Figure 2.

Effect of Zn²⁺ binding on proton uptake at pH 8.5 after a single laser flash in native (A and C) and Glu-L212→Gln mutant RCs (B and D) caused by formation of D⁺Q_A⁻ (A and B, 120 μM terbutryn added) and D⁺Q_B⁻ (C and D). The left ordinate shows the absorbance changes at 580 nm of cresol purple, the right ordinate the corresponding proton uptake. Experimental conditions were: 2 μM RC, 50 mM KCl, 0.04% β-d-dodecyl maltoside, 40 μM cresol purple, pH 8.5. The traces shown are the differences between the traces obtained in the unbuffered solution and the traces obtained after adding 7 mM Tris. Note that the traces with and without Zn²⁺ overlap in B and D.