Alternative high-performance liquid chromatographic peptide separation and purification concept using a new mixed-mode reversed-phase/weak anion-exchange type stationary phase

A stationary phase with a positively charged surface allows for minimizing formic acid concentration in the mobile phase, enhancing electrospray ionization in LC-MS proteomic experiments

The default choice of mobile phase acidifier for bottom-up LC-MS proteomic analyses is 0.10% formic acid because of its decent acidity, decent ion pairing ability, and low suppression of electrospray ionization. In recent years, state-of-the-art columns have been designed specifically to provide efficient separation even when using an MS-friendly mobile phase of low ionic strength. Despite this, no attempts have been made to improve the sensitivity of the MS-based analytical methods by reducing the amount of formic acid in the mobile phase. In this study, we evaluated the effect of reduced formic acid concentration in the mobile phase on the chromatographic behavior and MS response of peptides when separated using columns packed with a C18 stationary phase with a positively charged surface. Using 0.01% formic acid in the mobile phase maintained excellent chromatographic performance and increased MS signal response compared to the standard of 0.10%. The enhanced MS response translated to about 50% improved peptide identifications depending on the complexity and amount of sample injected. The increased retention of peptides at a reduced formic acid concentration was directly proportional to the number of acidic residues in the peptide sequence. The study was carried out by covering a spectrum of protein samples with varied complexity using analytical flow, micro-, and nanoflow regimes to expand the applicability in routine practice.

Selectivity optimization in liquid chromatography via stationary phase tuning

In striving for the best possible separation, the selectivity of stationary phases as an optimization parameter is often underestimated although there are many ways to influence this powerful tool. This review serves to provide an insight into the various ways of adapting the selectivity of a separation in liquid chromatography. Approaches via temperature and flow rate tuning are discussed as a basis followed by focusing on the stationary phase as the superior optimization parameter. Highly selective stationary phases hereby provide an advantage for groups of similar analytes. For more complex mixtures, separations can be improved using mixed-mode technologies where different retention mechanisms are combined. Serial coupling, mixed-bed columns, and stationary phase optimized selectivity liquid chromatography provide solutions to various degrees. Finally, the advantages of stationary phase tuning over adaption of mobile phase and/or temperature are presented in terms of optimum application range.

Benefits of a Mixed-Mode Stationary Phase to Address the Challenging Purification of an Industrially Relevant Peptide: A Proof-of-Concept Study

Peptides are a class of biomolecules with a great potential from the therapeutic point of view, because of their unique biological properties. Industrially, the production stategies adopted produce both the target peptide and a series of impurities that must be removed. Preparative chromatography is the technique of choice for the large-scale purification of biomolecules, generally performed in reversed-phase mode, using hydrophobic adsorbents (e.g., C8 stationary phases). A promising and innovative alternative is represented by mixed-mode columns, which bear two different ligands on the particle surface, exploiting two different retention mechanisms to improve the separation. This work represents a proof-of-concept study focused on the comparison of a hydrophobic adsorbent and a mixed-mode one (bearing both hydrophobic groups and charged ones) for the purification of a crude peptide mixture. Thanks to more-favourable thermodynamics, it was found that, when collecting the whole peak excluding fractions of the peak tail, the mixed-mode column led to an increase in the recovery of roughly +15%, together with a slight improvement in purity at the same time, with respect to the traditional hydrophobic column. In addition, if the whole peak, including the tail, is collected, the performance of the two columns are similar in terms of purity and recovery, but the pepetide elutes as a narrower peak with the mixed mode. This leads to a collection pool showing a much-higher peptide concentration and to lower solvent volumes needed, which is a beneficial achievement when targeting more sustainable processes. These results are very advantageous from the industrial viewpoint, because they also involve a decrease in the peptide amount contained in the peak tail, which must be reprocessed again to satisfy purity requirements.

