Zwitterionic stationary phase with covalently bonded phosphorylcholine type polymer grafts and its applicability to separation of peptides in the hydrophilic interaction liquid chromatography mode

Preparation of small peptoid-modified zwitterion-exchange/reversed-phase mixed-mode chromatography stationary phases for the separation of arenes and antibiotics

In this study, three small peptoids with different structures, named Sil-peptoids, were developed to improve the separation selectivity of zwitterion-exchange/reversed-phase mixed-mode chromatography stationary phases for multi-component complex drugs. Nonpolar, amphoteric, and alkaline drugs were used as test samples to demonstrate their retention behaviors in reversed-phase, ionic, and mixed-mode interactions. It was observed that different carboxyl anions in the small peptoids of the Sil-peptoids had vast differences in their stereo-selectivity. The stereo-selectivity and the influence of Sil-peptoids on the retention behavior of complex drugs and their interaction mechanism for the drug molecules were effectively evaluated through the combination of chromatographic analysis and molecular modeling. Finally, a mixture of drugs consisting of two polar and six non-polar drugs was used to obtain a separation effect with a resolution >1.5. Two other groups of polar antibiotics were used to verify the separation ability of the Sil-peptoids. The results indicated that the Sil-peptoids could separate multiple substances simultaneously. These novel stationary phases can be applied to the analysis of complex multi-component drugs.

Determination of 5-methyldeoxycytosine and oxidized derivatives by nano-liquid chromatography with zwitterionic monolithic capillary column

DNA methylation is one of the epigenetic modifications at the 5-carbon of cytosine to form 5-methyl-2'-deoxycytidine (5mdC). In mammalian cells, 5mdC can be transferred to 5-hydroxymethyl-2'-deoxycytidine (5hmdC) by ten-eleven translocation (TET), and 5hmdC is further oxidized to 5-formyl-2'-deoxycytidine (5fodC) and 5-carboxyl-2'-deoxycytidine (5cadC). In the present work, we developed a highly sensitive nano liquid chromatographic method for the determination of 5mC and 5hmC with zwitterionic monolithic capillary column. The conditions for the preparation of zwitterionic monolithic capillary column were systematically optimized. The monolithic capillary column displayed high column efficiency for nucleoside dA (190,000 plates/m) and allowed the baseline separation of 10 standard nucleosides in HILIC mode. The detection sensitivity was improved significantly by using the large volume injection combined with sample stacking onto the head of the column when sample was dissolved in high content organic solvent (ACN: H₂O = 97:3). The limit of detection for 5mdC and 5hmdC were determined as 4 nM and 3 nM, respectively, and the corresponding limit of quantification were determined as 12 nM and 10 nM, respectively. Further, the zwitterionic monolithic capillary column can be easily utilized in nano-LC and mass spectrometry coupling for qualitative analysis of 5mdC, 5hmdC, 5fodC and 5cadC in real mouse brain sample. The whole genomic DNA methylation levels in mouse brain sample can be easily determined with UV detection.

Zwitterionic polymer-terminated porous silica stationary phases for highly selective separation in hydrophilic interaction chromatography

We described two novel zwitterionic polymer-terminated porous silica stationary phases containing the same pyridinium cation and anions of different side chains (carboxylate and phosphonate groups) for use in hydrophilic interaction liquid chromatography (HILIC). These two novel columns were prepared by polymerizing 4-vinylpyridine and grafting it onto a silica surface, followed by quaternization reaction with 3-bromopropionic acid (Sil-VPC₂₄) and (3-bromopropyl) phosphonic acid (Sil-VPP₂₄), which possess positively charged pyridinium groups, and negatively charged carboxylate and phosphonate groups, respectively. The products obtained were verified through relevant characterization techniques such as elemental analysis, Fourier-transform infrared spectroscopy, thermogravimetric analysis, Zeta potential analysis, and Brunauer-Emmett-Teller analysis. The retention properties and mechanisms of different types of compounds (neutral, cationic, and anionic) on the two zwitterionic-modified silica stationary phases were studied by varying the buffer salt concentration and pH of the eluent. The separation of phenol and aromatic acids, disubstituted benzene isomers, sulfonamide drugs, as well as nucleosides/nucleobases were investigated on the two packed novel columns and a commercial zwitterionic column in identical HILIC mode, ensuring a thorough comparison between both novel columns and with a commercial standard. The results illustrated that various compounds could be separated up to various efficiencies based on the mechanism of hydrophilic interaction-based retention between the solutes and the two zwitterionic polymer stationary phases. The Sil-VPP₂₄ column demonstrated the best separation performance out of the three, as well as flexible selectivity and excellent resolution. Both novel columns exhibited excellent stability and chromatographic repeatability for the separation of seven nucleosides and bases.

