Fig. 2.

Experimental UV-Vis titration curves of ${\mathrm{K}\mathrm{S}\mathrm{I}}^{D40N}$ (circles) and tyrosine (triangles) in buffered H₂O (red) and D₂O (blue). Fractional ionization of the phenolic moiety is monitored by measuring the change in absorption at 300 nm. All datasets were well-fit to a sigmoid function, admitting four ${\mathrm{p}K}_{\text{a}}$ values and two ${\Delta \mathrm{p}K}_{\text{a}}$ values (the acidity isotope effect).