Counting polymers moving through a single ion channel

THE change in conductance of a small electrolyte-filled capillary owing to the passage of sub-micrometre-sized particles has long been used for particle counting and sizing. A commercial device for such measurements, the Coulter counter, is able to detect particles of sizes down to several tenths of a micrometre1–3. Nuclepore technology (in which pores are etched particle tracks) has extended the lower limit of size detection to 60-nm particles by using a capillary of diameter 0.45 μm (ref. 4). Here we show that natural channel-forming peptides incorporated into a bilayer lipid membrane can be used to detect the passage of single molecules with gyration radii as small as 5–15 Å. From our experiments with alamethicin pores we infer both the average number and the diffusion coefficients of poly(ethylene glycol) molecules in the pore. Our approach provides a means of observing the statistics and mechanics of flexible polymers moving within the confines of precisely defined single-molecule structures.

[1]  F J Sigworth,et al.  Estimation of Na+ dwell time in the gramicidin A channel. Na+ ions as blockers of H+ currents. , 1989, Biochimica et biophysica acta.

[2]  T. Allen Particle size analysis , 1979 .

[3]  Howard G. Barth,et al.  Modern methods of particle size analysis , 1984 .

[4]  G. Feher,et al.  Fluctuation spectroscopy: determination of chemical reaction kinetics from the frequency spectrum of fluctuations. , 1973, Proceedings of the National Academy of Sciences of the United States of America.

[5]  G. Blobel,et al.  A protein-conducting channel in the endoplasmic reticulum , 1991, Cell.

[6]  M Montal,et al.  Formation of bimolecular membranes from lipid monolayers and a study of their electrical properties. , 1972, Proceedings of the National Academy of Sciences of the United States of America.

[7]  V A Parsegian,et al.  Probing alamethicin channels with water-soluble polymers. Size-modulated osmotic action. , 1993, Biophysical journal.

[8]  H. E. Kubitschek,et al.  Electronic Counting and Sizing of Bacteria , 1958, Nature.

[9]  R. Tsien,et al.  Mechanism of ion permeation through calcium channels , 1984, Nature.

[10]  Shigenoki Kuga Pore size distribution analysis of gel substances by size exclusion chromatography , 1981 .

[11]  R. Stepto,et al.  Diffusion of low-molecular weight poly(ethylene oxide) in water , 1969 .

[12]  S. Bezrukov,et al.  Probing alamethicin channels with water-soluble polymers. Effect on conductance of channel states. , 1993, Biophysical journal.

[13]  T. M. Balasubramanian,et al.  Alamethicin. A rich model for channel behavior. , 1984, Biophysical journal.

[14]  O. Krasilnikov,et al.  A simple method for the determination of the pore radius of ion channels in planar lipid bilayer membranes. , 1992, FEMS microbiology immunology.

[15]  C. P. Bean,et al.  Electrokinetic measurements with submicron particles and pores by the resistive pulse technique , 1977 .

[16]  D. Haydon,et al.  Potential-dependent conductances in lipid membranes containing alamethicin. , 1975, Philosophical transactions of the Royal Society of London. Series B, Biological sciences.