Lipid transfer between vesicles: Effect of high vesicle concentration

The problem of the desorption of a lipid molecule from a lipid vesicle (donor) and its incorporation into another vesicle (acceptor) at high acceptor concentrations, which has been investigated experimentally (Jones, J. D. and Thompson, T. E., 1990. Biochemistry, 29:1593-1600), is analyzed here from a theoretical point of view, formulated in terms of the diffusion equation with appropriate boundary conditions. The goal is to determine whether or not the observed acceleration of the off-rate from a donor is caused by interaction with an acceptor vesicle at short range, or is simply the result of statistical effects due the proximity of the acceptor and its influence on the probability of the test lipid returning to the donor. We establish a correspondence between the theoretical parameters and the experimental, thermodynamic and dynamic variables entering the problem. The solution shows that, because of the extremely high Gibbs activation energy for desorption of a phospholipid, the process would always be first-order, even at very high vesicle concentrations. This means that acceleration of the off-rate must be due to donor-acceptor interactions at short distances, as proposed in the experimental work.