Preparation and characterization of prototypes for multi-modal separation media aimed for capture of negatively charged biomolecules at high salt conditions

Several prototypes of multi-modal ligands suitable for the capture of negatively charged proteins from high conductivity (28 mS/cm) mobile phases were coupled to Sepharose 6 Fast Flow. These new prototypes of multi-modal anion-exchangers were found by screening a diverse library of multi-modal ligands and selecting anion-exchangers resulting in elution of test proteins at high ionic strength. Candidates were then tested with respect to breakthrough capacity of BSA in a buffer adjusted to a high conductivity (20 mM Piperazine and 0.25 M NaCl, pH 6.0). The recovery of BSA was also tested with a salt step (from 0.25 to 2.0 M NaCl using 20 mM Piperazine as buffer, pH 6.0) or with a pH-step to pH 4.0. We have found that non-aromatic multi-modal anion-exchange ligands based on primary or secondary amines (or both) are optimal for the capture of proteins at high salt conditions. Furthermore, these new multi-modal anion-exchange ligands have been designed to take advantage not only of electrostatic but also hydrogen bond interactions. This has been accomplished through modification of the ligands by the introduction of hydroxyl groups in the proximity of the ionic group. Experimental evidence on the importance of the relative position of the hydroxyl groups on the ligand in order to improve the breakthrough capacity of BSA has been found. Compared to strong anion-exchangers such as Q Sepharose Fast Flow the new multi-modal weak anion-exchangers have breakthrough capacities of BSA at mobile phases of 28 mS/cm and pH 6.0 that are 20-30 times higher. The new multi-modal anion-exchangers can also be used at normal anion-exchange conditions and with either a salt step or a pH-step to acidic pH can accomplish the elution of proteins. In addition, the functional performance of the new anion-exchangers was found to be intact after treatment in 1.0 M sodium hydroxide solution for 1 week. A number of multi-modal anion-exchange ligands based on aromatic amines exhibiting high breakthrough capacity of BSA have been found. With these ligands recovery was often found to be low due to strong non-electrostatic interactions. However, for phenol derived anion-exchange media the recovery can be improved by desorption at high pH.     

Phase behavior of aqueous polyion-surfactant ion complex salts: effects of polyion charge density

The effect of varying the fraction of charged monomer units of the polyion in aqueous polyion-oppositely charged surfactant complex salts has been investigated. The complex salts used were based on cetyltrimethylammonium (C16TA+) with three different polymeric counterions: poly(acrylate) (PA-) or poly(acrylate) copolymerized with either dimethylacrylamide (PA-/DAM) or N-isopropylamide (PA-/NIPAM). The charge density of the polyion was varied by either adding poly(acrylic) acid (PAA) to the C16TAPA complex salt (annealed charges) or by varying the fraction of uncharged units in the C16TAPA/DAM or C16TAPA/NIPAM complex salts (quenched charges). The formed phases were studied visually between crossed polarizers and by small angle X-ray scattering (SAXS). Both types of complex salts (annealed and quenched) formed hexagonal phases at high fractions of charged monomers and low water contents. Upon increasing the water content, a cubic phase of the Pm3n space group was found. Upon further addition of water, a miscibility gap with the cubic phase in equilibrium with pure water was found. Decreasing the fraction of charged monomers in the annealed complex salt resulted in an increase of the curvature of the surfactant aggregates. Only at very low (<0.05) fractions of charged monomers did the packing of the surfactant aggregates lose long-range order, and eventually, the miscibility gap disappeared. For the quenched complex salts, the changes upon decreasing the fraction of charged monomers in the polyion were similar, but the loss of long-range order occurred at much higher fractions of charged monomers. The average surfactant aggregation number in the surfactant aggregates, which was similar for the annealed and quenched systems, decreased when the fraction of charged monomers was decreased.