Survey of existing examples of synthetic linchpin for post translational macrocyclization of phage-displayed natural amino acid peptide libraries shows that most linchpins are designed to serve a single purpose: Class I forms monocyclic peptides; Class II offers possibilities for diversification of monocycles by introducing unnatural pharmacophores; Class III creates bicyclic topologies. Figure provides examples for each type: 6, 36-40 2, 7 convert linear peptides to simple monocycles; 3,4 transform linear peptides to macrocycle with built in pharmacophores; 5, 6, 8 form bicycles and cannot be stopped at monocycle state. To fulfill all 3 purposes, we envisioned a multipurpose a C2-linchpin, 3,5-bis(bromomethyl)benzaldehyde, containing two thiol reactive moieties to form monocycle, and a tuned reactive aldehyde group to engage with either exogenous nucleophiles to introduce pharmacophores or with N-terminus to yield bicycles. The engagement of N-terminus in topological transformations yields single product from low symmetry C2-linchpin and removes potential liabilities associated with free Nterminus. Aldehydes as controllable and versatile electrophiles have been used to from peptide monocycles in water, and intact N-terminus is known to pose unique aldehyde reactivity as elegantly explored as Malins and Raj. 3,5-bis(bromomethyl)benzaldehyde (referred as KYL) can be applied on both synthetic peptides and phage-displayed libraries (Figure ). Tuning of monocyclization and bicyclization in an aqueous environment is performed by simply adjusting pH and grafting of pharmacophores is controlled by concentration. We see no barriers for expanding the utility of this linchpin to any other GEL or DEL libraries that contain a pair of thiol residues and a low basicity primary amine.