A Compendium of the Principal Stationary Phases Used in Hydrophilic Interaction Chromatography: Where Have We Arrived?

Hydrophilic interaction liquid chromatography (HILIC) today is a well-known and largely applied technique to analyse polar compounds such as pharmaceuticals, metabolites, proteins, peptides, amino acids, oligonucleotides, and carbohydrates. Due to the large number of stationary phases employed for HILIC applications, this review aims to help the reader in choosing a proper stationary phase, which often represents the critical point for the success of a separation. A great offer is present for achiral applications in contrast to the chiral phases developed for HILIC enantioseparations. In the last case, up-to-date solutions are presented.

A Survey of Polar Stationary Phases for Hydrophilic Interaction Chromatography and Recent Progress in Understanding Retention and Selectivity

Various polar stationary phases have become available for hydrophilic interaction chromatography (HILIC) and help drive continuous applications in biomedical, environmental and pharmaceutical areas in the past decade. Although the stationary phases for HILIC have been reviewed previously, it is an appropriate time to take another look at the progresses during the past five years. The current review provides an overview of the polar stationary phases commercially available for HILIC applications in an effort to assist scientists in selecting suitable columns. New types of stationary phase that were published in literature in the past five years are summarized and discussed. The trend in stationary phase research and development is also highlighted. Of particular interest is the experimental evidence for direct interactions of polar analytes with the ligands of the stationary phases under HILIC conditions. In addition, two different approaches have been developed to delineate the relative significance of the partitioning and adsorption mechanisms in HILIC, representing an important advancement in our understanding of the retention mechanisms in HILIC.

Evaluation of multiple hydrophilic interaction chromatography columns and surrogate matrix for arginine quantification in saliva by high-resolution mass spectrometry

Arginine, a pivotal ingredient in many biochemical synthetic pathways, can be used as a biomarker for many oral care clinical applications. It is still a challenge to develop a sensitive and reliable chromatographic method to quantify arginine as a biomarker in saliva, with or without arginine product pretreatment. The current method solved two critical issues for arginine quantitation in human saliva. The first issue was how to optimize arginine peak shape. A hydrophilic interaction chromatography method based on the column selection, pH and pK_a relationship, mobile phase ionic strength, organic solvent consideration, and temperature effects was developed. An optimized chromatographic condition for arginine quantitation in the saliva matrix was obtained. The second issue was how to build confidence in the use of a simple surrogate matrix methodology to replace the more complex traditional standard addition methodology. The surrogate matrix methodology we developed is applicable to the measurement of arginine as a potential non-invasive biomarker in human saliva. The method detection and quantification limit reached 2 and 6 ng/mL. The tailing factor was within the 0.9-1.1 range even though arginine had three pK_a values at 2.18, 9.09, and 13.2.

Separation of multiphosphorylated cyclopeptides and their positional isomers by hydrophilic interaction liquid chromatography (HILIC) coupled to electrospray ionization mass spectrometry (ESI-MS)

Peptides are efficient models used in different fields such as toxicology to study the interactions of several contaminants at the molecular scale, requiring the development of bio-analytical strategies. In this context, Hydrophilic interaction liquid chromatography (HILIC) coupled to electrospray ionization mass spectrometry (ESI-MS) was used to separate synthetic multiphosphorylated cyclopeptides and their positional isomers at physiological pH. We assessed (i) the selectivity of eleven HILIC columns, from different manufacturers and packed with diverse polar sorbents, and (ii) the effect of mobile phase composition on the separation selectivity. The best selectivity and baseline resolution were achieved with the columns grafted by neutral sorbents amide and diol. Furthermore, we investigated the HILIC retention mechanism of these peptides by examining the effect of the number of phosphorylated residues in the peptide scaffold on their retention. The peptide behavior followed the classical hydrophilic partitioning mechanism exclusively on amide and diol columns. This trend was not fully respected on bare and hybrid silica due to the attractive/repulsive interactions of the deprotonated surface silanol groups with the Arginine or Glutamate residues in the peptide scaffold according to the peptide sequence. The position of the phosphorylated amino acid in the peptide backbone also showed to have an impact on the retention, making possible the separation of positional isomers of these multiphosphorylated cyclic peptides using HILIC.

