Helix−Turn−Helix Motifs in Unsolvated Peptides

The conformations of unsolvated Ac-A14KG3A14K + 2H+ (Ac = acetyl, A = alanine, K = lysine, G = glycine) have been examined by ion mobility measurements and molecular dynamics simulations. This peptide was designed as a model helix-turn-helix motif. It was found to adopt three distinct geometries which were assigned to an extended helical conformation which is only stable at low temperatures (<230 K), a relatively high energy but metastable structure with exchanged lysines, and a coiled-coil. The coiled coil (which consists of an antiparallel arrangement of two helical alanine sections linked by a flexible glycine loop) is the dominant conformation. For temperatures >350 K, the experimental results indicate the helices uncouple and the loop randomizes. From equilibrium constants determined for this helix coupling right arrow over left arrow uncoupling transition, we found DeltaH degrees = -45 kJ mol-1 and DeltaS degrees = 114 J K-1 mol-1. -DeltaH degrees is essentially the enthalpy change for docking the two helices together while DeltaS degrees is essentially the entropy change for freeing up the glycine loop.