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Principles of High-Performance Ion-Exchange Chromatography

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Principles of High-Performance Ion-Exchange Chromatography PrinciplesPrinciples andand ApplicationsApplications ofof High-PerformanceHigh-Performance Ion-ExchangeIon-Exchange ChromatographyChromatography forfor BioseparationsBioseparations Introducing Vydac VHP High Performance Ion-Exchange Columns Principles of High-Performance Ion-Exchange Chromatography Why use ion-exchange chromatography to purify proteins? Ion-Exchange Ion-exchange chromatography separates Chromatography of proteins by charge under near physio- Polypeptides logical and non-denaturing conditions and ion-exchange resins generally have Figure 1. Protein anion exchange Ion-exchange chromatography a high loading capacity. Polymeric separates proteins by charge primarily O based ion-exchange resins are very through electrostatic interactions C CH NH robust. They are stable in strong acid or between charged amino acid side chains base and are resistant to urea and CH NH C CH and the surface charge of the ion- guanidine-HCl. Ion-exchange chroma- O exchange resin. CH2 CH2 tography is an excellent complement to Protein retention has been explained as a such high resolution techniques as C O CH O 2 "net charge" phenomenon in which a reversed-phase chromatography (see C O protein is considered to be a point Reference 3 and Page 14 of this aspartic acid O charge and retention is a function of the publication) pK = 3.9 net charge of the protein at the pH of the glutamic acid mobile phase. Kopaciewicz and col- pK = 4.3 What is the effect of leagues, however, have developed a more comprehensive mechanism of the mobile phase pH? ion-exchange chromatography of Anion-exchange chromatography polypeptides by showing that significant primarily retains biomolecules by the retention of proteins often occurs at the interaction of amine groups on the ion- Figure 2. Protein cation exchange pI, where the net charge is zero, and that exchange resin with aspartic or glutamic O O the correlation between net acid sidechains, which have pKs of ~ 4.4 C CH NH C CH NH charge and protein retention is (Figure 1). The mobile phase is buffered CH C CH NH C CH often poor (Reference 1). They (see table of recommended buffers) at O O showed that charge assymetry pH > 4.4, below which acid sidechains CH2 CH2 CH2 better explains protein retention begin to protonate and retention decreas- CH2 CH2 CH CH in ion-exchange chromatogra- es. Above pH 4.4 retention is largely HN + NH phy and that this accounts for dependent on the number of anionic CH2 CH2 C the fact that protein tertiary sidechains present in the protein. CH2 NH H structure affects retention in Proteins containing the same number of + + NH3 C NH ion-exchange chromatography. anionic sidechains can often be separat- 2 histidine For instance, structural isomers ed by adjustment of the mobile phase NH pK = 6.0 lysine 2 with identical pI's sometimes pH between 7 and 10 where histidine is pK = 10.5 arginine can be separated by ion- not protonated and lysine begins to de- pK = 12.5 exchange chromatography. protonate. Subtle changes occur to proteins in this pH region which affect the The pK's are given as the pK of the free amino acid. The actual pK of an Types of Ion-Exchange Resins amino acid sidechain inside a protein Type Functional group Common Term Vydac Column depends on the microenvironment of the protein where it is found. Cation exchange sulfonic acid S 400VHP Series Anion exchange quaternary amine Q 300VHP Series Anion exchange tertiary amine DEAE 301VHP Series Contents Pages 1-2 Basic principles of ion-exchange Page 9 Oligonucleotides (including phosphorothioates) chromatography in bioseparations Page 10 Hemoglobins Pages 3-4 VHP ion-exchange columns Page 11 Antibodies Page 5 Cation exchange: 400VHP Page 12 Complementary techniques: reversed phase Page 6 Anion exchange: 300VHP and 301VHP Page 13 Technical and product information Pages 7-8 Influence of pH on ion-exchange separations Page 14 Ordering information Page 1 VHP High-Performance Ion-Exchange Columns interaction of the protein with the resin acid are partially protonated. Subtle How do hydrophobic and which allow fine-tuning of the changes occur to proteins in this pH anion-exchange separation. A mobile region which affect the interaction of the interactions affect ion phase pH > 10 is not generally recom- protein with the resin and allow fine- exchange separations? mended because of possible protein tuning of the ion-exchange separation. Ion-exchange resins with hydrophobic degradation, such as deamidation, at Proteins differing in a single sidechain - character may result in multiple-mode higher pH's. Examples of adjusting pH for instance, aspartic acid in one versus separations. Multiple mode separations and gradient conditions to optimize isoaspartate in the other - can sometimes are sometimes beneficial but are more separations are shown in this publication be separated by careful adjustment of likely to be