Effect of 4-fluoro-L-proline on the SH3 binding affinity

Short proline-rich peptides corresponding to well-known SH3 ligands exhibit little or no secondary structure before their binding to the cognate protein-targets. Under these conditions the association of a proline-rich peptide with the SH3 domain indicates unfavorable binding entropy, likely resulting from a loss of rotational freedom on the formation of the PPII helix. With the aim of stabilizing the PPII helix conformation in SH3 binding motifs we replaced the proline residues of the HPK1 proline-rich decapeptide, PPPLPPKPKF (P2), either with the 4-R-fluoro-L-proline (FPro) or with the 4-S-fluoro-L-proline (fPro) at different i, i+3 positions. The interactions of the fluoro-proline peptides with the SH3 domain of the protein cortactin were analyzed quantitatively by non-immobilized ligand interactions assay by circular dichroism (NILIA-CD). The conformation of each peptide in both aqueous and organic solvents was investigated as a function of temperature using CD spectroscopy.