Optimisation and Characterisation of Novel Angiotensin-Converting Enzyme Inhibitory Peptides Prepared by Double Enzymatic Hydrolysis from Agaricus bisporus Scraps

Food-derived hypotensive peptides have attracted attention in the field of active peptide research in recent years. In this study, based on ACE inhibition rate and using the Box–Behnken central combination design principle to optimise the process of ACE inhibitor peptides prepared by double-enzyme hydrolysis. The amino acid sequences of ACE inhibitor peptides were determined by liquid chromatography mass spectrometry (LC-MS/MS), and their binding to ACE was studied by molecular docking. The optimal processing conditions were 1:1 alkaline protease: compound protease, pH was 8.43, enzymolysis temperature was 44.32 °C, and enzymolysis time was 3.52 h. Under these conditions, the ACE inhibition rate reached 65.12%, and the inhibition rate after separation and purification was 80.68% (IC₅₀ = 0.9 mg/mL). Three novel peptides with ACE inhibitory activity were detected by LC-MS/MS, with sequences LVYP (Leu-Val-Tyr-Pro), VYPW(Val-Tyr-Pro-Trp) and YPWT(Tyr-Pro-Trp-Thr). Molecular docking revealed that the three novel peptides all established hydrogen bonds with the S1(Tyr523, Glu384, Ala354) and S2 (His353) pockets of ACE. Among them, LVYP, VYPW and YPWT, respectively, formed eleven hydrogen bonds, six hydrogen bonds and nine hydrogen bonds with ACE. The study revealed that these peptides have the potential for the development of novel ACE inhibitor drugs and provide a new avenue for high-value utilisation of mushrooms scraps.

Theoretical study of the efficiency of liquid chromatography columns with particle size gradient

Modern analytical applications of liquid chromatography require more and more efficient columns. In this work, the possibility of utilizing particle size gradient in the chromatographic column was studied by a theoretical approach. In the course of our work three different scenarios of particle size gradients were considered with different shapes (linear, convex and concave). The evolution of bandwidth inside the column was plotted for each scenario. As a reference point, the bandwidth of the uniform column was used, which had the same pressure drop as the non-uniform column. According to our calculations, in isocratic elution mode, the non-uniform column does not offer any advantage compared to the uniform column, regardless the type of the particle size gradient. In gradient elution mode, however, extra band compression occurs was found. For negative particle size gradients, the final physical bandwidth was found to be approximately 1-4 % smaller than for uniform columns. This slight gain in efficiency in terms of bandwidth compression can be expanded to 5-8 % by the optimization of the limiting particle sizes. These optimized results are obtained when the final particle size is approximately 40% of the initial particle diameter.

Static vs. Dynamic Electrostatic Repulsion Reversed Phase Liquid Chromatography: Solutions for Pharmaceutical and Biopharmaceutical Basic Compounds

Many efforts have been made to separate basic compounds, which are challenging to resolve in reversed phase liquid chromatography. In this process, they are strongly retained and the peak shape undergoes significant distortion. The principal origin of this has been identified with the non-negligible interaction with residual deprotonated silanols. Consequently, all solutions that efficiently shield silanols are being sought. This review is an upgrade on the use of the electrostatic repulsion reversed phase (ERRP) approach: retention of bases, in protonated form, can be achieved by modulating the charge repulsion caused by the presence of positive charges in the chromatographic system. This study successfully (i) introduced fixed positive charges in the structure of stationary phases, (ii) used cationic and hydrophobic additives in the mobile phase, and (iii) used the ERRP-like approach employed at the preparative level for peptide purification.

Characterization and comparison of mixed-mode and reversed-phase columns; interaction abilities and applicability for peptide separation

