Supplementary Figure 7: The Debye–Hückel model described MCS and MCS⁺ FRET efficiencies for a wide range of ionic strengths, demonstrating a linear relationship between FRET efficiency and the effective electrostatic potential of the membrane.

(a-d) Ionic strength titration curve of MCS and MCS⁺ is fitted to the Debye–Hückel model (a, c):

K⁻¹ = M+N/sqrt(I)

and extended Debye–Hückel model (b, d):

K⁻¹ = M+N/{10^[-0.5*I^0.5/(1+0.98*I^0.5)]*I^0.5}^0.5

where, K⁻¹ is electric charge potential, I is ionic strength, M is the shifting factor and N the scaling factor. Data are mean and standard error of maxima normalized R/G curves for MCS and MCS⁺ averaged over n = 7 membrane sheets (black) with each measurement taken at equilibrium. The plot of Debye–Hückel model and extended Debye–Hückel model are shown in red. Fitting residuals is plotted in black dots (R² = 0.988 in a, R² = 0.981 in b, R² = 0.989 in c and R² = 0.977 in d).