OPTICAL-CHANGES IN UNILAMELLAR VESICLES EXPERIENCING OSMOTIC-STRESS

Membrane properties that vary as a result of isotropic and transmembra ne osmolality variations (osmotic stress) are of considerable relevanc e to mechanisms such as osmoregulation, in which a biological system ' 'senses'' and responds to changes in the osmotic environment. In this paper the light-scattering behavior of a model system consisting of la rge unilamellar vesicles of dioleoyl phosphatidyl glycerol (DOPG) is e xamined as a function of their osmotic environment. Osmotic downshifts lead to marked reductions in the scattered intensity, whereas osmotic upshifts lead to strong intensity increases. It is shown that these c hanges in the scattering intensity involve changes in the refractive i ndex of the membrane bilayer that result from an alteration in the ext ent of hydration and/or the phospholipid packing density, By consideri ng the energetics of osmotically stressed vesicles, and from explicit analysis of the Rayleigh-Gans-Debye scattering factors for spherical a nd ellipsoidal shells, we quantitatively demonstrate that although cha nges in vesicle volume and shape can arise in response to the impositi on of osmotic stress, these factors alone cannot account for the obser ved changes in scattered intensity.