EARLY AND DELAYED STAGES IN THE SOLUBILIZATION OF PURPLE MEMBRANE BY A POLYOXYETHYLENIC SURFACTANT

The purpose of this paper is to explore the reasons by which purple me mbrane solubilization by detergents takes hours, or even days, to reac h equilibrium, while most biomembranes are solubilized in a matter of seconds, or minutes. With that aim, changes in the purple membrane abs orption spectrum produced by hydrogenated Triton X-100 under equilibri um conditions (24 h) have been compared to those caused by the same su rfactant in the minute, second and sub-second time scale. It is found that the various processes that accompany, or lead to, solubilization are already detected, and even reach an apparent equilibrium, in the 1 0 s that follow detergent addition. No new phenomena are detected in t he following minutes, or hours, that are relevant to the process under study. This leads to the conclusion that the long solubilization proc ess consists of the repeated operation of simple phenomena that are re latively fast in themselves. A hypothesis is proposed according to whi ch the tight crystalline organization of the purple membrane prevents the insertion of detergent monomers in the lipid bilayer; instead, the surfactant would bind the periphery of the patches, i.e., the hydroca rbon-water contact region, and solubilization would take place gradual ly, from the periphery towards the core of the membrane patches, at a progressively lower rate as the amounts of free detergent and detergen t-binding sites are decreased by the previous solubilization steps.