In this work, two mixed-mode columns from a different manufacturers and one marketed as a reversed-phase column were characterized and compared in the terms of their interaction abilities, retentivity, peak symmetry, and applicability for peptide separation. All the tested columns contain octadecyl ligand and positively charged modifier, i.e. pyridyl group for the reversed-phase column XSelect CSH C18, quaternary alkylamine for mixed-mode column Atlantis PREMIER BEH C18 AX, and permanently charged moiety (details not available from the manufacturer) for mixed-mode column Luna Omega PS C18. For detailed characterization and comparison of their interaction potential, several approaches were used. First, a simple Walters test was performed to estimate hydrophobic and silanophilic interactions of the tested columns. The highest values of both parameters were observed for column Atlantis PREMIER BEH C18 AX. To investigate the effect of pH and buffer concentration on retention, mobile phases composed of acetonitrile and buffer (ammonium formate, pH 3.0; ammonium acetate pH 4.7 and pH 6.9) in various concentrations (5mM; 10mM; 15mM and 20mM) were used. The analysis of permanently charged compounds was used to describe the electrostatic interaction abilities of the stationary phases. The most significant contribution of electrostatic interactions to the retention was observed for Atlantis PREMIER BEH C18 AX column in the mobile phase with buffer of pH 3.0. A set of ten dipeptides, three pentapeptides and one octapeptide was used to investigate the effects of pH and buffer concentration on retention and peak symmetry. Each of the tested columns provides the optimal peak shape under different buffer pH and concentration. The gradient separation of the 14 tested peptides was used to verify the application potential of the tested columns for peptide separation. The best separation was achieved within 4 minutes on column Atlantis PREMIER BEH C18 AX.

The Role of Counter-Ions in Peptides-An Overview

Peptides and proteins constitute a large group of molecules that play multiple functions in living organisms. In conjunction with their important role in biological processes and advances in chemical approaches of synthesis, the interest in peptide-based drugs is still growing. As the side chains of amino acids can be basic, acidic, or neutral, the peptide drugs often occur in the form of salts with different counter-ions. This review focuses on the role of counter-ions in peptides. To date, over 60 peptide-based drugs have been approved by the FDA. Based on their area of application, biological activity, and results of preliminary tests they are characterized by different counter-ions. Moreover, the impact of counter-ions on structure, physicochemical properties, and drug formulation is analyzed. Additionally, the application of salts as mobile phase additives in chromatographic analyses and analytical techniques is highlighted.

A Critical Review of Bottom-Up Proteomics: The Good, the Bad, and the Future of this Field

Proteomics is the field of study that includes the analysis of proteins, from either a basic science prospective or a clinical one. Proteins can be investigated for their abundance, variety of proteoforms due to post-translational modifications (PTMs), and their stable or transient protein–protein interactions. This can be especially beneficial in the clinical setting when studying proteins involved in different diseases and conditions. Here, we aim to describe a bottom-up proteomics workflow from sample preparation to data analysis, including all of its benefits and pitfalls. We also describe potential improvements in this type of proteomics workflow for the future.

Multiple heart-cutting mixed-mode chromatography-reversed-phase 2D-liquid chromatography method for separation and mass spectrometric characterization of synthetic oligonucleotides

Until today, ion-pair reversed-phase chromatography is still the dominating method for analytical characterization of synthetic oligonucleotides. Its hyphenation with mass spectrometry, however, has some drawbacks such as ion-suppression in electrospray ionization. To overcome this problem, we present in this work a multiple heart-cutting (MHC) two-dimensional liquid chromatography (2D-LC) method with ultra-violet (UV) and electrospray ionization (ESI) mass spectrometry (MS) detection. A reversed-phase/weak anion-exchange (RP/WAX) stationary phase in the first dimension (¹D) provides the selectivity for separation of structurally closely related oligonucleotide sequences and deletions (shortmers), respectively, using a mixed pH/triethylammonium phosphate buffer gradient at constant organic modifier content. Heart cuts of the oligonucleotide peaks are transferred to the second dimension (²D) via a multiple heart-cutting valve which is equipped with two loop decks. The ²D RP column is used for desalting via a diverter valve. Active solvent modulation enables to refocus the oligonucleotide peak into a sharp zone by ²D RP entirely free of non-volatile buffer components and ion-pair agents. Oligonucleotides can thus be sensitively detected by ESI-QTOF-MS under MS-compatible conditions.

