A, F-D curves were collected from disc membranes in SMFS assay buffer supplemented with 200 μm ZnCl2, CaCl2, CdCl2, or CoCl2. The average force required to unfold each stable structural segment in rhodopsin observed in the presence of bivalent metal ions is shown relative to the average force observed in the absence of added bivalent metal ions. B, F-D curves were collected from disc membranes in SMFS assay buffer (25 mm MgCl2), in SMFS assay buffer devoid of MgCl2 (0 mm MgCl2), or in SMFS assay buffer devoid of MgCl2 and supplemented with 1 mm EDTA (0 mm MgCl2, 1 mm EDTA). The average force obtained from each condition is shown. Error bars represent the standard deviation. Values represented in the figure are given in Table 1.