Performance evaluation of silica microspheres functionalized by different amine-ligands for hydrophilic interaction chromatography

In this work, for the first time five amine-ligands including mono-amine, di-amine, tri-amine, secondary and tertiary amine, were functionalized on mesoporous micro-silicas and developed as stationary phases for hydrophilic interaction liquid chromatography (HILIC). The investigations about the retention mechanisms, effects of different chromatographic conditions and stability were systematically conducted. Three kinds of polar and hydrophilic compounds (saccharides, sulfonamides, nucleosides and nucleobases) were selected as probe molecules to evaluate their separation performances. Among the five stationary phases, only aminopropyl-bonded silica has already gained wide developments and applications. Whereas, there are no related researches about the other four to be utilized as separation media. By a series of chromatographic evaluations, the results revealed the other four mesoporous micro-silica materials functionalized with di-amine, tri-amine, secondary and tertiary amine, had great potential to be explored as novel stationary phases of HILIC. Particularly, the two stationary phases functionalized with di-amine and tri-amine exhibited outstanding separation and retention abilities. This work offered some insights on the understanding of retention in HILIC mode and provided us possibility to explore other amine-based HILIC stationary phases.

An exploratory approach for an oriented development of an untargeted hydrophilic interaction liquid chromatography-mass spectrometry platform for polar metabolites in biological matrices

The analysis of polar metabolites based on liquid chromatography-mass spectrometry (LC-MS) methods should take into consideration the complexity of interactions in LC columns to be able to cover a broad range of metabolites of key biological pathways. Therefore, in this study, different chromatographic columns were tested for polar metabolites including reversed-phase and hydrophilic interaction liquid chromatography (HILIC) columns. Based on a column screening, two new generations of zwitterionic HILIC columns were selected for further evaluation. A tree-based method optimization was applied to investigate the chromatographic factors affecting the retention mechanisms of polar metabolites with zwitterionic stationary phases. The results were evaluated based on a scoring system which was applied for more than 80 polar metabolites with a high coverage of key human metabolic pathways. The final optimized methods showed high complementarity to analyze a wide range of metabolic classes including amino acids, small peptides, sugars, amino sugars, phosphorylated sugars, organic acids, nucleobases, nucleosides, nucleotides and acylcarnitines. Optimized methods were applied to analyze different biological matrices, including human urine, plasma and liver cell extracts using an untargeted approach. The number of high-quality features (< 30% median relative standard deviation) ranged from 3,755 for urine to 5,402 for the intracellular metabolome of liver cells, showing the potential of the methods for untargeted purposes.

A novel hydrophilic interaction liquid chromatography method for the analysis of arterolane; Isolation and structural elucidation of its major degradation product by LC-HRMS and NMR

Adequate retention of arterolane (ART) has not been published in the literature till date; the hydrophilic interaction liquid chromatographic (HILIC) method with improved retention and separation of arterolane from its degradation products are reporting here for the first time. The present study discusses the comparative retention of the drug-using Reverse Phase C₁₈ and HILIC columns, indicating the effective method (Syncronis HILIC-150 x 4.6 mm, 5μ). It was observed that the buffer concentration (ammonium acetate) in the mobile phase plays a crucial role in the retention of ART. Forced degradation studies of ART were conducted as per ICH Q1 (R2) prescribed conditions. The drug is not stable in hydrolytic (acid, base and neutral), oxidative and photolytic conditions and observed four unique degradation products (DPs). The optimized method for the analysis of degraded samples comprises of Liquid Chromatography-High Resolution Mass Spectrometry (LC-HRMS) technique having Agilent HILIC plus (100 × 4.6 mm, 3.5μ) column and acetonitrile (ACN) and 10 mM ammonium acetate 75:25 (% v/v) mobile phase with 0.4 mL/minute flow. Initially, the structure of all four DPs was proposed on the basis of accurate mass and their fragmentation pattern. The major degradation product (DP4) was isolated, and its structure was confirmed by HRMS and ¹H NMR, ¹³C NMR, HMBC, DEPT 135 and Deuteriated NMR spectroscopy. The formation of DPs might be due to the breakdown of (1 r,3r,5 r,7r)-2-methoxyadamantan-2-ol from ART (DP4), and subsequent diol formation at 1,2,4-trioxolane moiety (DP3).

A click tyrosine zwitterionic stationary phases for hydrophilic interaction liquid chromatography

New zwitterionic (ZIC) stationary phases (SPs) are synthesized with the click and conventional bonding of tyrosine to silica gel. Infrared spectra and elemental analysis demonstrate the successful click and conventional bonding of this ZIC group on silica particles by the surface coverage including 2.36 and 0.75 µm m^-2, respectively. Given the above-mentioned explanation, the present study evaluated the retention mechanism and chromatographic manners of polar compounds on these new materials under hydrophilic interaction liquid chromatography (HILIC) conditions. Based on the results, the Click-Tyrosine Stationary Phase provided good HILIC characteristics when it was applied to separate phenolic compounds, amino acids, alkaloids, and nucleobases compared to bare silica gel SP and even conventional tyrosine SPs. Further, this new Click-Tyrosine-SP represented appropriate HILIC features and column efficiency (the theoretical plate number was up to 50,000 plates m^-1 for thebaine). Furthermore, the study investigated the effect of solute polarity (the number of the hydroxyl group of phenolic compounds) and hydrophobicity (the number of the side chain of aliphatic amino acids) on retention behaviors. Finally, some important factors were studied as the potential variables for guiding the retention behavior of the polar compound in HILIC condition including solvent composition, salt concentration, and the buffer pH of the mobile phase.