A crosslinked, low pH-stable, mixed-mode cation-exchange like stationary phase made using the thiol-yne click reaction

Mixed-mode cation-exchange stationary phases are useful for the separation of mixtures containing hydrophobic, acidic, and basic molecules. To ensure that weak organic bases are protonated and carboxylic acids are neutral low pH mobile phases are required. Mixed-mode stationary phases that are stable at pH < 3 are needed. We synthesized a crosslinked structure along the surface of thiol functionalized silica gel particles using the thiol-yne click reaction. The alkyne, 1,7-octadiyne, was added to the 3-mercaptopropyl silica gel, then crosslinked using 1,6-hexanedithiol. Elemental analysis showed low octadiyne ligand surface coverage, but, stoichiometrically, three sulfurs were added to each octadiyne ligand during the crosslinking step, indicating that crosslinking occurred. The effect of the crosslinking on the stability was tested with a 50:50 (v/v) pH 0.50 5% TFA aqueous:acetonitrile mobile phase at 70 °C for six days, over 35,000 column volumes. The stationary phase showed good stability with the retention of triphenylene decreasing only 20% during that time. The Tanaka test showed that the phase has a methylene selectivity of 1.20 ± 0.04, a high shape selectivity of 2.71 ± 0.03, and a 3.98 ± 0.05 cation-exchange factor at pH 2.70. The phase has a selectivity factor for nitrobenzene and benzene of 1.41 ± 0.01, indicating the electron donating charge transfer characteristic of the phase. The mixed-mode characteristics of the phase were investigated using a mixture of the monoamine neurotransmitters norepinephrine, dopamine, and serotonin. Baseline resolution of the monoamines could be obtained using a simple 20 mM potassium phosphate (pH 2.70)/methanol mobile phase. Altering both the methanol content and the potassium ion concentration altered the retention of the monoamines indicating mixed-mode cation exchange characteristic of the crosslinked stationary phase.

Sustainability Challenges in Peptide Synthesis and Purification: From R&D to Production

In recent years, there has been a growing interest in therapeutic peptides within the pharmaceutical industry with more than 50 peptide drugs on the market, approximately 170 in clinical trials, and >200 in preclinical development. However, the current state of the art in peptide synthesis involves primarily legacy technologies with use of large amounts of highly hazardous reagents and solvents and little focus on green chemistry and engineering. In 2016, the ACS Green Chemistry Institute Pharmaceutical Roundtable identified development of greener processes for peptide API as a critical unmet need, and as a result, a new Roundtable team formed to address this important area. The initial focus of this new team is to highlight best practices in peptide synthesis and encourage much needed innovations. In this Perspective, we aim to summarize the current challenges of peptide synthesis and purification in terms of sustainability, highlight possible solutions, and encourage synergies between academia, the pharmaceutical industry, and contract research organizations/contract manufacturing organizations.

Column Characterization and Selection Systems in Reversed-Phase High-Performance Liquid Chromatography

Reversed-phase high-performance liquid chromatography (RP-HPLC) is the most popular chromatographic mode, accounting for more than 90% of all separations. HPLC itself owes its immense popularity to it being relatively simple and inexpensive, with the equipment being reliable and easy to operate. Due to extensive automation, it can be run virtually unattended with multiple samples at various separation conditions, even by relatively low-skilled personnel. Currently, there are >600 RP-HPLC columns available to end users for purchase, some of which exhibit very large differences in selectivity and production quality. Often, two similar RP-HPLC columns are not equally suitable for the requisite separation, and to date, there is no universal RP-HPLC column covering a variety of analytes. This forces analytical laboratories to keep a multitude of diverse columns. Therefore, column selection is a crucial segment of RP-HPLC method development, especially since sample complexity is constantly increasing. Rationally choosing an appropriate column is complicated. In addition to the differences in the primary intermolecular interactions with analytes of the dispersive (London) type, individual columns can also exhibit a unique character owing to specific polar, hydrogen bond, and electron pair donor-acceptor interactions. They can also vary depending on the type of packing, amount and type of residual silanols, "end-capping", bonding density of ligands, and pore size, among others. Consequently, the chromatographic performance of RP-HPLC systems is often considerably altered depending on the selected column. Although a wide spectrum of knowledge is available on this important subject, there is still a lack of a comprehensive review for an objective comparison and/or selection of chromatographic columns. We aim for this review to be a comprehensive, authoritative, critical, and easily readable monograph of the most relevant publications regarding column selection and characterization in RP-HPLC covering the past four decades. Future perspectives, which involve the integration of state-of-the-art molecular simulations (molecular dynamics or Monte Carlo) with minimal experiments, aimed at nearly "experiment-free" column selection methodology, are proposed.

