High-performance liquid chromatography of amino acids, peptides and proteins

Kinetics and thermodynamics of rebaudioside A adsorption on a strongly acidic cation exchange resin

To explain the mechanism underlying the adsorption of stevia's polar component rebaudioside A in hydrophilic interaction liquid chromatography mode, the characteristics of rebaudioside A adsorption on various resins in an organic-solvent-rich system were studied. Among the tested resins, the strongly acidic cation resin FPC11 showed the best adsorption behavior for rebaudioside A. The factors affecting the adsorption kinetics of the resin for rebaudioside A are discussed. The results showed that the pseudo-second-order reaction model and intra-particle diffusion model best described the adsorption kinetics of rebaudioside A on the resin. The adsorption rate was controlled by physical sorption, mainly via electron sharing or electron transfer between the adsorbent and the adsorbate. The adsorption process with multiple stages involved weak initial adsorption behavior. Thermodynamic studies showed that the adsorption of rebaudioside A on the resin was not an ideal monolayer adsorption, but mutual adsorption effects between the adsorbates. The adsorption was a spontaneous, entropy-increasing endothermic process. The synergistic effect of hydrogen bonding and ion-dipole was a possible driving force.

Separation of peptides by HPLC using a surface-confined ionic liquid stationary phase

A butylimidazolium bromide surface-confined ionic liquid stationary phase was synthesized in-house. The synthesized phase was investigated for the separation of five peptides (Gly-Tyr, Val-Tyr-Val, leucine enkephalin, methionine enkephalin, and angiotensin-II). The peptides were successfully separated in less than 5 min. The effect of trifluoroacetic acid (TFA) on the separation of peptides was evaluated with results confirming that TFA was not acting as ion-pairing agent in separation of peptides on this phase.

Considerations on HILIC and polar organic solvent-based separations: use of cyclodextrin and macrocyclic glycopetide stationary phases

There is a natural tendency in science to prefer straightforward, logical classification systems. The use of mobile phase-stationary phase combinations that do not fit neatly into the standard "normal phase" or "reversed-phase" categories has been going on for over 50 years. The term "hydrophilic interaction chromatography" (HILIC) is sometimes being used as a general category for these "other" separations. In some cases, it may be appropriate and in others, not. Indeed the mechanistic constrains used to define the method seem to be varying with time. Given the name HILIC, it is assumed that water is not only present in the mobile phase, but also plays an essential role in the retention mechanism. However, there is residual water present in all organic solvents. Regardless, the number of reported separations in this alternative mode has increased tremendously in the last two decades. This is due to the advent of new stationary phases and an emphasis on polar, biologically important molecules. We discuss the relationships between HILIC and other chromatographic modes. We then examine two classes of stationary phases that have played a major role in these separations. These particular stationary phases can be used to provide appreciable mechanistic information as well.

Hydrophilic interaction chromatography

Separation of polar compounds on polar stationary phases with partly aqueous eluents is by no means a new separation mode in LC. The first HPLC applications were published more than 30 years ago, and were for a long time mostly confined to carbohydrate analysis. In the early 1990s new phases started to emerge, and the practice was given a name, hydrophilic interaction chromatography (HILIC). Although the use of this separation mode has been relatively limited, we have seen a sudden increase in popularity over the last few years, promoted by the need to analyze polar compounds in increasingly complex mixtures. Another reason for the increase in popularity is the widespread use of MS coupled to LC. The partly aqueous eluents high in ACN with a limited need of adding salt is almost ideal for ESI. The applications now encompass most categories of polar compounds, charged as well as uncharged, although HILIC is particularly well suited for solutes lacking charge where coulombic interactions cannot be used to mediate retention. The review attempts to summarize the ongoing discussion on the separation mechanism and gives an overview of the stationary phases used and the applications addressed with this separation mode in LC.

The measurement and meaning of void volumes in reversed-phase liquid chromatography

The seemingly simple process of measuring the mobile phase volume, V0, in reversed-phase liquid chromatography has eluded unambiguous agreement for over 25 years. Examples exist in the literature where the reported volume is physically impossible, either equal to or larger than the empty column volume, or being so small that it would represent a total porosity of half the theoretical limit for well-packed columns. Here we review the many proposals for methods of measurement, and compare and critique them. At this time, there is still no consensus for the best method of measurement, and workers are urged to critically examine values they measure, to insure they are at least physically possible.