Superhydrophilicity and strong salt-affinity: Zwitterionic polymer grafted surfaces with significant potentials particularly in biological systems

In recent years, zwitterionic polymers have been frequently reported to modify various surfaces to enhance hydrophilicity, antifouling and antibacterial properties, which show significant potentials particularly in biological systems. This review focuses on the fabrication, properties and various applications of zwitterionic polymer grafted surfaces. The "graft-from" and "graft-to" strategies, surface grafting copolymerization and post zwitterionization methods were adopted to graft lots type of the zwitterionic polymers on different inorganic/organic surfaces. The inherent hydrophilicity and salt affinity of the zwitterionic polymers endow the modified surfaces with antifouling, antibacterial and lubricating properties, thus the obtained zwitterionic surfaces show potential applications in biosystems. The zwitterionic polymer grafted membranes or stationary phases can effectively separate plasma, water/oil, ions, biomolecules and polar substrates. The nanomedicines with zwitterionic polymer shells have "stealth" effect in the delivery of encapsulated drugs, siRNA or therapeutic proteins. Moreover, the zwitterionic surfaces can be utilized as wound dressing, self-healing or oil extraction materials. The zwitterionic surfaces are expected as excellent support materials for biosensors, they are facing the severe challenges in the surface protection of marine facilities, and the dense ion pair layers may take unexpected role in shielding the grafted surfaces from strong electromagnetic field.

Selective enrichment of sialylated glycopeptides with mesoporous poly-melamine-formaldehyde (mPMF) material

Analysis of glycoprotein sialylation is challenging due to the relatively low abundance of sialylated glycopeptides (SGPs) in complex biosamples and low signals of SGPs in mass spectrometry. In this study, a mesoporous poly-melamine-formaldehyde (mPMF) polymer was prepared and utilized as the high-efficiency sorbent for SGPs. The mPMF polymer featured high surface area (755.4 m² g^-1) and high density of amine and triazine functional groups. This polymer demonstrated high enrichment selectivity (resistant to 100 molar fold interference of BSA) and superior adsorption capacity (560 mg g^-1) for SGPs. The high performance of mPMF toward SGPs ascribes to the unique physicochemical properties of mPMF and high density of accessible binding sites for glycopeptides. Further application of mPMF to HeLa S3 cell lysate resulted in 576 characterized glycopeptides with 218 unique glycosylation sites. This finding provides a new choice of promising extraction approach for characterization of protein glycosylation. Graphical abstract A mesoporous poly-melamine-formaldehyde (mPMF) polymer was prepared and utilized as the high-efficiency enrichment sorbent for sialylated glycopeptides (SGPs).

Peptide retention time prediction in hydrophilic interaction liquid chromatography: Zwitter-ionic sulfoalkylbetaine and phosphorylcholine stationary phases

Peptide retention time prediction models have been developed for zwitter-ionic ZIC-HILIC and ZIC-cHILIC stationary phases (pH 4.5 eluents) using proteomics-derived retention datasets of ~30 thousand tryptic peptides each. Overall, hydrophilicity of these stationary phases was found to be similar to the previously studied Amide HILIC phase, but lower compared to bare silicas. Peptide retention is driven by interactions of all charged (hydrophilic) residues at pH 4.5 (Asp, Glu, Arg, Lys, His), but shows specificity according to orientation of functional groups in zwitter-ionic pair. Thus, ZIC-cHILIC exhibits an increased contribution of negatively charged Asp and Glu due to the distal positioning of positively charged quaternary amines on the stationary phase. These findings confirm that HILIC interactions are driven by both peptide distribution between water layer adsorbed on the stationary phase and by interactions specific to functional groups of the packing material. Sequence-Specific Retention Calculator HILIC models were optimized for these columns showing 0.967-0.976 R²-values between experimental and predicted retention values. ZIC-HILIC separations represent a good choice as a first dimension in 2D LC-MS of peptide mixtures with correlations between retention values of ZIC-HILIC against RPLC found at 0.197 (ZIC-HILIC) and 0.137 (ZIC-cHILIC) R²-values, confirming a good orthogonality.

