Studies on phosphatidylcholine model membranes. I. Size-heterogeneity effect on permeability measurements

The osmotic shrinking rate of unsonicated egg phosphatidylcholine (PC) liposomes in hypertonic NaCl was studied by determining the initial time rate of change of the reciprocal of the optical density, $\text{d(OD)}{}^{\mathrm{?1}}\text{d}\text{t}$, in a stopped-flow kinetics apparatus, $\text{d(OD)}{}^{\mathrm{?1}}\text{dt}$ was found to be a linear function of reciprocal OD and reciprocal PC concentration, where the linear parameters were quite different depending on the size distribution of liposomes in the dispersion. An approximate theoretical calculation of relative shrinking rates suggests that the larger liposomes mask the osmotic activity of smaller liposomes in the same dispersion. It is concluded that this method should only be used for comparing osmotic permeabilities of liposomes dispersions when both the OD and liposome size distribution of the dispersions are the same.