(a) Displacement isotherms at 602 nm obtained upon the addition of l- or d-histidine (2.02 mM) into a solution containing CAS (10 μM), Cu(OTf)2 (200 μM), and (S,S)-1 (2.5 mM) in 1:1 MeOH:H2O, 50 mM HEPES buffered to pH 7.5. (b) Histidine (480 μM) was added to a solution containing CAS (21 μM), Cu(OTf)2 (375 μM), and (S,S)-1 (4.7 mM) in 1:1 MeOH:H2O, 50 mM HEPES buffered to pH 7.5. (c) Valine (124 μM) was added to a solution containing CAS (30 μM), Cu(OTf)2 (120 μM), and (R,R)-2 (50 mM) in 1:1 MeOH:H2O, 50 mM HEPES buffered to pH 7.0.