Figure 4.

The R*H⁺·ArrFL-1 binding mode stabilized at low pH. Difference spectra recorded in the absence of ArrFL-1 peptide are shown in black, in the presence of 20 mm ArrFL-1 in gray. The resulting PBS are shown in blue (WT), red (in ²H₂O), or purple (R3.50L mutant). Note the difference in band pattern compared with the deprotonated complex. The small effect of ²H₂O on the positive 1657 cm⁻¹ indicates that this band is a structurally sensitive amide I band.