Exploration of structure--activity relationships among foldamer ligands for a specific protein binding site via parallel and split-and-mix library synthesis

We describe the use of parallel and split-and-mix library synthesis strategies for exploration of structure-activity relationships among peptidic foldamer ligands for the BH3-recognition cleft of the anti-apoptotic protein Bcl-xL. This effort began with a chimeric (alpha/beta+alpha)-peptide oligomer (composed of an alpha/beta-peptide segment and an alpha-peptide segment) that we previously identified to bind tightly to the target cleft on Bcl-xL. The side chains that interact with Bcl-xL were varied in a 1000-member one-bead-one-compound library. Fluorescence polarization (FP) screening identified four new analogues with binding affinities similar to that of the lead compound but no analogues with enhanced affinity. These results suggested that significant improvements in affinity were unlikely in this series. We then used library synthesis to examine backbone variations in the C-terminal alpha-peptide segment of the lead compound. These studies provided an opportunity for direct comparison of parallel and split-and-mix synthesis formats for foldamer libraries with respect to synthetic variability and assay sensitivity. We found that compounds from both the parallel and one-bead-one-compound libraries could be reliably screened in a competition FP assay without purification of library members. Our findings should facilitate the use of combinatorial library synthesis for exploration of foldamers as inhibitors of protein-protein interactions.