One-pot preparation of zwitterionic sulfoalkylbetaine monolith for rapid and efficient separation of lysozyme from egg white

A porous zwitterionic monolithic column was prepared to rapidly and efficiently separate lysozyme from egg white. The monolith was synthesized in a stainless steel HPLC column (5 cm × 4.6 mm i.d.) by in-situ thermal initiated co-polymerization of N,N-dimethyl-N-methacryloxyethyl-N-(3-sulfopropyl) ammonium betaine (MSA) and ethylene dimethacrylate (EDMA). Due to the combination of quaternary ammonium and sulfonic groups on the monolithic matrix in one-pot process, the hydrophobic carbon chain and hydrophilic radical were obtained, which provided multiple driving forces for neutral, basic and acidic analytes, thus mix-mode chromatography mechanism contributed to the retention of different charged proteins. Properties such as composition, morphology and stability of the MSA-co-EDMA monolithic column were characterized by various analytical methods and the results showed that the monolith has large through-pores, good hydrophilicity and permeability. The effects of mobile phase pH and ionic strength on proteins were investigated, drawing the conclusion that the main adsorption and elution mechanism of lysozyme on MSA-co-EDMA monolith was electrostatic interaction, while those of other proteins included hydrophobic, hydrophilic and electrostatic interactions. Therefore, efficient separation of lysozyme and other proteins could be successfully achieved by switching the pH of mobile phase. Lysozyme can be adsorbed using 20 mmol/L phosphate buffer (pH 7.0) and eluted with 20 mmol/L phosphate buffer (pH 2.0). To prove the practicality of the monolithic column, it was also applied in the separation of lysozyme in egg white, which means the work has the potential for further development in proteome analysis of real biological samples.

Interaction Mechanisms of Zwitterions with Opposite Dipoles in Aqueous Solutions

Zwitterionic groups have been widely used in antibiofouling surfaces to resist nonspecific adsorption of proteins and other biomolecules. The interactions among zwitterionic groups have attracted considerable attention in bioengineering, whereas the understanding of their nanomechanical mechanism still remains limited. In this work, the interaction mechanisms between two zwitterionic groups with opposite dipoles, i.e., phosphorylcholine (PC) and sulfobetaine (SB), have been investigated via direct force measurements using an atomic force microscope (AFM) and dynamic adsorption tests using the quartz crystal microbalance with dissipation monitoring technique (QCM-D) in aqueous solutions. The AFM force measurements show that the adhesive forces between contacted zwitterionic surfaces during separation in both symmetric and asymmetric configurations were close, mainly due to the enforced alignment of opposing dipole pairs via complementary orientations under confinement. The solution salinity and pH had almost negligible influence on the adhesion measured during surface separation. The QCM-D adsorption tests of PC-headed lipid on PC and SB surfaces showed some degree of adsorption of lipid molecules on the SB surface, whereas not on the PC surface. The different adsorption behaviors indicate that because the outermost negatively charged sulfonic group on the SB faced the aqueous solution, this configuration could facilitate it to form an attractive electrostatic interaction with the PC head of lipid molecules in the solution. This work shows that in addition to hydration and steric interactions, the zwitterionic dipole-induced interactions play an important role in the adhesion and antifouling behaviors of the zwitterionic molecules and surfaces. The improved fundamental understanding provides useful insights into the development of new functional materials and coatings with antifouling applications.

Systematic Comparison of Zwitterionic and Non-Zwitterionic Antifouling Polymer Brushes on a Bead-Based Platform

Nonspecific adsorption of biomolecules to solid surfaces, a process called biofouling, is a major concern in many biomedical applications. Great effort has been made in the development of antifouling polymer coatings that are capable of repelling the nonspecific adsorption of proteins, cells, and micro-organisms. In this respect, we herein contribute to understanding the factors that determine which polymer brush results in the best antifouling coating. To this end, we compared five different monomers: two sulfobetaines, a carboxybetaine, a phosphocholine, and a hydroxyl acrylamide. The antifouling coatings were analyzed using our previously described bead-based method with flow cytometry as the read-out system. This method allows for the quick and automated analysis of thousands of beads per second, enabling fast analysis and good statistics. We report the first direct comparison made between a sulfobetaine with opposite charges separated by two and three methylene groups and a carboxybetaine bearing two separating methylene groups. It was concluded that both the distance between opposite charges and the nature of the anionic groups have a distinct effect on the antifouling performance. Phosphocholines and simple hydroxyl acrylamides are not often compared with the betaines. However, here we found that they perform equally well or even better, yielding the following overall antifouling ranking: HPMAA ≥ PCMA-2 ≈ CBMAA-2 > SBMAA-2 > SBMAA-3 ≫ nonmodified beads (HPMAA being the best).

