Figure 2.

Histograms of FRET efficiency values, E, taken from single molecules exposed to buffer solutions with 0, 0.625, 1.25, 2.5, 5, 10, 20, 40 and 100 mM Mg²⁺ concentration. The lowest panel shows the histogram for molecules exposed to 0 mM Mg²⁺ immediately after exposure to 40 mM Mg²⁺. Three subpopulations, denoted as U (〈E_U〉 ∼ 0), I (〈E_I〉 ∼ 0.7) and F (〈E_F〉 ∼ 0.9), can be distinguished. Dotted, dashed and solid thick lines represent fit results using model distributions for the U, I and F states, respectively. The solid line represents the sum over the three distributions.