HPLC of peptides and proteins: standard operating conditions

The standard operating conditions for the eight basic modes of HPLC are presented in this unit. They include: size-exclusion chromatography (HP-SEC), ion-exchange chromatography (HP-IEX), normal phase chromatography (HP-NPC), hydrophobic interaction chromatography (HP-HIC), reversed-phase chromatography (RP-HPLC), hydrophilic interaction chromatography (HP-HILIC), immobilized metal ion affinity chromatography (HP-IMAC), and biospecific/biomimetic affinity chromatography (HP-BAC). In addition, some subsets of these chromatographic modes, e.g., mixed mode chromatography (HP-MMC), charge transfer chromatography (HP-CTC), or ligand-exchange chromatography (HP-LEC) are described. Procedures for multimodal column switching are also included, as are guidelines for a systematic approach to method development. Example separations help illustrate the procedures. The standard operating conditions for the eight basic modes of HPLC are presented in this unit.

Systematic, computer-assisted optimisation of the isolation of Thermus thermophilus 50S ribosomal proteins by reversed-phase high-performance liquid chromatography

Isolation of the 50S ribosomal proteins from Thermus thermophilus has been achieved for the first time using reversed-phase high-performance liquid chromatography based on the use of the non-end-capped LiChrospher RP-18 sorbent and computer-assisted method development for optimisation of the resolution. The separation approach for these basic ribosomal proteins utilised mobile phases of high ionic strength, to suppress silanophilic interactions with this type of reversed-phase sorbent. These conditions were found to be a key requirement for achieving good resolution with minimal peak-tailing. The retention times of the 50S ribosomal proteins of Thermus thermophilus were observed to be in very close agreement with the values predicted by computer simulation procedures based on linear solvent strength concepts, with an average error of only 0.5%, whilst base-line resolution was achieved for most of the adjacent peak zones. Following N-terminal sequencing, the proteins TthL5, TthL9, TthL18, TthL24, TthL29, TthL32, TthL34, TthL35 and TthL36 of Thermus thermophilus were readily identified. This approach thus provided a readily optimised strategy for the isolation of the 50S ribosomal proteins from Thermus thermophilus and should be generally applicable to the corresponding ribosomal proteins from various other species, as well as other classes of basic proteins present in crude extracts derived from other biological sources.

Hydrophilic-interaction chromatography of peptides on hydrophilic and strong cation-exchange columns

Hydrophilic-interaction chromatography (HILIC) was recently introduced as a potentially useful separation mode for the purification of peptides and other polar compounds. The elution order of peptides in HILIC, which separates solutes based on hydrophilic interactions, should be opposite to that obtained in reversed-phase chromatography, which separates solutes based on hydrophobic interactions. Three series of peptides, two of which consisted of positively charged peptides (independent of pH at pH less than 7) and one of which consisted of uncharged or negatively charged peptides (dependent on pH), and which varied in overall hydrophilicity/hydrophobicity, were utilized to examine the separation mechanism and efficiency of HILIC on hydrophilic and strong cation-exchange columns.

Strategies for internal amino acid sequence analysis of proteins separated by polyacrylamide gel electrophoresis

An evaluation has been made of various strategies for obtaining internal amino acid sequence data from electrophoretically separated proteins. Electroblotting, in situ proteolysis and extraction, and direct electroelution are compared. Electroblotting of protein or peptides from gels resulted in poor yields (typically, 1-7%). However, higher yields (3-67%) were achieved by in situ enzymatic cleavage followed by acid extraction of the peptides from the gel. Peptides extracted from the gel were separated by reversed-phase high-performance liquid chromatography (RP-HPLC), on short, small-bore columns (100 x 2.1 mm I.D.), to enable recovery of peptides in small volumes (ca. 50 microliters) suitable for microsequence analysis. Capillary zone electrophoresis under acidic conditions (pH 2.5) was used to assess peptide purity before sequence analysis. Cysteine residues were identified in unmodified proteins or peptides by a characteristic phenylthiohydantoin (PTH)-amino acid derivative during sequence analysis. This derivative does not co-chromatograph with any known PTH-amino acid. Direct electrophoretic elution of protein from gels yielded between 45-50% of applied protein. Proteins recovered from gels by electrophoretic elution required further purification by inverse-gradient RP-HPLC [R. J. Simpson, R. L. Moritz, E. C. Nice and B. Grego, Eur. J. Biochem., 165 (1987) 21] to remove sodium dodecylsulphate and acrylamide-related contaminants for sequence analysis.

