Quantification of Water Flux in Vesicular Systems

Abstract

Water transport across lipid membranes is fundamental to all forms of life and plays a major role in health and disease. However, not only typical water facilitators like aquaporins facilitate water flux, but also transporters, ion channels or receptors represent potent water pathways. The efforts directed towards a mechanistic understanding of water conductivity determinants in transmembrane proteins, the development of water flow inhibitors, and the creation of biomimetic membranes with incorporated membrane proteins or artificial water channels depend on reliable and accurate ways of quantifying water permeabilities Pf. A conventional method is to subject vesicles to an osmotic gradient in a stopped-flow device: Fast recordings of scattered light intensity are converted into the time course of vesicle volume change. Even though an analytical solution accurately acquiring Pf from scattered light intensities exists, approximations potentially misjudging Pf by orders of magnitude are used. By means of computational and experimental data we point out that erroneous results such as that the single channel water permeability pf depends on the osmotic gradient are direct results of such approximations. Finally, we propose an empirical solution of which calculated permeability values closely match those calculated with the analytical solution in the relevant range of parameters.

Water-ion permselectivity of narrow-diameter carbon nanotubes

“…S4) where V r ( t ) is the relative volume of the vesicle at time t , P f is the osmotic water permeability coefficient, A / V 0 is the surface area–to–volume ratio of a vesicle, C in and C out are initial solute concentrations inside and outside the vesicle, respectively, and V W is the molar volume of water. We also compared the water permeability coefficients obtained with this procedure with the values determined with a different protocol ( 44 ) that computes permeabilities directly from the time constants and found that both approaches give permeability values that differ by less than 10% (with the second approach producing systematically higher values). The unitary water permeability P w of CNTPs was calculated using the following expression after subtracting the background vesicle permeability where A is the surface area of the CNTP-LUVs, and N is the average number of CNTPs in the vesicle.…”

Positively charged residues at the channel mouth boost single-file water flow

“…As previously described, we monitored the intensity of the scattered light at 90° at a wavelength of 546 nm. 28 The integral water permeability ( P f ) of the LUVs was obtained by fitting eqn (1) to the experimental data: 8 , 31 I ( t ) = a + b [ αV bare ( t ) + (1 – α ) V AQP ( t )] + d [ αV bare ( t ) + (1 – α ) V AQP ( t )] 2 where α in eqn (1) is the fraction of bare vesicles, which did not contain AQP4. V ( t ) depends on P f according to: 8 , 31 where V w , V 0 , A , c in,0 , c out , c Δ , and L are the molar volume of water, vesicle volume at time zero, surface area of the vesicle, the initial osmolyte concentration inside the vesicles, the outer and the incremental osmolyte concentration in the external solution due to sucrose addition, and the Lambert function L ( x )e L ( x ) = x , respectively.…”

Single-file transport of water through membrane channels

“…The approximate solutions (β) may lead to a large error in P f – more than an order of magnitude in size. 89 Moreover, P f erroneously seems to depend on the osmotic gradient. In contrast, scanning electrochemical microscopy 58 and fluorescence self-quenching experiments (analyzed using a numerical solution) 90 show that P f does not depend on the osmotic gradient.…”

“…Eqn (14) has been used to calculate the p f values of several AQPs, 22 , 37 hSGLT1, 36 and KcsA. 91 It may be substituted by the following approximation that ensures acceptable accuracy: 89 …”

“… 92 Consequently, two P f values and two V ( t ) distributions have to be considered. 22 , 89 Adapting the Rayleigh-Gans-Debye relation and comprising the change in size and refractive index of the lipid vesicles revealed a second order dependency with coefficients a, b, d : 22 …”