Abstract
Peptide-appended Pillar[5]arene (PAP) is an artificial water channel that can be incorporated into lipid and polymeric membranes to achieve high permeability and enhanced selectivity for angstrom-scale separations [Shen et al. Nat. Commun. 9:2294 (2018)]. In comparison to commonly studied rigid carbon nanotubes, PAP channels are conformationally flexible, yet these channels allow a high water permeability [Y. Liu and H. Vashisth Phys. Chem. Chem. Phys. 21:22711 (2019)]. Using molecular dynamics (MD) simulations, we study water dynamics in PAP channels embedded in biological (lipid) and biomimetic (block-copolymer) membranes to probe the effect of the membrane environment on water transport characteristics of PAP channels. We have resolved the free energy surface and local minima for water diffusion within the channel in each type of membrane. We find that water follows single file transport with low free-energy barriers in regions surroundings the central ring of the PAP channel and the single file diffusivity of water correlates with the number of hydrogen bonding sites within the channel, as is known for other sub-nm pore-size synthetic and biological water channels [Horner et al. Sci. Adv. 1:e1400083 (2015)].
Highlights
Water transport in sub-nm size pores occurs in various structures including carbon nanotubes (CNTs), derivatives of imidazoles and pillararenes, and biological channel proteins (Corry, 2008; Majumder et al, 2005; Tunuguntla et al, 2017; Licsandru et al, 2016; Cheruzel et al, 2003; Hu et al, 2012; Shen et al, 2015; Barboiu and Gilles, 2013; Si et al, 2011; Shen et al, 2015; Grzelakowski et al, 2009; Kumar et al, 2007)
We study the dynamics of water molecules within peptide appended pillar[5]arene (PAP) and map the free energy surface of water transport in PAP for different membrane environments, thereby quantifying the effect of membrane on water structure and dynamics, especially single-file water transport
Our results collectively show that the water structure and dynamics in the rigid structural parts of the PAP channel are consistent with the single file water transport with low free energy barriers, as observed in several other narrow pores including CNTs, while the water structure near the flexible peptide arms is affected by unique conformational dynamics of the channel in lipid and polymeric membranes
Summary
Water transport in sub-nm size pores occurs in various structures including carbon nanotubes (CNTs), derivatives of imidazoles and pillararenes, and biological channel proteins (Corry, 2008; Majumder et al, 2005; Tunuguntla et al, 2017; Licsandru et al, 2016; Cheruzel et al, 2003; Hu et al, 2012; Shen et al, 2015; Barboiu and Gilles, 2013; Si et al, 2011; Shen et al, 2015; Grzelakowski et al, 2009; Kumar et al, 2007). Water Dynamics in Artificial Channels structures, synthetic flexible channels based upon pillararene derivatives, in particular peptide appended pillar[5]arene (PAP) water channels, are emerging as novel biomimetic pores (Hu et al, 2012; Shen et al, 2015) to study the interplay of water transport and conformational flexibility of the channel in different membrane environments. We have previously reported that PAP can be successfully inserted into lipid and block-copolymer (BCP) based membranes with the channel permeability between ∼108–109 water molecules s−1 (Shen et al, 2018; Barden and Vashisth, 2018). We study the dynamics of water molecules within PAP and map the free energy surface of water transport in PAP for different membrane environments, thereby quantifying the effect of membrane on water structure and dynamics, especially single-file water transport
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