in aqueous solution, the polar/apolar interface facilitates the interconversion to the closed conformation, which is required for membrane permeation. Comparison to unbiased MD simulations with a POPC bilayer reveals that not only the conformations but also the orientations are relevant in a membrane system. These findings allow us to propose a permeability model that includes both 'prefolding' and 'non-prefolding' cyclic peptides - an extension that can lead to new design considerations for permeable cyclic peptides.