1.D.64 The Carbon Nanotube (CarNT) Family
Carbon nanotubes (CarNTs), small segments of carbon nanotubes capable of forming defined pores in lipid membranes, are important future components for bionanoelectronic devices as they can provide a robust analog of biological membrane channels. 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 demonstrated 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.
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).