Direct analysis of chiral active pharmaceutical ingredients and their counterions by ultra high performance liquid chromatography with macrocyclic glycopeptide-based chiral stationary phases

In this work the simultaneous separation of chiral active pharmaceutical ingredients (API) in salt form from their counterions has been performed by using different high-efficiency macrocyclic glycopeptide-based chiral stationary phases (CSPs). Not only a new zwitterionic vancomycin-based CSP has been prepared (similarly to what was done for teicoplanin) but macrocyclic selectors have also been bonded to sub-2 μm fully porous silica particles through traditional ureidic linkage to obtain versions of CSPs suitable for ultra-high performance applications. The direct separation of chiral APIs and counterions is particularly attracting since it simplifies the workflow traditionally used with reduction of analysis time and costs. The wide selection of macrocyclic antibiotics CSPs now available has allowed to manage different cases that can happen in the simultaneous separation of APIs and their counterions (either cations or anions). Indeed, while inorganic cations are retained on traditional vancomycin- and teicoplanin-based CSPs, inorganic anions are almost unretained (due to Donnan's effect). On the other hand, cations and anions can be both retained on the zwitterionic versions of these CSPs. Afterwards, zwitterionic CSPs allowed the separation of other compounds including N-derivative amino-acids, profens, polyols, sugar anomers, oligosaccharides and inorganic anions/cations opening new perspectives in the use of this family of CSPs.

Preparation and evaluation of maltose modified polymer-silica composite based on cross-linked poly glycidyl methacrylate as high performance liquid chromatography stationary phase

A new maltose modified polymer-silica composite was fabricated and applied as high performance liquid chromatography (HPLC) stationary phase. The cross-linked poly glycidyl methacrylate (pGMA) layer was chemically bonded to the outer surface as well as pore inner surface of silica beads via in-situ polymerization, and then maltose was modified onto the polymer layer via a [3 + 2] "click" reaction. The porous spherical silica (4 μm diameter) with 300 Å pore size was selected as the matrix so that the 3.25 nm-thick polymer layer fabricated on the pore inner surface would not affect its permeability. The typical 'U-shape' retention curves indicated a mixed-mode retention mechanism of the as-synthesized stationary phase. Both polar and non-polar analytes could be well separated on the stationary phase with column efficiency reaching 123809 plates/m for guanosine in hydrophilic interaction liquid chromatography (HILIC) mode and 46808 plates/m for fluorene in reversed-phase liquid chromatography (RPLC) mode, respectively. Nucleotides and their bases were baseline separated with good peak shape without any buffer salt in mobile phase, suggesting the effective shielding of the silanol groups. The packing material also showed excellent chromatographic repeatability with intraday RSDs of the retention time of five nucleosides less than 0.048% (n = 3) and interday RSDs less than 0.33% (n = 7) and great pH stability (from 1.5 to 10.2). Finally, the stationary phase was applied to the separation of ginseng extract.

Hyperbranched Polyglycerol Functionalized Silica Stationary Phase for Hydrophilic Interaction Liquid Chromatography

Surface-initiated anionic-ring-opening multibranching polymerization was employed to prepare a hyperbranched polyglycerol (HPG) functionalized silica stationary phase for hydrophilic interaction liquid chromatography (HILIC). The obtained stationary phase was characterized by Fourier-transform infrared spectrometry (FT-IR) and thermogravimetric analysis (TGA). The chromatographic properties of the prepared stationary phase were systematically investigated. The abundance and multitude distribution of hydroxyl groups in HPG endowed the stationary phase with improved hydrophilicity and enhanced separation performance compared with the stationary phase functionalized with monolayer of hydroxyl groups. The stationary phase showed excellent retention of various polar compounds, such as nucleosides, necleobases, phenols and sulfanilamides, indicating great potential in the separation of complex biosamples.

Surface Active to Non-Surface Active Transition and Micellization Behaviour of Zwitterionic Amphiphilic Diblock Copolymers: Hydrophobicity and Salt Dependency