Enhancing adsorption capacity while maintaining specific recognition performance of mesoporous silica: a novel imprinting strategy with amphiphilic ionic liquid as surfactant

In order to facilitate the broad applications of molecular recognition materials in biomedical areas, it is critical to enhance their adsorption capacity while maintaining their excellent recognition performance. In this work, we designed and synthesized well-defined peptide-imprinted mesoporous silica (PIMS) for specific recognition of an immunostimulating hexapeptide from human casein (IHHC) by using amphiphilic ionic liquid as the surfactant to anchor IHHC via a combination of one-step sol-gel method and docking oriented imprinting approach. Thereinto, theoretical calculation was employed to reveal the multiple binding interactions and dual-template configuration between amphiphilic ionic liquid and IHHC. The fabricated PIMS was characterized and an in-depth analysis of specific recognition mechanism was conducted. Results revealed that both adsorption and recognition capabilities of PIMS far exceeded that of the NIMS's. More significantly, the PIMS exhibited a superior binding capacity (60.5 mg g^-1), which could increase 18.9% than the previous work. The corresponding imprinting factor and selectivity coefficient could reach up to 4.51 and 3.30, respectively. The PIMS also possessed lickety-split kinetic binding for IHHC, where the equilibrium time was only 10 min. All of these merits were due to the high surface area and the synergistic effect of multiple interactions (including hydrogen bonding, π-π stacking, ion-ion electrostatic interactions and van der Waals interactions, etc) between PIMS and IHHC in imprinted sites. The present work suggests the potential application of PIMS for large-scale and high-effective separation of IHHC, which may lead to their broad applications in drug/gene deliver, biosensors, catalyst and so on.

N-Propyl-N'-2-pyridylurea-modified silica as mixed-mode stationary phase with moderate weak anion exchange capacity and pH-dependent surface charge reversal

Herein, we present a novel silica-based stationary phase modified with N-propyl-N'-2-pyridylurea selector. Due to the weakly basic properties of the pyridine selector and the presence of residual silanols after selector immobilization, a zwitterionic surface with a pI observed at approximately pH 5.5 was measured by electrophoretic light scattering in pH-dependent ζ-potential determinations. The capability of the new N-propyl-N'-2-pyridylurea-modified silica to serve as mixed-mode stationary phase was investigated. For this purpose, it was characterized under RP and HILIC conditions using test mixtures. Subsequent classification of this stationary phase in comparison to in-house and commercial benchmarks was carried by principal component analysis of resultant retention factors from chromatographic tests. The results show a relatively unique mixed-mode character amongst the tested stationary phases. The chromatographic retention characteristics of acidic compounds matched well the ζ-potential determinations. The application of anion-exchange at low pH values (e.g. pH 5) and ion exclusion chromatography at pH 7 for the separation of uridine 5'-mono-, di- and triphosphate demonstrated a pH-dependent umpolung of the stationary phase surface. The combination of these separation principles in a pH gradient from 5 to 7 gave rise to weak anion-exchange selectivity with a charge-inducted elution due to repulsive interactions at higher pH and resulted in a significant faster separation with improved peak shape under mild elution conditions.

Quantitative analysis of acid-catalyzed levulinic acid product mixture from cellulose by mixed-mode liquid chromatography

A mixed-mode weak anion-exchange/reversed-phase liquid chromatography (LC) column was successfully applied for the analysis of levulinic acid (4-oxopentanoic acid, LA) product mixture derived from cellulose. Due to the existence of ionic and neutral byproducts, the analysis of the product mixture usually requires ion chromatography, LC and gas chromatography simultaneously. The new method enables accomplishment of the analysis in one LC run within 6min. LC mobile phase of 10mM phosphate buffer containing 5% acetonitrile with pH=5.5 was used. The linear regression coefficients for the UV signal of standard compounds with the corresponding mass concentrations were greater than 0.999. The method recoveries were between 98.57-103.48%. The limits of quantification were 5, 10,1000, 1500 and 3000ng/mL for 5-hydroxymethylfurfural, furfural, acetic acid, formic acid and LA respectively. The mixed-mode column exhibits comprehensive separation mechanism of both reversed-phase and ion-exchange interactions. The mobile phase with different pH, organic modifier solvent and buffer concentration provided flexible LC method for the sample with different complexity.

