Retention model for proteins in reversed-phase liquid chromatography

Thermodynamic analysis of adsorption and retention behaviors in normal-phase liquid chromatography

This study investigated the adsorption mechanisms in a normal-phase system using a cyano-based stationary phase as the sorbent. The minor disturbance method was used to measure the adsorption isotherms of acetone and alcohols with various structures. Excluding data in pure n-hexane revealed that the adsorption behaviors on cyano sites were well described by the Langmuir model. The adsorption equilibrium constants, ranging from 8.86 to 11.15 at 25 °C, showed no significant differences across alcohol structures and decreased with increasing temperature. The saturation adsorption concentration decreased with increasing alcohol molecule size, with branched-chain alcohols showing a lower saturation adsorption amount compared to straight-chain alcohols. The standard state adsorption enthalpies and entropies calculated from the equilibrium constants for various alcohols ranged from -29 to -22 kJ/mol and -78 to -55 J/K·mol, respectively, showing enthalpy-entropy compensation. A discrepancy was observed between these adsorption enthalpies and those obtained from the retention factors of alcohols using pure n-hexane as the mobile phase. This discrepancy may result from the affinity energy distribution of the adsorbent. In pure n-hexane, the adsorption behaviors of adsorbates were considerably affected by high-affinity sites. Moreover, acetone and these alcohol molecules were used as solvent modifiers to investigate the relationship between the retention factor, modifier concentration, and temperature for various solutes with distinct functional groups. The retention curves were converted to enthalpic curves using the van't Hoff equation. A theoretical model was proposed to describe the relationship between the van't Hoff enthalpy change and mobile phase composition. The proposed model effectively described the enthalpic curves, indicating that the enthalpy change follows a saturation curve with increasing modifier concentration. This trend is primarily due to competitive adsorption and complexation behaviors between the solute and modifier molecules.

Quantitative analysis of therapeutic proteins in biological fluids: recent advancement in analytical techniques

Pharmaceutical application of therapeutic proteins has been continuously expanded for the treatment of various diseases. Efficient and reliable bioanalytical methods are essential to expedite the identification and successful clinical development of therapeutic proteins. In particular, selective quantitative assays in a high-throughput format are critical for the pharmacokinetic and pharmacodynamic evaluation of protein drugs and to meet the regulatory requirements for new drug approval. However, the inherent complexity of proteins and many interfering substances presented in biological matrices have a great impact on the specificity, sensitivity, accuracy, and robustness of analytical assays, thereby hindering the quantification of proteins. To overcome these issues, various protein assays and sample preparation methods are currently available in a medium- or high-throughput format. While there is no standard or universal approach suitable for all circumstances, a liquid chromatography-tandem mass spectrometry (LC-MS/MS) assay often becomes a method of choice for the identification and quantitative analysis of therapeutic proteins in complex biological samples, owing to its high sensitivity, specificity, and throughput. Accordingly, its application as an essential analytical tool is continuously expanded in pharmaceutical R&D processes. Proper sample preparation is also important since clean samples can minimize the interference from co-existing substances and improve the specificity and sensitivity of LC-MS/MS assays. A combination of different methods can be utilized to improve bioanalytical performance and ensure more accurate quantification. This review provides an overview of various protein assays and sample preparation methods, with particular emphasis on quantitative protein analysis by LC-MS/MS.

Computer-aided design of proline-rich antimicrobial peptides based on the chemophysical properties of a peptide isolated from Olivancillaria hiatula

The chemophysical properties of a peptide isolated from Olivancillaria hiatula were combined with computational tools to design new antimicrobial peptides (AMPs). The in silico peptide design utilized arbitrary sequence shuffling, AMP sequence prediction and alignments such that putative sequences mimicked those of proline-rich AMPs (PrAMPs) and were potentially active against bacteria. Molecular modelling and docking experiments were used to monitor peptide binding to some intracellular targets like bacteria ribosome, DnaK and LasR. Peptide candidates were tested in vitro for antibacterial and antivirulence activities. Chemophysical studies of peptide extract suggested hydrophobic, acidic and proline-rich peptide properties. The amino acid signature of the extract matched that of AMPs that inhibit intracellular targets. Two of the designed PrAMP peptides (OhPrP-3 and OhPrP-5) had high affinity for the ribosome and DnaK. OhPrP-1, 2 and 4 also had favorable interactions with the biomolecular targets investigated. Peptides had bactericidal activity at the minimum inhibitory concentration against Pseudomonas aeruginosa. The designed peptides docked strongly to LasR suggesting possible interference with quorum sensing, and this was corroborated by in vitro data where sub-inhibitory doses of all peptides reduced pyocyanin and pyoverdine expression. The designed peptides can be further studied for the development of new anti-infective agents.

Ultra-short columns for the chromatographic analysis of large molecules

Today, reverse phase liquid chromatography (RPLC) analysis of proteins is almost exclusively performed on conventional columns (100-150 mm) in gradient elution mode. However, it was shown many years ago that large molecules present an on/off retention mechanism, and that only a very short inlet segment of the chromatographic column retains effectively the large molecules. Much shorter columns - like only a few centimetres or even a few millimetres - can therefore be used to efficiently analyse such macromolecules. The aim of this review is to summarise the historical and more recent works related to the use of very short columns for the analysis of model and therapeutic proteins. To this end, we have outlined the theoretical concepts behind the use of short columns, as well as the instrumental limitations and potential applications. Finally, we have shown that these very short columns were also possibly interesting for other chromatographic modes, such as ion exchange chromatography (IEX), hydrophilic interaction chromatography (HILIC) or hydrophobic interaction chromatography (HIC), as analyses in these chromatographic modes are performed in gradient elution mode.

