We then tested whether the intramolecular reaction can be intercepted by the exogenous amine introduced during the reductive amination. Aniline derivative (pKa around 5), aliphatic amine (pKa around 11) and N-terminus of glycine amine with pKa around 7.7 exhibited contrasting reactivity when interacting with the aldehyde-peptide. Mixing 1 mM aniline derivative and 0.8 mM of aldehyde-peptide at pH 4.6, 70% of product was monocyclic product. At 10 mM, the aniline derivative converted all the aldehyde-peptide to desired product (Figure ). Glycine derivative at 1 mM and 10 mM yielded 12 % and 16 % monocyclic product (Figure ); The reaction was undetectable for aliphatic amine at 1 mM, but at 10 mM, 35 % of product was desired monocycle (Figure ). Outcomes are expected based on the protonation these amines at pH 4.6. The observation shows that the reaction of less reactive amines can still be driven forward by increasing their concentration. These findings provided valuable insights for the future design of phage display library and its modification by pharmacophores that contain amines.