Unexpected elution behaviour of peptides with various reversed-phase columns

Gastrin and cholecystokinin peptides were separated by reversed-phase chromatography on conventional bristle-type and polymer-coated stationary phases. The retention of the sulphated and non-sulphated isomeric forms of both peptides is governed by the structure of the peptide, the net charge and additional polar interactions with the stationary phases. Polymer-coated phases are optimum for separations according to chain length, whereas polar interactions are required for the separation of sulphated and non-sulphated peptides of identical chain length.

Peptide mapping and internal sequencing of proteins electroblotted from two-dimensional gels onto polyvinylidene difluoride membranes. A chromatographic procedure for separating proteins from detergents

Direct sequence analysis of proteins electroblotted from two-dimensional polyacrylamide gels onto immobilizing matrices provides an efficient technique for obtaining N-terminal sequence data for proteins not amenable to purification by reversed-phase high-performance liquid chromatography (RP-HPLC). We present in this paper a procedure for obtaining peptide fragments from electroblotted proteins for internal amino acid sequence analysis. First, Coomassie Blue-stained proteins are extracted from polydivinylidene difluoride membranes, using a detergent mixture of sodium dodecylsulfate and Triton X-100. Proteins are then separated from the detergent mixture by a chromatographic procedure which relies on the ability of proteins to interact with certain reversed-phase sorbents at high organic solvent concentrations. Under these conditions, detergents and Coomassie Blue are not retained and pass through the column. Proteins are recovered by simultaneously: (i) introducing trifluoroacetic acid into the mobile phase and (ii) decreasing the organic solvent concentration. After proteolytic fragmentation, peptides are purified by microbore-column (1-2 mm I.D.) RP-HPLC for microsequence analysis.

General strategies in the separation of proteins by high-performance liquid chromatographic methods

General fractionation strategies for the high-resolution purification of proteins are described. The impact of different separation parameters and resolution optimisation approaches with tandem-based systems on retention and recovery behaviour is reviewed. Procedures for the successful linkage of different chromatographic steps into a preferred sequence of operations are discussed in terms of the underlying principles and modus operandi of high-performance liquid chromatographic purification of proteins and related biomacromolecules.

Evaluation of advanced silica packings for the separation of biopolymers by high-performance liquid chromatography. III. Retention and selectivity of proteins and peptides in gradient elution on non-porous monodisperse 1.5-microns reversed-phase silicas

Following previous studies of the use of non-porous monodisperse 1.5-microns n-octyl- and n-octadecyl-bonded silicas in gradient elution of proteins, this work was aimed at elucidating further the properties of this novel column material for peptide and protein separations in comparison with wide-pore silicas. First, it is demonstrated that with short columns (e.g., 35 X 8 mm I.D.) packed with these non-porous reversed-phase materials, mixtures of small peptides and mixtures of proteins can be very efficiently resolved. When the chain length of the bonded ligand was varied, the retention of a test set of proteins in gradient elution followed the ligand sequence C18 greater than C8 approximately C4 approximately phenyl greater than C2 under constant elution conditions, and the selectivity remained unchanged. Comparison of the S values of these proteins, as determined from evaluation of the log k' vs. phi dependences with non-porous silicas and with a LiChrospher Si 1000 C8 with identical accessible ligand surface areas per unit column volume, indicated lower values for the non-porous materials (k' = capacity factor; phi = molar fraction of organic solvent; S = slope of the plot of log k' vs. phi). The origin of this behaviour is discussed.

A high-performance liquid chromatography procedure for recovering subnanomole amounts of protein from SDS-gel electroeluates for gas-phase sequence analysis

A high-performance liquid chromatographic procedure for recovering subnanomole amounts of protein from SDS/polyacrylamide gel electroeluates in a form suitable for gas-phase sequence analysis has been developed. By a judicious choice of reversed-phase column packing, proteins can be retained at high concentrations of n-propanol (90-100%) where sodium dodecylsulfate and acrylamide gel-related contaminants are washed through the column. Retained proteins can be recovered from the column in high yield (greater than 90%) by the simultaneous adding of an ion-pairing reagent into the mobile phase and elution with a gradient of decreasing n-propanol concentration (i.e. an 'inverse or negative gradient'). Furthermore, by using a steep gradient (e.g. 50%/min) at a low flow rate (20-200 microliters/min) the proteins can be recovered in less than 100 microliters and can be used for gas-phase sequence analysis without further manipulation. This procedure is independent of sodium dodecylsulfate concentration (up to 1.2% w/v) in sample loading volumes of up to 1.5 ml. Microbore columns (2.1 mm internal diameter) have been employed for recovering small amounts of protein (1-100 micrograms from electroeluates of protein-containing gel spots while conventional columns (4.6 mm internal diameter) were used for isolating larger amounts of protein (greater than 500 micrograms) from electroeluates of preparative gel bands. The general utility of this inverse-gradient high-performance liquid chromatography procedure has been demonstrated by its successful application in recovering a wide variety of proteins from sodium dodecylsulfate gel electroeluates in a form suitable for N-terminal sequence analysis in the 10-500 pmol range.

