COUNTING POLYMERS MOVING THROUGH A SINGLE-ION CHANNEL

THE change in conductance of a small electrolyte-filled capillary owin g to the passage of sub-micrometre-sized particles has long been used for particle counting and sizing. A commercial device for such measure ments, the Coulter counter, is able to detect particles of sizes down to several tenths of a micrometre(1-3). Nuclepore technology (in which pores are etched particle tracks) has extended the lower limit of siz e detection to 60-nm particles by using a capillary of diameter 0.45 m u m (ref. 4). Here we show that natural channel-forming peptides incor porated into a bilayer lipid membrane can be used to detect the passag e of single molecules with gyration radii as small as 5-15 Angstrom. F rom our experiments with alamethicin pores we infer both the average n umber and the diffusion coefficients of poly(ethylene glycol) molecule s in the pore. Our approach provides a means of observing the statisti cs and mechanics of flexible polymers moving within the confines of pr ecisely defined single-molecule structures.