Preparation of a silica stationary phase co-functionalized with Wulff-type phenylboronate and C12 for mixed-mode liquid chromatography

A silica stationary phase was designed and synthesized through the co-functionalization of silica with Wulff-type phenylboronate and C12 for mixed-mode liquid chromatography applications. The as-synthesized stationary phase was characterized by elemental analysis and Fourier Transform-InfraRed Spectroscopy (FT-IR). Retention mechanisms, including boronate affinity (BA), reversed-phase (RP) and anion-exchange (AE), were involved. Retention mechanism switching was easily realized by adjustment of the mobile phase constitution. Cis-diol compounds could be selectively captured under neutral conditions in BA mode and off-line separated in RP mode. Neutral, basic, acidic and amphiprotic compounds were chromatographed on the column in RP chromatography, while inorganic anions were chromatographed in AE chromatography to characterize the mixed-mode nature of the prepared stationary phase. In addition, the RP performance was compared with an octadecyl silica column in terms of column efficiency (N/m), asymmetry factor (A_f), retention factor (k) and resolution (Rs). The prepared stationary phase offered multiple interactions with analytes in addition to hydrophobic interactions under RP elution conditions. Based on the mixed-mode properties, off-line 2D-LC, for selective capture and separation of urinary nucleosides, was successfully realized on a single column, demonstrating its powerful application potential for complex samples.

Mixed-mode chromatography in pharmaceutical and biopharmaceutical applications

Mixed-mode chromatography (MMC) is a fast growing area in recent years, thanks to the new generation of mixed-mode stationary phases and better understanding of multimode interactions. MMC has superior applications in the separation of compounds that are not retained or not well resolved by typical reversed-phase LC methods, especially for polar and charged molecules. Due to the multiple retention modes that a single MMC column can offer, often MMC provides additional dimension to a separation method by adjusting the mobile phase conditions. Mixed-mode media is also an effective way to clean up complex sample matrices for purification purposes or for sensitive detection of trace amounts of analytes. In this article, we discuss mixed-mode stationary phases and separation mechanisms and review recent advances in pharmaceutical and biopharmaceutical applications including the analysis and/or purification of counterions, small molecule drugs, impurities, formulation excipients, peptides and proteins.

Synthesis and characterization of a multimode stationary phase: Congo red derivatized silica in nano-flow HPLC

Congo red derivatized silica has been developed as a mixed mode stationary phase and used for nano-flow HPLC.

Retention prediction of highly polar ionizable solutes under gradient conditions on a mixed-mode reversed-phase and weak anion-exchange stationary phase

In the present work the retention of three highly polar and ionizable solutes - uric acid, nicotinic acid and ascorbic acid - was investigated on a mixed-mode reversed-phase and weak anion-exchange (RP/WAX) stationary phase in buffered aqueous acetonitrile (ACN) mobile phases. A U-shaped retention behavior was observed for all solutes with respect to the eluent organic modifier content studied in a range of 5-95% (v/v). This retention behavior clearly demonstrates the presence of a HILIC-type retention mechanism at ACN-rich hydro-organic eluents and an RP-like retention at aqueous-rich hydro-organic eluents. Hence, this column should be promising for application under both RP and HILIC gradient elution modes. For this reason, a series of programmed elution runs were carried out with increasing (RP) and decreasing (HILIC) organic solvent concentration in the mobile phase. This dual gradient process was successfully modeled by two retention models exhibiting a quadratic or a cubic dependence of the logarithm of the solute retention factor (lnk) upon the organic modifier volume fraction (φ). It was found that both models produced by gradient retention data allow the prediction of solute retention times for both types of programmed elution on the mixed-mode column. Four, in the case of the quadratic model, or five, in the case of the cubic model, initial HILIC- and RP-type gradient runs gave satisfactory retention predictions of any similar kind elution program, even with different flow rate, with an overall error of only 2.5 or 1.7%, respectively.

