Preparative purification of peptides by reversed-phase chromatography. Sample displacement mode versus gradient elution mode

In this study, we describe a novel method of preparative liquid chromatography, applicable to analytical columns and instrumentation, where the reversed-phase column is operated in sample displacement mode (SDM). This technique takes advantage of the different relative hydrophobicities of components of a sample mixture, so that when a column is optimally loaded with an aqueous solution of the sample mixture, there is competition among the sample components for the adsorption sites on the hydrophobic stationary phase. The more hydrophobic components compete more successfully for these sites than more hydrophilic components, which are displaced and immediately eluted from the column. Thus, the major separation takes place in water. Subsequent treatment with an aqueous organic eluent is only required to wash retained components off the column and takes no part in the major separation process. A two-column (precolumn and main column, in series) SDM strategy was applied to the preparative purification of a peptide product from neighbouring hydrophilic and hydrophobic impurities. Hydrophobic impurities were isolated on a shorter precolumn trap while hydrophilic impurities were displaced from the main column in the aqueous mobile phase (0.05% aq. trifluoroacetic acid), leaving the main column filled with homogeneous product. Gradient elution was then used to remove the peptide product from the main column. The researcher can regulate the size of the precolumn trap depending on the amount of hydrophobic impurities in a particular sample, or the size of the main column depending on the amount of product desired. The simplicity and flexibility of the SDM approach to preparative-scale purification enabled rapid separation of a single peptide component from a complex multicomponent mixture and should prove to be valuable for researchers in the peptide/protein field.

High-performance liquid chromatography of amino acids, peptides and proteins. LXXXV. Evaluation of the use of hydrophobicity coefficients for the prediction of peptide elution profiles

The gradient elution behaviour of five synthetic decapeptide analogues has been investigated using an octadecylsilica stationary phase and trifluoroacetic acid-water-acetonitrile mobile phases. The influence of gradient time and flow-rate on the relative retentions and bandwidths of these peptides was assessed using quantitative expressions derived from linear solvent strength theory and general plate height theory. Linear relationships between logarithmic median capacity factors, log k, and the mole fraction of organic solvent modifier, phi, were observed over the experimental range of conditions used. The slopes of these plots were different for all peptides, which indicates that divergences will occur in the prediction of peptide retention times due to conformation dependent changes in hydrophobic contact area occupancy at the stationary phase surface. However, the differences in S values (tangent to the curve obtained in a plot of log k versus phi) for these peptides were not substantial enough to seriously affect the prediction of peptide retention times at one gradient slope from those observed at another. In addition, significant differences existed between experimental and theoretical peak capacity data of these peptide analogues of similar molecular weight and overall polarity, particularly at lower flow-rates or longer residence times. These results once again demonstrate that additional diffusional and interactive processes occur during the reversed-phase separation of peptides and proteins which are not yet adequately formalized by current chromatographic theory.

Size-exclusion high-performance liquid chromatography of peptides. Requirement for peptide standards to monitor column performance and non-ideal behaviour

A series of five synthetic peptide polymers with the sequence Ac-(G-L-G-A-K-G-A-G-V-G)n-amide, where n = 1-5, was employed to assess the resolving power of high-performance size-exclusion columns in peptide separations. The peptide standards showed great versatility in monitoring both ideal (no interactions of solutes with the column material) and non-ideal (hydrophobic and/or ionic interactions of solutes with the column material) size-exclusion behaviour in volatile and non-volatile mobile phases. The effectiveness of adding salts or organic solvents to overcome non-specific interactions of solutes with the column materials was well illustrated by the standards. In addition, the advantageous use of non-ideal size-exclusion behaviour was highlighted. The ability to predict the position and/or elution order of peptides during size-exclusion chromatography (SEC) requires peptides to be separated by a pure size-exclusion process. Although the peptide standards demonstrated similar ideal size-exclusion profiles in non-denaturing medium on all the columns studied, this study suggested that, if the conformational character of a peptide-protein mixture in a particular mobile phase is uncertain and ideal size-exclusion behaviour is required, SEC should be carried out under highly denaturing conditions.

