Experimental procedure for peptides 5 and 6. Scheme S1 Synthesis of H-Gly-Gly-Arg-AMC (5) and butynoyl-functionalized Gly-Gly-Arg-AMC (6) (i) AMC, POCl 3 , pyridine (dry), -15 °C (2 h) to r.t. (1 h) (68%); (ii) 2.6 M HCl in EtOAc, r.t., 2 h for B (97%) and 4 h for 5 (98%); (iii) Boc-Gly-Gly-OH (1.02 equiv.), H-Arg-AMC (B), DMF, EDC.HCl (1.1 equiv.) DMAP (2 equiv.), 0 °C to r.t., 16 h (86%); (iv) 2 M NaOH, dioxane/H 2 O (1:1 v/v), r.t., 18 h (63%). Materials Unless otherwise stated, all

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... were obtained from commercial sources and used without further purification. Analytical thin layer chromatography (TLC) was performed on Merck precoated silica gel 60 F-254 plates (layer thickness 0.25 mm) with