Fig. 4. (A) Quantification strategy for the reaction between DFS and M13 phage displaying a peptide ACPARSPLEC.13 (B) Reaction of reduced phage with biotin-iodoacetamide (BIA) biotinylates the phage. (C) Capture assay measures the yield of biotinylation. (D) Capture assay determined that 95% of phage is reduced (reactive with BIA). Diluting the reducing agent 10-fold does not change the number of reduced thiols, but exposing the reduced phage to 0.5 mM DFS in 30% MeCN/Tris buffer for 30 min consumes reactive thiols yielding 80% OFS-modified phage. Error bars represent standard deviations from two experiments. (E) Capture can be summarized as stack bar: bottom bar shows that ∼60% of phage is modified by 0.5 mM solution of DFS in aqueous Tris buffer containing 5% DMF as co-solvent.