Studies on the batch adsorption of plasmid DNA onto anion-exchange chromatographic supports

The adsorption of a supercoiled 4.8 kbp plasmid onto quaternary ammonium an ion-exchangers was studied in a finite bath. Equilibrium experiments were p erformed with pure plasmid, at 25 degrees C, using commercial Q-Sepharose m atrices differing in particle diameter (High Performance, 34 mu m; Fast Flo w, 90 mu m; and Big Beads, 200 mu m) and a recently commercialized ion-exch anger, Streamline QXL (d(p) = 200 mu m) at different salt concentrations (0 .5, 0.7, and 1M NaCl). Plasmid adsorption was found to follow second-order kinetics (Langmuir isotherm) with average association constants K-A = 0.32 +/- 0.12 mL mu g(-1) and K-A = 0.25 +/- 0.15 mL mu g(-1) at 0.5 and 0.7 M N aCl, respectively. The maximum binding capacities were not dependent on the ionic strength in the range 0.5-0.7 M but decreased with increasing partic le diameter, suggesting that adsorption mainly occurs at the surface of the particles. No adsorption was found at 1 M NaCl. A nonporous model was appl ied to describe the uptake rate of plasmid onto Streamline QXL at 0.5 M NaC l. The overall process rate was controlled by mass transfer in the regions of low relative amounts of adsorbent (initial stages) and kinetically contr olled in the later stages of the process for high relative amounts of adsor bent. The forward reaction rate constant (k(1) = 0.09 +/- 0.01 mL mg(-1) s( -1)) and film mass transfer coefficient (K-f = (6 +/- 2) x 10(-4) cm s(-1)) were calculated. Simulations were performed to study the effect of the rel ative amount of adsorbent on the overall process rate, yield, and media cap acity utilization.