Membrane fouling during ultrafiltration of plasmid DNA through semipermeable membranes

• Significant decline in flux and plasmid transmission in concentrated solutions.
• Filtrate flux and plasmid sieving coefficient are essentially independent of stirring.
• Simple fouling model is developed based on partial blockage of membrane pores.
• Model calculations are in good agreement with flux and sieving data.
• Plasmid capture probability is consistent with knot formation in long polymers.

Although several recent studies have demonstrated the potential of using ultrafiltration for the purification of plasmid DNA, these data were limited to dilute solutions where membrane fouling was negligible. Ultrafiltration experiments were performed using 2.9 and 16.9 kbp (kilo base pair) supercoiled plasmids over a range of plasmid concentrations and filtration conditions using different molecular weight cutoff polyethersulfone ultrafiltration membranes. The flux and plasmid sieving coefficient were constant in dilute solutions but showed a significant decline in the more concentrated solutions. The flux decline was not due to concentration polarization effects; the filtrate flux and plasmid sieving coefficient were both essentially independent of the stirring speed. A simple fouling model was developed based on the partial blockage of the membrane pores by individual plasmids, with these partially blocked pores allowing filtrate flow but being inaccessible to subsequent DNA. The model calculations were in good agreement with the experimental data, with the plasmid capture probability increasing with increasing plasmid size. This behavior is consistent with independent results for the probability of knot formation in long plasmids. These results provide important insights into the fouling behavior of plasmid DNA during ultrafiltration through semipermeable membranes.

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