The first direct observation of silent translocation events - Burden Lab

Researchers have long suspected certain molecules can pass through ion channels and nanopores without generating characteristic current signatures. But, detecting electrically silent transport has remained elusive.
Investigators from the Chemistry Department at Wheaton College reported the first direct observation of silent translocation events.
Assisted by microchip technology and electronics from Nanion and Ionera, investigators from the Burden lab used a specialized optical microscope to measure light and electrical patterns simultaneously, as fluorescent molecules moved through a nanopore.
Repeated bursts of fluorescence from single molecules in a focused laser were observed in proportion to the translocation rate. However, when a voltage was applied across the membrane, corresponding electrical patterns were absent, confirming scientists’ beliefs.
The Wheaton team also found that the silent translocation rate changes with voltage. A version of Arrhenius theory, which describes energy barriers inherent to many kinetic molecular processes, explains the result.
Diffusion pushes molecules in the direction of the concentration gradient. With a positive potential below the membrane, electrophoretic and electroosmotic forces push in the same direction, making transport through the pore rapid.
But as the applied voltage drops, so does the electrophoretic and electroosmotic forces. Thus, the silent translocation rate declines. At zero potential, electrophoretic and electroosmotic forces vanish completely. Diffusion alone pushes molecules with the correct orientation past the nanopore barrier.
At the slightest negative potential below the membrane, the direction of electrophoretic and electroosmotic forces flips to oppose net diffusion. This creates a sudden drop in the translocation rate.
At large negative potentials, translocation decreases exponentially, as electrophoretic and electroosmotic forces grow in opposition to diffusion. Diffusion can occasionally overcome, but the number of molecules with proper orientation and velocity to translocate grows vanishingly small. Using these trends with applied voltage, researchers determined the size of the Arrhenius energy barrier for silent translocation to be ~8kT.