Atypical mobilities of single native DNA molecules in microchip electrophoresis revealed by differential interference contrast microscopy.

A transmitted-light optical microscope using differential interference contrast (DIC) was employed to follow the real-time dynamics of different kb-sized single native dsDNA molecules without fluorescent-dye labeling. In a PDMS/glass microchip, the electrophoretic migration velocities of large dsDNA molecules are lower than small dsDNA molecules in a running buffer of 0.25% v/v nonionic polymeric surfactant C16E6 (n-alkyl polyoxyethylene ether) in 100 mM N-tris(hydroxymethyl)methyl-2-aminoethanesulfonic acid (TES) buffer because the C16E6 behaved as a dynamic polymer. However, the order of migration reversed in 50 mM Gly-Gly buffer. The radial positions of individual DNA molecules (i.e., center or walls of the microchip) did not change the migration order. The atypical migration order correlated well with the results in CE. The alignment of the deformable molecules due to viscous drag is likely responsible for these observations.