Transport of Long Neutral Polymers in the Semidilute Regime through a Protein Nanopore

We investigate the entrance of single poly(ethylene glycol) chains into an $\alpha$-hemolysin channel. We detect the frequency and duration of the current blockades induced by large neutral polymers, where chain radius is larger than pore diameter. In the semidilute regime, these chains pass only if the monomer concentration is larger than a well-defined threshold. Experiments are performed in a very large domain of concentration and molecular mass, up to $35\%$ and 200 kDa, respectively, which was previously unexplored. The variation of the dwell time as a function of molecular mass shows that the chains are extracted from the semidilute solution in contact with the pore by a reptation mechanism.

Figure 1

Left: $\alpha$-hemolysin channel inserted into lipid bilayers in the presence of long polymers in a semidilute regime. $\xi$ is the mesh size of this solution. Right: Single-channel recordings of the pore in the presence of PEG 2 kDa $20\%$ ($w/v$) applied to both sides of the bilayer. The applied voltage is 100 mV.

Figure 2

Event frequency of long neutral polymers. Left: Histogram of the interevent time of a PEG 35 kDa solution ($33\%$). The event frequency ($75\pm 6\mathrm{Hz}$) is estimated from the exponential fit (dotted line). Right: Event frequency for the PEG 20 kDa [open (blue) circles] and 35 kDa [filled (red) circles] according to their mass concentration.

Figure 3

Dynamics of neutral chains as a function of molecular mass. Left: Histogram of the dwell time of PEG 35 kDa ($c=33\%$) and its integrated distribution. From the exponential fit (solid red curve), we estimate the dwell time to be $\tau =1250\pm 100\mu \mathrm{s}$. Right: Dwell time $\tau$ versus PEG molecular mass $\mathcal{M}$. $c\le 20\%$ for $\mathcal{M}<10\mathrm{kDa}$; otherwise, $c>c_{\mathrm{th}}$. Below: Reduced dwell time ($\frac{\tau }{c^{1.5}}$) versus PEG molecular mass. The dotted line corresponds to a power fit function ($\mathcal{M}\ge 20\mathrm{kDa}$). The error bars are deduced from the exponential fits.

Figure 4

Left: Single-channel current trace through an $\alpha$-hemolysin pore. We observe the dwell time of the 200 kDa [left-hand (blue) circle] and $-35\mathrm{kDa}$ [right-hand (red) circle] ($2\%–98\%$) mixture. Right: Normalized current blockade ($1-\frac{I_b}{I_0}$) as a function of molecular mass. The dotted lines correspond to the average values $0.61\pm 0.02$ [lower (blue) line] and $0.76\pm 0.01$ [upper (red) line].