Switchable Proline Derivatives: Tuning the Conformational Stability of the Collagen Triple Helix by pH Changes

(4S)-Aminoproline is introduced as a pH-sensitive probe for tuning the conformational properties of peptides and proteins. The pH-triggered flip of the ring puckering and the formation/release of a transannular H bond were used to switch the formation of collagen triple helices on and off reversibly.