Encapsulation of Inorganic Nanoparticles in Block Copolymer Vesicle Wall Driven by the Interfacial Instability of the Emulsion Droplets

In this work, we proposed an effective route, i.e., three-dimensional (3D) confined co-assembly of block copolymers (BCPs) and inorganic nanoparticles (NPs) within the organic emulsion droplet, to efficiently encapsulate high-density...

Lower limit of the gas permeability coefficient of gas vesicles

The gas vesicles found in various planktonic prokaryotes are hollow, rigid structures permeable to gases. They collapse when the difference between the external hydrostatic pressure and internal gas pressure exceeds their critical pressure (usually about 0.6 MPa). It was found that dried gas vesicles would survive exposure to gas pressures considerably in excess of this value (4 MPa or more), because gas diffused into them as the pressure was raised and the pressure difference required to cause collapse was not established. They survived the most rapid rates of pressure rise, 0–4.6 MPa in less than 2.5 ms, to which they were exposed. From this it can be calculated that the gas permeability coefficient of the average gas vesicle ( α ) exceeds 22 x 10 3 s -1 and the permeability of the gas vesicle wall ( k ) is greater than 332 μm s -1 . Gas molecules may diffuse through fixed pores in the gas vesicle wall. Since a gas molecule of collision diameter 0.63 nm is known to penetrate the gas vesicle, this would be the minimum diameter of such a fixed pore. It is shown by kinetic theory that the permeability coefficient of an average gas vesicle with one pore of this size would be 2.1 x 10 3 s -1 : there would, therefore, have to be at least 11 such pores to account for the observed minimum permeability coefficient. Gas vesicles in aqueous suspension will also survive a rapid rise in the overlying gas pressure in excess of their critical pressure if they are near enough to the gas-water interface for sufficient gas to reach them by diffusion during the pressure rise. The distance from the interface at which the gas vesicles survive can be used to calculate the diffusivity of the gas through the suspension. A modification of this method can be used to measure the gas-permeability of certain types of cells containing gas vesicles.

The elastic compressibility of gas vesicles

Theelastic compressibility of gas vesicles isolated from Anabaena flos-aquae has been measured with a specially constructed apparatus. The gas vesicle suspension was contained in a glass tube, closed at one end with a piston allowing volume adjustment and attached at the other end to a microcapillary, and was subjected to pressure from compressed air. The elastic compressibility of the gas vesicle suspension was determined by applying or removing pressure and measuring the ensuing displacement of the meniscus in the capillary with a vernier microscope. After allowing for the compressibility of the compression tube and of water in the suspension, the compressibility of the intact gas vesicles has been calculated to be 0.00155 bar -1 , and the elastic bulk modulus 645 bar. The elastic modulus of the protein that forms the gas vesicle wall can also be calculated from these measurements; it is 27500 bar. These measurements confirm that the gas vesicle is a rigid structure and show that the buoyancy provided by them will be relatively unaffected by pressures that do not actually cause gas vesicle collapse. The apparatus described can also be used to provide a direct measure­ment of the volume of gas vesicle gas space present in a suspension of a gas-vacuolate organism, and to investigate the gas vesicle critical collapse pressure. Gas vesicles appear to collapse by instability failure but the pressure at which this occurs, about 6 bar, is higher than would be predicted from knowledge of the dimensions and elastic modulus of the gas vesicle wall. This supports the idea that the orientation of the ribs, which form the structure, provides ring-stiffening support.