Pseudo-proline building blocks exert a dual functionality in enhancing and stabilizing the relevant polyproline II (PPII) conformation and increasing and optimizing van der Waals contacts and hydrogen bonding to the receptor mols. thus modifying affinity and specificity. They are highly useful in studying ligand recognition mediated by Src homol. 3 domains essential in cellular regulation and protein-protein interactions. The 2-C substituents promote the induction of the required PPII helix and allow for optimal complementation of the SH3 topog. [on SciFinder (R)]