1.D.64 The Carbon Nanotube (CarNT) Family
Carbon nanotubes (CarNTs; CNTs), small segments of carbon nanotubes capable of forming defined pores in lipid membranes, are important components for bionanoelectronic devices as they can provide a robust analog of biological membrane channels. They can transport, for example water and protons. In order to control the incorporation of these CarNT channels into lipid bilayers, Tran et al. 2016; employed a noninvasive in situ probe, small-angle X-ray scattering, to study the integration of CarNTs into dioleoylphosphatidylcholine bilayers. They showed that CarNTs in solution are stabilized by a monolayer of lipid molecules wrapped around their outer surface. They also showed that insertion of CarNTs into the lipid bilayer results in decreased bilayer thickness with the magnitude of this effect increasing with the concentration of CarNTs. CarNTs have been used to treat brain tumors and degenerative diseases because of their abilities to cross the blood-brain barrier (Kafa et al. 2016). Single-walled carbon nanotubes are used in the near infrared-mediated thermal ablation of tumor cells because they efficiently convert absorbed readiation into heat (Murali et al. 2016). Subcellular locations determine the efficacy of thermal ablation. For example, incorporation into the plasma membrane is not as effective as an equivalent amount internalized within endosomal/lysosomal vesicles.
Carbon nanotube porins (CarNTPs) are 10- to 20-nm-long segments of lipid-stabilized single-walled carbon nanotubes that insert into phospholipid membranes to form nanometer-scale-diameter pores that approximate the geometry and many key transport characteristics of biological membrane channels. Tunuguntla et al. 2016 described protocols for CarNTP synthesis by ultrasound-assisted cutting of long CarNTs in the presence of lipid amphiphiles, and for validation of CarNTP incorporation into a lipid membrane using a proton permeability assay. They also described protocols for measuring conductance of individual CarNTPs in planar lipid bilayers and plasma membranes of live cells. These CarNTPs remain stable and active for at least 10-12 weeks. Chen et al. 2018 integrated silicon nanoribbon transistor sensors with an antifouling lipid bilayer coating that contains proton-permeable carbon nanotube porin (CNTP) channels and demonstrated robust pH detection in a variety of complex biological fluids.
Membrane-spanning CarNTs can trigger spontaneous fusion of small lipid vesicles (Bhaskara et al. 2017). In coarse-grained molecular dynamics simulations, CarNT bridging between two vesicles locally perturbs their lipid structure. Their outer leaflets merge as the CarNT pulls lipids out of the membranes, creating an hourglass-shaped fusion intermediate with intact inner leaflets. As the CarNT moves away from the symmetry axis connecting the vesicle centers, the inner leaflets merge, forming a pore that completes fusion (Bhaskara et al. 2017).
CarNTs form pores that can transport water (and other molecules such as protons) by continuously filling the nonpolar carbon nanotube with a one-dimensionally ordered chain of water molecules. Tight hydrogen-bonding networks inside the tube, ensures that density fluctuations in the surrounding bath lead to concerted and rapid motion along the tube axis (Hummer et al. 2001). A small reduction in the attraction between the tube wall and water dramatically affects pore hydration, leading to sharp, two-state transitions between empty and filled states on a nanosecond timescale. The water-water interactions, determined by dipole orientation configurations, influence the transport rate (Zuo et al. 2010).
Carbon nanotubes represent examples of nanofluidic channels that combines extremely small diameters with atomically smooth walls and well-defined chemical functionalities at the pore entrance. Sub-1 nm diameter carbon nanotube porins (CNTPs) embedded in a lipid membrane matrix demonstrate extremely high water permeabilities and strong ion selectivities. Tunuguntla et al. 2018 examined chemical modifications of the CNT rims and chaotropic polyethyleneglycol (PEG) additives on CNTP water permeability and Arrhenius activation energy barriers for water transport. They showed that PEG, especially in its more chaotropic coiled configuration, enhances water transport by reducing the associated activation energy. Removal of the static charges on the CNTP rim by converting -COOH groups to neutral methylamide groups also reduces the activation energy barriers and enhances water transport rates.
Artificial channels made of carbon nanotube (CNT) porins have symmetric shape and high mechanical, chemical, and thermal stability that ensure well-defined transport properties, and at the same time, make them ideal model systems for more complicated membrane protein pores. Vögele et al. 2018 found that the interaction with lipid acyl chains is independent of the CNT chirality and the chemical details of functional groups at the CNT rims. The latter, however, are important for the interactions with lipid head groups, and for water permeability. The orientation and permeability of the pore are mainly determined by how well its hydrophobicity pattern matches the membrane.