We have synthesized a range of zwitterionic amphiphilic diblock copolymers with the same hydrophilic block (carboxybetaine) but with different hydrophobic blocks (n-butylmethacrylate (n-BMA) or 2-ethylhexylacrylate (EHA)) by the reversible addition⁻fragmentation chain transfer (RAFT) polymerization method. Herein, we systematically examined the role of hydrophobicity and salt concentration dependency of surface activity and micellization behaviour of block copolymer. Transition from surface active to non-surface active occurred with increasing hydrophobicity of the hydrophobic block of block copolymer (i.e., replacing P(n-BMA) by PEHA). Foam formation of block copolymer slightly decreased with the similar variation of the hydrophobic block of block copolymer. Block copolymer with higher hydrophobicity preferred micelle formation rather than adsorption at the air⁻water interface. Dynamic light scattering studies showed that block copolymer having P(n-BMA) produced near-monodisperse micelles, whereas block copolymer composed of PEHA produced polydisperse micelles. Zimm plot results revealed that the value of the second virial coefficient (A₂) changed from positive to negative when the hydrophobic block of block copolymer was changed from P(n-BMA) to PEHA. This indicates that the solubility of block copolymer having P(n-BMA) in water may be higher than that of block copolymer having PEHA in water. Unlike ionic amphiphilic block copolymer micelles, the micellar shape of zwitterionic amphiphilic block copolymer micelles is not affected by addition of salt, with a value of packing parameters of block copolymer micelles of less than 0.3.

Optimization and Investigation of Zwitterionic Monolithic Stationary Phases for Capillary Ion Chromatography

Zwitterionic monolithic columns were synthesized by a one-pot reaction using [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide, ethylene dimethacrylate, methanol and 2,2'-azobis(isobutyronitrile) as the monomer, cross-linker, porogen and initiator, respectively. The optimum conditions for polymerization and the efficiency of the prepared columns were examined for ion chromatography. The separation of five kinds of inorganic anions was achieved. The back pressures were monitored as increasing flow-rate, and the resulting plate heights (i.e. height equivalent of a theoretical plate, HETP) of SCN^- were calculated at the inspected flow-rates. It was found that the increment rates of both the back pressure and HETP were rather slight. Mobile phases containing various cations or acid increased the retention times of the anions. Divalent cations could be separated, while monovalent cations could not be resolved due to their weak retention on the stationary phases.

The effect of charged groups on hydrophilic monolithic stationary phases on their chromatographic properties

In order to investigate the effect of charged groups present in hydrophilic monolithic stationary phases on their chromatographic properties, three charged hydrophilic monomers, i.e. N,N-dimethyl-N-acryloyloxyethyl-N-(3-sulfopropyl)ammonium betaine (SPDA), [2-(acryloyloxy)ethyl]trimethylammonium chloride (AETA), and 3-sulfopropyl acrylate potassium salt (SPA) were co-polymerized with the crosslinker N,N'-methylenebisacrylamide (MBA), respectively. The physicochemical properties of the three resulting charged hydrophilic monolithic columns were evaluated using scanning electron microscopy, ζ-potential analysis and micro-HPLC. High column efficiency was obtained on the three monolithic columns at a linear velocity of 1mm/s using thiourea as test compound. Comparative characterization of the three charged HILIC phases was then carried out using a set of model compounds, including nucleobases, nucleosides, benzoic acid derivatives, phenols, β-blockers and small peptides. Depending on the combination of stationary phase/mobile phase/solute, both hydrophilic interaction and other potential secondary interactions, including electrostatic interaction, hydrogen-bonding interaction, molecular shape selectivity, could contribute to the over-all retention of the analytes. Because of the strong electrostatic interaction provided by the quaternary ammonium groups in the poly (AETA-co-MBA) monolith, this cationic HILIC monolith exhibited the strongest retention for benzoic acid derivatives and small peptides with distorted peak shapes and the weakest retention for basic β-blockers. The sulfonyl groups on the poly (SPA-co-MBA) hydrophilic monolith could provide strong electrostatic attraction and hydrogen bonding for positively charged analytes and hydrogen-donor/acceptor containing analytes, respectively. Therefore, basic drugs, nucleobases and nucleotides exhibited the strongest retention on this anionic monolith. Because of the weak but distinct cation exchange properties of the zwitterionic poly (SPDA-co-MBA) hydrophilic monolith, it exhibited the best separation for most test analytes (including phenols, β-blockers and small peptides) in terms of selectivity, peak shape and analysis time. The poly (AETA-co-MBA) hydrophilic monolithic column provides the best separation of nucleobases and nucleosides. These results could guide the selection and application of these charged HILIC monoliths in the future.

Synthesis of Zwitterionic Polymer Particles via Combined Distillation Precipitation Polymerization and Click Chemistry for Highly Efficient Enrichment of Glycopeptide