complicating or detrimental on pages 7, 8, 9 and 11. the mobile phase around pH 4. Exam- to ion-exchange chromatography. In Cation-exchange chromatography ples of adjusting pH and salt gradient addition hydrophobic adsorption may retains biomolecules by the interaction conditions to optimize separations are lead to reduced recovery, band broaden- of sulfonic acid groups on the surface of shown in this publication on pages 7, 8, ing and/or protein denaturation. Many the ion-exchange resin with histidine 9 and 11. proteins, such as bovine serum albumin (pK ~ 6.5), lysine (pk ~ 10) and arginine and ovalbumin, are particularly sensitive (pK ~ 12) (Figure 2). The mobile phase What is the effect of to ion-exchange resins with hydrophobic is buffered (see table of recommended displacing ions? character. To minimize hydrophobic buffers) to maintain the mobile phase adsorption and avoid mixed-mode below pH 6 or 7 in order to keep the Polypeptide separations are somewhat separations in protein ion-exchange basic sidechains protonated (Table 1). dependent on the displacer ion. Al- chromatography, Vydac developed the At higher pH the basic sidechains begin though sodium and choride are the most VHP matrix by modifying PS-DVB to deprotonate and retention decreases. common displacer ions, differences in beads with a hydrophilic surface (see Below pH 6 retention is dependent on retention and selectivity for some Page 3). the number of basic amino acids present polypeptides have been noted for in the protein. various organic and inorganic anions in anion-exchange chromatography and What is the effect of Proteins with the same number of basic various alkali and alkaline earth metals temperature? amino acids can often be separated by for cation-exchange chromatography Temperature affects ion-exchange adjusting the mobile phase pH between (see Ref. 1 and 2). 3 and 5 where aspartic acid and glutamic chromatography separations through its effect on the structure of the protein. Although temperature does not affect Recommended Buffers for Polypeptide Ion-Exchange the electrostatic interaction, it often Chromatography affects the structure of a protein and therefore the interaction of the protein A wide range of buffers are available for use with ion-exchange chromatography. with the ion-exchange resin. Subtle Recommended buffers for various ranges of pH are listed below. variations in selectivity with temperature Anion-Exchange Chromatography Buffers may result from temperature induced changes in protein structure. Buffers for anion exchange are generally basic amines. Buffer Concentration Anion pKa Buffering Region References L-histidine 20 mM Cl- 6.15 5.5 - 6.8 1. Retention Model for High-Perfor- bis-Tris 20 mM Cl- 6.50 5.8 - 7.0 mance Ion-Exchange bis-Tris propane 20 mM Cl- 6.80 6.4 - 7.3 Chromatography, W. Kopaciewicz, Triethanolamine 20 mM Cl- 7.77 7.3 - 8.2 M.A. Rounds, J. Fausnaugh and F.E. Tris 20 mM Cl- 8.16 7.5 - 8.8 Regnier, J. Chrom. 266, 3-21 (1983) diethanolamine 20 mM Cl- 8.88 8.4 - 9.4 2. Characterization of the influence of displacing salts on retention in Cation Exchange Chromatography Buffers gradient elution ion-exchange chro- Buffers for cation-exchange chromatography are acids. matography of proteins and peptides, G. Malmquist and N. Lundell, J. Chrom. Buffer Concentration Cation pKa Buffering Region 627, 107-124 (1992) formate 20 mM Na+ 3.75 3.3 - 4.3 3. Two-Dimensional Liquid Chroma- acetate 20 mM Na+ 4.76 4.2 - 5.2 tography of Peptides: An MES 20 mM Na+ 6.15 5.5 - 6.7 Optimization Strategy, N. Lundell and phosphate 20 mM Na+ 2.1/7.2 2.0 - 7.6 K. Markides, Chromatographia 34, 369- 375 (1992) HEPES 20 mM Na+ 7.55 7.6 - 8.2 VHP High-Performance Ion-Exchange Columns Page 2 Vydac VHP Ion-Exchange Columns Vydac VHP Ion-exchange columns are high-performance columns for the ion-exchange separation and purification of polypeptides and polynucleotides. VHP ion-exchange columns combine spherical polystyrene-divinylbenzene (PS-DVB) copolymer beads with a robust hydrophilic surface and stable derivatization chemistry to produce three types of high perfor- mance ion-exchange resins for bioseparations. ■ 400VHP sulfonic acid (strong - 'S' type) cation exchange (Page 5) ■ 300VHP quaternary amine (strong - 'Q' type) anion exchange (Page 6) ■ 301VHP tertiary amine (moderate - 'DEAE' type) anion exchange (Page 6) VHP Ion-exchange columns VHP Ion-exchange columns offer offer superior resolution superior column efficiency The selectivity resulting from Vydac's unique Vydac VHP Ion-exchange columns are the first surface chemistry is illustrated by the separation truly high-performance ion-exchange columns of small impurities in a lysozyme sample. for the separation of proteins and polypeptides. ■ The Vydac 400VHP (strong cation-exchange) Five or eight micron beads and excellent surface column separates three impurities from chemistry combine to offer higher separation lysozyme efficiencies than obtained with traditional protein ■ .... versus only one impurity on the traditional ion-exchange columns.
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