Diffusion, Joule heating, and band broadening in capillary gel electrophoresis of DNA

We calculated the longitudinal and transverse diffusion coefficients for a DNA molecule undergoing gel electrophoresis in the limit where it is reptating through a dense polymer matrix. Our results indicate that both diffusion coefficients increase with the electric field intensity. The transverse and longitudinal diffusion coefficients are roughly equal for regular field intensities, but the former dominates for the high field intensities normally used for capillary gel electrophoresis (CE). This has important implications for the optimization of CE. Our results clearly show that the naive use of the zero-field diffusion constant, the Einstein relation or the longitudinal diffusion constant when calculating the contribution of the parabolic temperature profile to band broadening may lead to large overestimates under typical CE conditions. Finally, we show that the field-dependent diffusion coefficients may be responsible for the existence of an optimal field intensity for CE, even if Joule heating is neglected.