How does a supercoiled DNA chain pass through a small conical glass pore?
We investigated the captioned question by a resistive pulse technique, in which a constant electrical potential was applied inside and outside of a conical glass capillary with its tip opening diameter (d) down to ∼10 nm. The insertion of a chain segment into the tip decreases the current (Ip). Our studies on transport dynamics of individual supercoiled DNA chains through such a tip with different openings at various potentials (V) reveal that when d ≈ 35 nm, they can pass through the conical glass capillary without much stretching; namely, the current pulse has a typical triangle shape and its half-height duration time (Δtd,1/2) decreases, but its occurring frequency (f) increases exponentially, as V increases. For a smaller tip opening (d ≈ 14 nm), we found that f strangely increases and then decreases with increasing V. In addition, the current pulse is significantly skewed with a long tail and its minimum occurs when V ≈ 200 mV. Typically, each current pulse is composed of four steps as follows: (1) Ip sharply decreases from its baseline (Io) when a DNA chain approaches the tip opening and inserts a segment to block the pore; (2) the inserted segment is stretched under the electric field gradient so that the pore is less blocked, resulting in a slight current increase; (3) then Ip slightly decreases once more, indicating that the pulling of the inserted segment is faster than the relaxation (unwinding) of the rest of the chain outside so that it clogs at the tip entrance; and (4) Ip gradually increases and finally returns to Io because more and more segments are gradually pulled in by the electrical field until the entire chain slips through the conical glass capillary. These steps can be well explained in terms of a big difference between times of pulling the first segment inside the tip and relaxing the rest of the segments outside.