Characterization of hepatitis B virus capsids by resistive-pulse sensing.

We report characterization of hepatitis B virus (HBV) capsids by resistive-pulse sensing through single track-etched conical nanopores formed in poly(ethylene terephthalate) membranes. The pores were ∼40 nm in diameter at the tip, and the pore surface was covalently modified with triethylene glycol to reduce surface charge density, minimize adsorption of the virus capsids, and suppress electroosmotic flow in the pore. The HBV capsids were assembled in vitro from Cp149, the assembly domain of HBV capsid protein. Assembled T = 3 (90 Cp149 dimer) and T = 4 (120 dimer) capsids are 31 and 36 nm in diameter, respectively, and were easily discriminated by monitoring the change in current as capsids passed through an electrically biased pore. The ratio of the number of T = 3 to T = 4 capsids transiting a pore did not reflect actual concentrations, but favored transport of smaller T = 3 capsids. These results combined with longer transit times for the T = 4 capsids indicated that the capsids must overcome an entropic barrier to enter a pore.