Highly permeable polymeric membranes based on the incorporation of the functional water channel protein Aquaporin Z

Abstract

The permeability and solute transport characteristics of amphiphilic triblock-polymer vesicles containing the bacterial water-channel protein Aquaporin Z (AqpZ) were investigated. The vesicles were made of a block copolymer with symmetric poly-(2-methyloxazoline)-poly-(dimethylsiloxane)-poly-(2-methyloxazoline) (PMOXA(15)-PDMS(110)-PMOXA(15)) repeat units. Light-scattering measurements on pure polymer vesicles subject to an outwardly directed salt gradient in a stopped-flow apparatus indicated that the polymer vesicles were highly impermeable. However, a large enhancement in water productivity (permeability per unit driving force) of up to approximately 800 times that of pure polymer was observed when AqpZ was incorporated. The activation energy (E(a)) of water transport for the protein-polymer vesicles (3.4 kcal/mol) corresponded to that reported for water-channel-mediated water transport in lipid membranes. The solute reflection coefficients of glucose, glycerol, salt, and urea were also calculated, and indicated that these solutes are completely rejected. The productivity of AqpZ-incorporated polymer membranes was at least an order of magnitude larger than values for existing salt-rejecting polymeric membranes. The approach followed here may lead to more productive and sustainable water treatment membranes, whereas the variable levels of permeability obtained with different concentrations of AqpZ may provide a key property for drug delivery applications.

Fig. 1.

Examination of polymer vesicles by using microscopy. The cryogenic transmission electron microscope images were used for size determination because regular transmission electron microscopy and atomic force microscopy influence the structure of the observed vesicles. (a) Cryogenic transmission electron micrograph of an ABA polymer vesicle. (Scale bar: 200 nm.) (b) Electron micrograph of a cluster of vesicles. (Scale bar: 50 nm.) (c) Atomic force micrograph of vesicles on mica in nontapping mode shows that a film of polymer is formed on the hydrophilic mica surface with vesicles located in the film. (Scale bar: 200 nm.)

Fig. 2.

Stopped-flow light-scattering experiments. (a) Increase in relative light scattering with and without reconstituted AqpZ into the ABA polymer at 5.5°C at a molar ratio of 200:1 (polymer:protein). Fits are shown as guides. The initial rise rates were used to calculate the permeability. As seen from a, a rise cannot be calculated for the pure ABA polymer vesicles. (b) Shown is the rise in scattering between 2 and 10 s for the ABA vesicles that was used in calculating permeability at 5.5°C.

Fig. 3.

Arrhenius plots for calculation of activation energy for osmotic transport of water across polymer membranes with incorporated AqpZ (AqpZ-ABA) and pure polymer membranes (ABA).

Fig. 4.

Effect of increasing AqpZ concentrations on the permeability of ABA polymers. (a) Light-scattering results normalized to fit between 0 and 1. Curves for different ratios have been offset for clarity and fits are shown as guides. Data at 0 AqpZ:ABA ratio shown at a fixed offset. (b) Increase in calculated permeability with increase in protein-to-polymer molar ratio.

Fig. 5.

Comparison of reported permeability values for polymeric membranes to those obtained in this study. FO is a commercial forward-osmosis membrane with data from McCutcheon and Elimelech (37) at 20°C. RO is a commercial reverse-osmosis desalination membrane with data from Matsura (38) at room temperature (assumed 25°C). EE-EO is a polyethylethylene-polyethylene oxide diblock polymer with data from Discher et al. (13) at 20°C. ABA represents the polymer vesicles used in this study with permeability calculated at 20°C. AqpZ-ABA represents the polymer vesicles with incorporated AqpZ at 1:50 molar ratio used in this study at 20°C. Data for ABA and AqpZ were obtained at 5.5°C and calculated at 20°C by using E_a values.