Single polymer molecules in a protein nanopore in the limit of a strong polymer-pore attraction.

The capture and release of single poly(ethylene glycol) molecules by the alpha-Hemolysin pore are observed as time-resolved reversible steps in ion conductance. The capture on rate, inferred from the step frequency, decreases monotonically with polymer size. However, the polymer residence time shows a crossover behavior, first increasing and then decreasing with molecular weight. Our interpretation is that, in the case of polymers which are too large to be accommodated within the pore, the out-of-the-pore part of the molecule pulls on the trapped part, thus acting as an entropic spring.