One Peptide Reveals the Two Faces of α-Helix Unfolding-Folding Dynamics

The understanding of fast folding dynamics of single alpha-helices comes mostly from studies on rationally designed peptides displaying sequences with high helical propensity. The folding/unfolding dynamics and energetics of alpha-helix conformations in naturally occurring peptides remains largely unexplored. Here we report the study of a protein fragment analogue of the C-peptide from bovine pancreatic ribonuclease-A, RN80, a 13-amino acid residue peptide that adopts a highly populated helical conformation in aqueous solution. H-1 NMR and CD structural studies of RN80 showed that alpha-helix formation displays a pH-dependent bell-shaped curve, with a maximum near pH 5, and a large decrease in helical content in alkaline pH. The main forces stabilizing this short alpha-helix were identified as a salt bridge formed between Glu-2 and Arg-10 and the cation-pi interaction involving Tyr-8 and His-12. Thus, deprotonation of Glu-2 or protonation of His-12 are essential for the RN80 alpha-helix stability. In the present study, RN80 folding and unfolding were triggered by laser-induced pH jumps and detected by time-resolved photoacoustic calorimetry (PAC). The photoacid proton release, amino acid residue protonation, and unfolding/folding events occur at different time scales and were clearly distinguished using time-resolved PAC. The partial unfolding of the RN80 alpha-helix, due to protonation of Glu-2 and consequent breaking of the stabilizing salt bridge between Glu-2 and Arg-10, is characterized by a concentration-independent volume expansion in the sub-microsecond time range (0.8 mL mol(-1), 369 ns). This small volume expansion reports the cost of peptide backbone rehydration upon disruption of a solvent-exposed salt bridge, as well as backbone intrinsic expansion. On the other hand, RN80 alpha-helix folding triggered by His-12 protonation and subsequent formation of a cation-pi interaction leads to a microsecond volume contraction (-6.0 mL mol(-1), similar to 1.7 mu s). The essential role of two discrete side chain interactions, a salt bridge, and in particular a single cation-pi interaction in the folding dynamics of a naturally occurring alpha-helix peptide is uniquely revealed by these data.