Effects of ion-pairing reagents on the prediction of peptide retention in reversed-phase high-performance liquid chromatography

We have examined the resolution, on reversed-phase columns, of a series of model synthetic peptides and commercially available synthetic peptide standards under gradient elution conditions, using a water-acetonitrile mobile phase containing hydrophilic (phosphoric acid) or hydrophobic (trifluoroacetic acid, heptafluorobutyric acid) ion-pairing reagents. Increasing hydrophobicity or concentration of the ion-pairing reagents increased peptide retention times. It was clearly shown that these reagents effected changes in peptide retention time solely through interaction with the basic residues in the peptide. In general, each positive charge, whether originating from a lysine, arginine or histidine side-chain, or from an N-terminal alpha-amino group, exerts an equal effect on peptide retention. Different counterions have different effects on the change in peptide retention time per positively charged residue due to their differences in hydrophobicity. However, increasing concentrations of a specific counterion have an essentially equal effect per positively charged residue. These effects are also column dependent (n-alkyl chain length and ligand density). These results, demonstrating a simple relationship between peptide retention in different ion-pairing systems, enabled the determination of rules for prediction of peptide retention times in one ion-pairing system from observed or predicted retention times in another system. The small average deviation of predicted and observed retention times for a series of basic peptides was good evidence for the value of this predictive method. This study provides a clear understanding of the effect of changing counterion hydrophobicity or concentration on peptide retention, and thus can be extremely beneficial in the purification of peptides and for providing proof of peptide homogeneity.

Separation of peptides by strong cation-exchange high-performance liquid chromatography

The effects of pH and gradient conditions on the separation of a series of ten peptides (9-36 residues) and carboxamidomethylated troponin I (CM-TnI, 178 residues) on a new commercially available strong cation-exchange silica based 300-A column (Synchropak S300) were examined. The elution times of the peptides were linear with respect to their net charge at pH 3.0 and pH 6.5. The basic protein CM-TnI (pI approximately 9.5) and peptides with net charges from +2 to +10 were separated with linear AB salt gradients varying from 5 mM to 10 mM B per min (A = 5 mM KH2PO4 buffer, pH 6.5 or 3.0; B = 5 mM KH2PO4 buffer, pH 6.5 or 3.0, containing 1 M KCl). All peptides and CM-TnI were eluted with KCl concentrations below ca. 0.6 M. The advantage of performing cation-exchange chromatography over anion-exchange chromatography was demonstrated for the separation of peptides which, while acidic or weakly basic at neutral pH, through protonation of the acidic functions results in positively charged peptides at pH 3.0.

General method for the separation of cyanogen bromide digests of proteins by high-performance liquid chromatography. Rabbit skeletal troponin I

In this study, we have demonstrated the necessity for a combination of size-exclusion, ion-exchange and reversed-phase high-performance liquid-chromatography to resolve completely a protein digest. Our approach minimizes the number of steps, and the column order provides the maximum information about the properties of the fragments. The order is: (1) size-exclusion (Bio-Rad TSK-250 column), (2) strong cation-exchange (Synchropak S300 column), and finally (3) reversed-phase chromatography (Ultrapore C3). It was desirable for the first step of the procedure to be size-exclusion chromatography to produce the least number of fractions. The volatile eluent used in size-exclusion eliminated the need for subsequent sample desalting. Volatile buffers were not necessary for the ion-exchange chromatography, since the fractions were both desalted and purified in the final reversed-phase step. All column effluents were compatible with absorbance measurements at 210 nm to provide maximum sensitivity for peptide detection. The results obtained in this study strongly suggest that the combined use of three methods of separation, which utilize different selectivities (size, charge, hydrophobicity), can provide excellent resolving power for peptide separations. We believe this fast, efficient procedure should be generally applicable to other protein digests.