Surface potential dependence of the distribution of charged dye molecules onto photosynthetic membranes

Partition of merocyanine dyes, which have a negative charge, onto photosynthetic membranes of chloroplasts and bacteria was analyzed by measuring the fluorescence intensity change, absorbance change, and amount of dye in the supernatant after centrifugation. The partition depended on the surface potential, which is a function of valence and concentration of ions in the medium. The distribution of dyes between the membrane and aqueous phase was determined after centrifugation. The logarithm of the ratio of distribution was linearly related to the logarithm of salt concentration as predicted from the Gouy-Chapman theory and the Boltzmann distribution. Plots of the logarithm of fluorescence intensity against the logarithm of KCl and MgSO4 concentrations gave two straight lines with a slope ratio of about two. The absorbance change upon salt addition was also explained by the Gouy-Chapman theory. The use of these dyes as probes of the surface potential of membranes is discussed.