Polymer Partitioning from Nonideal Solutions into Protein Voids

Abstract

Using the change in conductance of a single nanometer-wide protein pore of the α-hemolysin channel to detect pore occupancy by polymers, we measure the equilibrium partitioning of differently sized poly(ethylene glycol)s as a function of polymer concentration in the bulk solution. In the semidilute regime, increased polymer concentration results in a sharp increase in polymer partitioning. Quantifying solution nonideality by osmotic pressure and taking the free energy of polymer confinement by the pore at infinite dilution as an adjustable parameter allows us to describe polymer partitioning only at low polymer concentrations. At larger concentrations the increase in partitioning is much sharper than the model predictions. The nature of this sharp transition between strong exclusion and strong partitioning might be rationalized within the concepts of scaling theory predicting this kind of behavior whenever the correlation length of the monomer density in the semidilute bulk solution becomes smaller than the pore radius. Specific attractive interactions between the protein pore and the polymer that exist in addition to the entropic repulsion accounted for in the present study may also play a role.

Size-dependent forced PEG partitioning into channels: VDAC, OmpC, and α-hemolysin

“…The usual treatment is that the ion channel is a nanosize cylindrical cuvette filled with electrolyte solution, and thus total channel resistance is the integral resistance of the solution in the channel (13). The effect of polymer on channel conductance is then related to the effect of polymer on solution conductivity.…”

“…The fits for β are within 10% of what we obtain from the ion selective electrode measurements. Due to its small size, PEG200 is usually assumed to equipartition between the channel and the bathing solution (13). We refrain from adopting that assumption a priori and make an effort to deduce it from our experiments.…”

“…The partitioning of nonionic polymers such as PEG into α-hemolysin (aHL) from Staphylococcus aureus has been previously studied and shown to be size-dependent at relatively low salt concentrations (8)(9)(10)(11)(12)(13). In a different way, namely, as the amplitude of channel blockage, polymer size dependency has also been observed in single-molecule studies at high salt concentrations (4 M KCl) with aHL (14) and recently with aerolysin from Aeromonas hydrophila (15), and was shown to exhibit pronounced size sensitivity with resolution in the submonomer range.…”

“…We studied passive size-dependent partitioning and size discrimination that can be manipulated to force polymers into nanosize pores under strong nonideality, when polymer partitioning is qualitatively modified by polymer-polymer repulsion that allows polymers, which are excluded in dilute solutions, to enter the channel pore (13). This concentration-dependent partitioning was rationalized by an argument that the overlap concentration of the polymer in the pore is higher than that in the bath (16,17).…”

Probing single nanometer-scale pores with polymeric molecular rulers

“…In addition, Krasilnikov and others used PEGs to estimate the radii of ion channels from the ability of size-selected PEGs to partition into the channel pore. 11,[19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] Protein translocation across membranes, which plays a key role in many biological processes, can be facilitated by ion channels. [37][38][39] Other biomolecules can be detected as they transport through ion channels.…”