September 17, 2018 feature
A complex interplay between transport and dispersion across wiggling nanopores
Physicist Richard Feynman highlighted the importance of fluctuations in living matter when he stated, "Everything that living things do can be understood in terms of the jigglings and wigglings of atoms." This holds true for the widely investigated transport driven by fluctuations in biological nanopores and for similar observations in non-living fluid phases, where bulk hydrodynamic fluctuations dramatically affect nanoscale dynamics. Numerical simulations have also highlighted the impact of phonon modes in carbon nanotubes transporting confined particles within them, and studies at larger scales have investigated microfluidic surface acoustic waves that manipulate fluids at the microscale. While these observations show the quantitative impact of surface agitation on transport properties spanning considerable length scales, a general theory that predicts the dependence of transport properties on surface fluctuations is lacking. Such potential to actively or passively control molecular transport through nanopores will impact biosensing applications.
For instance, at present, it is surprisingly difficult to answer if pore surface fluctuations enhanced or diminished diffusive transport. While surface fluctuations are expected to enhance properties of diffusion through induced hydrodynamic flows, geometrical bulges may entrap particles for entropic slowing. As a result, the more general situation of transport with temporally varying geometry remains open. In response, writing in Nature Physics, Marbach et al. have now established a general relationship between diffusive transport and the dynamic spectrum of surface fluctuations. The framework applies to cases in which structural fluctuations of the confining pore are induced by thermal noise, and to active non-equilibrium fluctuations induced by external stimuli. The theory was applied to understand several situations relevant to nanopore transport and to larger-scale configurations such as active contractions in fungal species that influence nutrient transport.
The findings demonstrated a complex interplay between transport and surface wiggling. The theory was in full agreement with simulations of molecular dynamics and with existing observations from literature. The results illuminated the impact of pore agitation in a broad range of artificial and biological porins and at larger scales in vascular motion of fungi, intestinal contractions and microfluidic surface waves, opening the possibility to actively tune the transport across membranes via external stimuli. Such phenomena have potential applications for controlled nanoscale pumping, osmosis and dynamic ultrafiltration across membranes.
The theory began by analyzing diffusion of a particle confined between two fluctuating surfaces in a simple two-dimensional geometry, readily extendable to three dimensions. At first, the general theory applied to several scenarios, including fluctuations originating from thermal noise and active (non-equilibrium) fluctuations driven by external stimuli. In subsequent equations, the diffusion constant could be renormalized to be either positive or negative, since jiggling in the system could either speed things up or slow things down. Further scenarios were considered for situations in which the pore structure underwent non-equilibrium fluctuations due to an external stimulus by adding a force to an equation, leading to out-of-equilibrium transport within pores.
As an outcome of the theoretical framework, the physicists were able to quantify the impact of surface fluctuations on transport in the model systems considered in the study. Typically for high Péclet-like number (a dimensionless indicator of diffusion or advection in a system) the transport is expected to increase under structural jiggling through advection dominated distribution, as observed.
The transport of molecules through biological membranes is a tightly regulated process, absolutely vital for living organisms. The paradigm can therefore be used in an organism of interest to understand the dynamics of nutrient dispersal within contracting vascular networks, since alternative strategies for fast nutrient dispersal is essential for survival. For example, in the unicellular P. polycephalum organism composed of a connected network of veins containing cytoplasm, nutrients can be delivered across the body through non-stationary periodic shuttle flows driven by a peristaltic wave of contractions spanning the organism.
The study highlighted that a wide variety of situations covering a range of scales can be put in to perspective under the theorized framework. The theory will allow the identification of key components to design active channels. The results showed the possibility of actively tuning diffusion across nanopores. In addition, osmosis can be modified in fluctuating channels by harnessing out-of-equilibrium dynamics of the pore, for future exploration. Such fine-tuning capabilities will have significant applications in in-depth molecular studies in the future.
U. F. Keyser. Controlling molecular transport through nanopores, Journal of The Royal Society Interface (2011). DOI: 10.1098/rsif.2011.0222
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