Abstract

Continued advance of a new temperature-controlled chromatography system, comprising a column filled with thermoresponsive stationary phase and a travelling cooling zone reactor (TCZR), is described. Nine copolymer grafted thermoresponsive cation exchangers (thermoCEX) with different balances of thermoresponsive (N-isopropylacrylamide), hydrophobic (N-tert-butylacrylamide) and negatively charged (acrylic acid) units were fashioned from three cross-linked agarose media differing in particle size and pore dimensions.

Marked differences in grafted copolymer composition on finished supports were sourced to base matrix hydrophobicity. In batch binding tests with lactoferrin, maximum binding capacity (q_max) increased strongly as a function of charge introduced, but became increasingly independent of temperature, as the ability of the tethered copolymer networks to switch between extended and collapsed states was lost.

ThermoCEX formed from Sepharose CL-6B (A2), Superose 6 Prep Grade (B2) and Superose 12 Prep Grade (C1) under identical conditions displayed the best combination of thermoresponsiveness (q_max,50 °C/q_max,10 °C ratios of 3.3, 2.2 and 2.8 for supports ‘A2’, ‘B2’ and ‘C1’ respectively) and lactoferrin binding capacity (q_max,50 °C ∼ 56, 29 and 45 mg/g for supports ‘A2’, ‘B2’ and ‘C1’ respectively), and were selected for TCZR chromatography.

With the cooling zone in its parked position, thermoCEX filled columns were saturated with lactoferrin at a binding temperature of 35 °C, washed with equilibration buffer, before initiating the first of 8 or 12 consecutive movements of the cooling zone along the column at 0.1 mm/s. A reduction in particle diameter (A2 → B2) enhanced lactoferrin desorption, while one in pore diameter (B2 → C1) had the opposite effect. In subsequent TCZR experiments conducted with thermoCEX ‘B2’ columns continuously fed with lactoferrin or ‘lactoferrin + bovine serum albumin’ whilst simultaneously moving the cooling zone, lactoferrin was intermittently concentrated at regular intervals within the exiting flow as sharp uniformly sized peaks.

Halving the lactoferrin feed concentration to 0.5 mg/mL, slowed acquisition of steady state, but increased the average peak concentration factor from 7.9 to 9.2. Finally, continuous TCZR mediated separation of lactoferrin from bovine serum albumin was successfully demonstrated. While the latter's presence did not affect the time to reach steady state, the average lactoferrin mass per peak and concentration factor both fell (respectively from 30.7 to 21.4 mg and 7.9 to 6.3), and lactoferrin loss in the flowthrough between elution peaks increased (from 2.6 to 12.2 mg). Fouling of the thermoCEX matrix by lipids conveyed into the feed by serum albumin is tentatively proposed as responsible for the observed drops in lactoferrin binding and recovery.

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