High-performance liquid chromatography separation of cis-trans anthocyanin isomers from wild Lycium ruthenicum Murr. employing a mixed-mode reversed-phase/strong anion-exchange stationary phase

The cis-trans isomerism is a common phenomenon for acylated anthocyanins. Nevertheless, few studies reported effective methods for the preparation of isomeric anthocyanins from natural products. In this work, a high-performance liquid chromatography (HPLC) method was developed to efficiently purify anthocyanin isomers from Lycium ruthenicum Murr. based on a mixed-mode reversed-phase/strong anion-exchange column (named XCharge C8SAX). Four commercially available columns were evaluated with a pair of isomeric anthocyanins, and the results demonstrated that the XCharge C8SAX column exhibited improved selectivity and column efficiency for the isomers. The chromatographic parameters, including pH, organic content, and ionic strength, were investigated. Optimal separation quality for the anthocyanin isomers was achieved on the XCharge C8SAX column. Six pure anthocyanins, including two pairs of cis-trans isomeric anthocyanins with one new anthocyanin, were purified from L. ruthenicum and identified. All of the results indicated that this method is an effective way to separate anthocyanins, especially for cis-trans isomers.

Synthetic oligonucleotide separations by mixed-mode reversed-phase/weak anion-exchange liquid chromatography

Synthetic oligonucleotides gain increasing importance in new therapeutic concepts and as probes in biological sciences. If pharmaceutical-grade purities are required, chromatographic purification using ion-pair reversed-phase chromatography is commonly carried out. However, separation selectivity for structurally closely related impurities is often insufficient, especially at high sample loads. In this study, a "mixed-mode" reversed-phase/weak anion exchanger stationary phase has been investigated as an alternative tool for chromatographic separation of synthetic oligonucleotides with minor sequence variations. The employed mixed-mode phase shows great flexibility in method development. It has been run in various gradient elution modes, viz. one, two or three parameter (mixed) gradients (altering buffer pH, buffer concentration, and organic modifier) to find optimal elution conditions and gain further insight into retention mechanisms. Compared to ion-pair reversed-phase and mere anion-exchange separation, enhanced selectivities were observed with the mixed-mode phase for 20-23 nucleotide (nt) long oligonucleotides with similar sequences. Oligonucleotides differing by 1, 2 or 3 nucleotides in length could be readily resolved and separation factors for single nucleotide replacements declined in the order Cytosine (C)/Guanine (G)>Adenine (A)/Guanine∼Guanine/Thymine (T)>Adenine/Cytosine∼Cytosine/Thymine>Adenine/Thymine. Selectivities were larger when the modification was at the 3' terminal-end, declined when it was in the middle of the sequence and was smallest when it was located at the 5' terminus. Due to the lower surface area of the 200Å pore size mixed-mode stationary phase compared to the corresponding 100Å material, lower retention times with equal selectivities under milder elution conditions were achievable. Considering high sample loading capacities of the mixed-mode anion-exchanger phase, it should have great potential for chromatographic oligonucleotide separation and purification.

Preparation of a novel carboxyl stationary phase by "thiol-ene" click chemistry for hydrophilic interaction chromatography

A novel carboxyl-bonded silica stationary phase was prepared by "thiol-ene" click chemistry. The resultant Thiol-Click-COOH phase was evaluated under hydrophilic interaction liquid chromatography (HILIC) mobile phase conditions. A comparison of the chromatographic performance of Thiol-Click-COOH and pure silica columns was performed according to the retention behaviors of analytes and the charged state of the stationary phases. The results indicated that the newly developed Thiol-Click-COOH column has a higher surface charge and stronger hydrophilicity than the pure silica column. Furthermore, the chromatographic behaviors of five nucleosides on the Thiol-Click-COOH phase were investigated in detail. Finally, a good separation of 13 nucleosides and bases, and four water-soluble vitamins was achieved.