Figure 2. Mechanical unfolding of G6 at different [Ca²⁺].

(a) Representative force-extension profiles for unfolding of (GB1–G6)₄ in the presence of various [Ca²⁺] at a pulling speed of 400 nm s⁻¹. The [Ca²⁺] at which the traces were measured are shown on the same row in b. Red lines correspond to the WLC fitting to the G6 unfolding events. (b) The unfolding forces histograms at different [Ca²⁺]. The solid lines are Gaussian fittings of these data. The unfolding forces of G6 show unimodal distributions at all Ca²⁺ concentrations and the widths of the distributions remain constant. This indicates that the calcium-free and calcium-bound conformations are in fast equilibrium upon stretching.