strained peptides – understanding preorganisation and binding a ffi nity †

The development of constrained peptides represents an emerging strategy to generate peptide based probes and hits for drug-discovery that address challenging protein–protein interactions (PPIs). In this manuscript we report on the use of a novel a-alkenylglycine derived amino acid to synthesise hydrocarbon constrained BH3-family sequences (BIM and BID). Our biophysical and structural analyses illustrate that whilst the introduction of the constraint increases the population of the bioactive a-helical conformation of the peptide in solution, it does not enhance the inhibitory potency against proapoptotic Bcl-xL and Mcl-1 PPIs. SPR analyses indicate binding occurs via an induced fit mechanism whilst X-ray analyses illustrate none of the key interactions between the helix and protein are disturbed. The behaviour derives from enthalpy–entropy compensation which may be considered in terms of the ground state energies of the unbound constrained and unconstrained peptides; this has implications for the design of preorganised peptides to target protein–protein interactions.