Effect of mass loadability, protein concentration and n-alkyl chain length on the reversed-phase high-performance liquid chromatographic behaviour of bovine serum albumin and bovine follicular fluid inhibin

The chromatographic behaviour of bovine serum albumin and partially purified preparations of the 58 kilodalton form of bovine follicular fluid inhibin has been investigated using two different n-alkylsilica stationary phases. In particular, the effects of mass loadability, protein concentration and loading conditions on the relative retention, peak width and recovery of these proteins have been studied over a dynamic range up to 100 mg protein per g packing material per injection. The influence of variable amounts of more abundant contaminating proteins such as bovine serum albumin in crude inhibin preparations on the chromatographic purification of trace quantities of the 58 kilodalton inhibin protein has also been examined. Based on these observations, recommendations are offered for the selection of protein loading conditions with n-alkylsilica stationary phases, particularly for the trace recovery and purification of hydrophobic proteins similar to inhibin where self-aggregation, adsorption to glass or plastic surfaces, and aberrant chromatographic behaviour on size exclusion or adsorptive chromatographic supports may occur.

Semi-preparative high-performance reversed-phase displacement chromatography of insulins

A reversed-phase displacement chromatographic method has been developed for the purification of bovine and porcine insulin samples. Up to 500 mg of raw insulin could be purified on a Nucleosil C8 analytical column using a methanol-containing phosphate buffer carrier and a cetrimide-containing displacer. A proinsulin contamination level as low as 100 ppm in the collected fractions could be achieved.

High-performance liquid chromatography of amino acids, peptides and proteins. LXIII. Reversed-phase high-performance liquid chromatographic characterisation of several polypeptide and protein hormones

The chromatographic behaviour on alkylsilicas of a variety of hormonal proteins is described. Optimization of resolution and recovery of these protein hormones, which included porcine relaxins, human chorionic gonadotropin, human placental lactogen, pituitary derived growth hormone and adenohypophyseal glycoprotein hormones, was achieved by manipulation of both mobile and stationary phase parameters. With standard stainless-steel analytical columns (10-30 cm X 0.4 cm) packed with meso- or macro-porous n-alkylsilica supports these proteins can be readily fractionated at the semi-preparative level with separation times generally under 90 min using elution systems directly compatible with subsequent methods of primary structure determination or biological functional analysis. The effects of changes in several experimental parameters on peak symmetry, retention and recovery are described.

Separation of cereal proteins by reversed-phase high-performance liquid chromatography

Cereal proteins have been extremely difficult to purify and characterize owing to their heterogeneity, poor solubility and tendency to polymerize. High-performance liquid chromatography (HPLC) on a 300 A reversed-phase (RP)(C18) support (Syn Chropak RP-P), using acetonitrile as organic modifier in the presence of trifluoroacetic acid, has been found to be capable of high-resolution separations of these proteins; the resolution is often better than that obtained by any other chromatographic or electrophoretic method. Examples are presented showing separations of low-molecular-weight gliadins, omega-gliadins and ethanol-soluble reduced glutenin subunits from wheat and of zein from corn. In addition, proteins may be directly extracted from ground single kernels and subsequently analyzed by RP-HPLC; applications in genetic studies, in breeding programs and in varietal identification are proposed. In addition to its high resolution, RP-HPLC is superior to most other methods in speed, sensitivity, reproducibility and suitability for quantitation. Polypeptide chains of molecular weight up to 133,000 are recovered in high yields and the column capacity is high, demonstrating that RP-HPLC is suitable for both preparative and analytical separations of proteins. RP-HPLC resolves proteins primarily on the basis of differences in surface hydrophobicity, so it therefore complements, rather than duplicates, other techniques that separate proteins on the basis of size or charge. RP-HPLC promises to become an invaluable technique for the fractionation and characterization of proteins from cereals and other sources.