Because of the low abundance of glycopeptide in natural biological samples, methods for efficient and selective enrichment of glycopeptides play a significant role in mass spectrometry (MS)-based glycoproteomics. In this study, a novel kind of zwitterionic hydrophilic interaction chromatography polymer particles, namely, poly(N,N-methylenebisacrylamide-co-methacrylic acid)@l-Cys (poly(MBAAm-co-MAA)@l-Cys), for the enrichment of glycopeptides was synthesized by a facile and efficient approach that combined distillation precipitation polymerization (DPP) and "thiol-ene" click reaction. In the DPP approach, residual vinyl groups explored outside the core with high density, then the functional ligand cysteine was immobilized onto the surface of core particles by highly efficient thiol-ene click reaction. Taking advantage of the unique structure of poly(MBAAm-co-MAA)@l-Cys, the resulting particles possess remarkable enrichment selectivity for glycopeptides from the tryptic digested human immunoglobulin G. The polymer particles were successfully employed for the analysis of human plasma, and 208 unique glycopeptides corresponding to 121 glycoproteins were reliably identified in triple independent nano-LC-MS/MS runs. The selectivity toward glycopeptides of these particles poly(MBAAm-co-MAA)@l-Cys is ∼2 times than that of the commercial beads. These results demonstrated that these particles had great potential for large-scale glycoproteomics research. Moreover, the strategy with the combination of DPP and thiol-ene click chemistry might be a facile method to produce functional polymer particles for bioenrichment application.

Zwitterionic hydrophilic interaction solid-phase extraction and multi-dimensional mass spectrometry for shotgun lipidomic study of Hypophthalmichthys nobilis

Zwitterionic hydrophilic interaction liquid chromatography (ZIC-HILIC) material was used as solid-phase extraction sorbent for purification of phospholipids from Hypophthalmichthys nobilis. The conditions were optimized to be pH 6, flow rate 2.0mL·min(-1), loading breakthrough volume ⩽5mL, and eluting solvent 5mL. Afterwards, the extracts were analyzed by multi-dimensional mass spectrometry (MDMS) based shotgun lipidomics; 20 species of phosphatidylcholine (PC), 22 species of phosphatidylethanoamine (PE), 15 species of phosphatidylserine (PS), and 5 species of phosphatidylinositol (PI) were identified, with content 224.1, 124.1, 27.4, and 34.7μg·g(-1), respectively. The MDMS method was validated in terms of linearity (0.9963-0.9988), LOD (3.7ng·mL(-1)), LOQ (9.8ng·mL(-1)), intra-day precision (<3.64%), inter-day precision (<5.31%), and recovery (78.8-85.6%). ZIC-HILIC and MDMS shotgun lipidomics are efficient for studying phospholipids in H. nobilis.

Preparation of phosphorylcholine-based hydrophilic monolithic column and application for analysis of drug-related impurities with capillary electrochromatography

A hydrophilic monolithic CEC column was prepared by thermal copolymerization of zwitterionic monomer 2-methacryloyloxyethyl phosphorylcholine (MPC), pentaerythritol triacrylate (PETA), either methacrylatoethyl trimethyl ammonium chloride (META) or sodium 2-methylpropene-1-sulfonate (MPS) in a polar binary porogen consisting of methanol and THF. A typical hydrophilic interaction LC retention mechanism was observed for low-molecular weight polar compounds including amides, nucleotides, and nucleosides in the separation mode of hydrophilic interaction CEC, when high content of ACN (>60%) was used as the mobile phase. The effect of the electrostatic interaction between the analytes and the stationary phase was found to be negligible. The poly(MPC-co-PETA-co-META or MPS) monolithic columns have an average column efficiency of 40 000 plates/m and displayed with a satisfactory repeatability in terms of migration time and peak areas. Finally, the column was successfully applied to determine the impurities of a positively charged drug pramipexole which are often separated by ion pair RP chromatography due to their high hydrophilicity. All four components can be baseline separated within 5 min with BGE consisting of ACN/20 mM ammonium formate buffer (pH 3.0; 80/20).

Sample preparation for mass spectrometric analysis of human serum N-glycans using hydrophilic interaction chromatography-based solid phase extraction

Expression levels of N-linked glycans derived from human serum glycoproteins have been shown to change during the progression of many diseases. Generally, N-glycans released from human serum proteins co-exist with endogenous serum peptides, salts, and other contaminants. Effective removal of these contaminants is essential to obtain the glycan profile of human serum proteins. Here, we developed a sample preparation method for mass spectrometry (MS) analysis of N-linked glycans derived from human serum glycoproteins based on a zwitterionic hydrophilic material named Click TE-Cys. The high hydrophilicity of Click TE-Cys, resulting from its unique surface structure and charge distribution, facilitated removal of co-existing salts and endogenous serum peptides. Furthermore, the present enrichment approach was handled in parallel, thus saving time. Using this method, a total of 47 unique N-glycans released from human serum proteins were identified. The intrabatch and interbatch coefficients of variation for the 47 N-linked glycans were 8.57% ± 0.96% and 9.22% ± 1.03%, respectively. These results demonstrate that the present method is suitable for fast purification of N-linked glycans derived from human serum glycoproteins, and has potential for clinical application.