Following the validation of KYL linchpin on synthetic peptides, we accessed its capability to modify phage displayed peptides. NMR spectroscopy cannot be applied to phage displayed peptides, thus, we monitored KYL modification using previously reported ESI-MS and MALDI method. ESI-MS Spectra of modified peptides cleaved from pIII clone were inconclusive (Figure ), but modifications were evidenced on pVIII-displayed peptide in MALDI. A pVIII clone, SWCRPATVNC, was first reduced by TCEP, then treated by 0.1 mM of KYL for 30 min in TB buffer, pH 8.6. Formation of aldehyde was observed by 131 mass increase (Figure ) and further confirmed by incubating the phage with TAMRA hydrazine, leading to the formation of corresponding hydrazone (+610 observed increase, +615 theoretical increase) (Figure ). The aldehyde-installed phage clone was then treated with 10 mM NaCNBH3 overnight in NH4OAc buffer, pH 4.6 for Malins bicyclization. Reaction resulted in a minor decrease in mass (-11 observed decrease, -16 theoretical decrease), and most importantly the product no longer responded to TAMRA hydrazine (Figure ). Combined observations confirmed the consumption of aldehyde and formation of Malins bicycle.