High-Throughput Chromatographic Separation of Oligonucleotides: A Proof of Concept Using Ultra-Short Columns

Ion-pairing reversed-phase liquid chromatography (IP-RPLC) is the reference separation technique for characterizing oligonucleotides (ONs) and their related impurities. The aim of this study was to better understand the retention mechanism of ONs, evaluate the applicability of the linear solvent strength (LSS) retention model, and explore the potential of ultra-short columns having a length of only 5 mm for the separation of model ONs. First, the validity of the LSS model was evaluated for ONs having sizes comprised between 3 and 30 kDa, and the accuracy of retention time predictions was assessed. It was found that ONs in IP-RPLC conditions follow an "on-off" elution behavior, despite a molecular weight lower than that of proteins. For most linear gradient separation conditions, a column length between 5 and 35 mm was found to be appropriate. Ultra-short columns of only 5 mm were therefore explored to speed up separations by considering the impact of the instrumentation on the efficiency. Interestingly, the impacts of injection volume and post-column connection tubing on peak capacity were found to be negligible. Finally, it was demonstrated that longer columns would not improve selectivity or separation efficiency, but baseline separation of three model ONs mixtures was enabled in as little as 30 s on the 5 mm column. This proof-of-concept work paves the way for future investigations using more complex therapeutic ONs and their related impurities.

Heterogenous adsorption mechanisms for describing enantioselective retention in normal-phase liquid chromatography

In this study, the enantioselective retention behaviors of methyl mandelate (MM) and benzoin (B) were investigated using Chiralpak IB as a sorbent and ethanol, 1-propanol, and 1-butanol as solvent modifiers in the normal-phase mode. For both MM and B, similar chiral recognition mechanisms were observed, potentially involving at least two types of chiral adsorption sites. With a retention model describing local retention behaviors, an enantioselectivity model based on a three-site model was proposed to describe the data. Fitted parameters were also used to analyze the contributions of each type of adsorption site to the apparent retention behavior. Combining the local retention model with the three-site model provided a qualitative and quantitative explanation for the correlation between modifier concentration and enantioselectivity. Overall, our results indicated that heterogeneous adsorption mechanisms are a key aspect in understanding enantioselective retention behaviors. Distinct local adsorption sites contribute differently to apparent retention behaviors, with these contributions being influenced by the mobile phase composition to varying degrees. Hence, enantioselectivity changes with variations in modifier concentration.

Effect of stereoconfiguration of aromatic ligands on retention and selectivity of terphenyl isomer-bonded stationary phases

The structure of ligands has a significant influence on the separation properties of alkyl and aromatic phases in reversed-phase liquid chromatography. Compared with alkyl phases, the effect of stereoconfiguration of aromatic ligands on the retention and selectivity of stationary phases has rarely been addressed. To illustrate the issue, three terphenyl isomer-bonded stationary phases were prepared via the coupling chemistry of isocyanate with terphenyl amine isomers, 3,4-diphenylaniline, 2,4-diphenylaniline and 4-amino-p-terphenyl, respectively. The retention behaviors of stationary phases were assessed in terms of retention strength, selectivity, hydrophobic and π-π interaction by five kinds of solutes. It is found that the selectivity towards the solutes is slightly larger on the branched m-terphenyl-bonded phase (m-π³) than o-terphenyl-bonded phase (o-π³) but is significantly improved on the chain p-terphenyl-bonded phase (p-π³). The results can be interpreted by the ease self-adjustment of the conformation of the chain p-terphenyl ligand and the smaller steric effect of p-π³ towards the insertion of solutes into the ligand brushes. In addition, the p-π³ yields excellent selective separation towards aromatic solutes. These findings are of significance in the design of aromatic stationary phases.

Pressure-Enhanced Liquid Chromatography, a Proof of Concept: Tuning Selectivity with Pressure Changes and Gradients

Many chromatographers have observed that the operating pressure can dramatically change the chromatographic retention of solutes. Small molecules show observables changes, yet even more sizable effects are encountered with large biomolecules. With this work, we have explored the use of pressure as a method development parameter to alter the reversed-phase selectivity of peptide and protein separations. An apparatus for the facile manipulation of column pressure was assembled through a two-pump system and postcolumn flow restriction. The primary pump provided an eluent flow through the column, while the secondary pump provided a pressure-modulating flow at a tee junction after the column but ahead of a flow restrictor. Using this setup, we were able to quickly program various constant pressure changes and even pressure gradients. It was reconfirmed that pressure changes impact the retention of large molecules to a much greater degree than small molecules, making it especially interesting to consider the use of pressure to selectively separate solutes of different sizes. The addition of pressure to bring the column operating pressure beyond 500 bar was enough to change the elution order of insulin (a peptide hormone) and cytochrome C (a small serum protein). Moreover, with the proposed setup, it was possible to combine eluent and pressure gradients in the same analytical run. This advanced technique was applied to improve the separation of insulin from one of its forced degradation impurities. We have referred to this method as pressure-enhanced liquid chromatography and believe that it can offer unseen selectivity, starting with peptide and protein reversed-phase separations.

Development and evaluation of a hydrogenated rosin (β-acryloxyl ethyl) ester-bonded silica stationary phase for high-performance liquid chromatography separation of paclitaxel from yew bark

Paclitaxel (PTX) is a complex diterpenoid anticancer drug whose separation from yew biomass poses a significant challenge. In this study, a new stationary phase comprising hydrogenated rosin (β-acryloxyl ethyl) ester (HRE)-bonded silica (HRE@SiO₂) is developed to separate and purify PTX from crude yew-bark extract using high-performance liquid chromatography. In HRE@SiO₂, HRE molecules, which are functional ligands, are bonded to the surface of a silica gel matrix using a coupling agent, (3-mercaptopropyl)trimethoxysilane. The proposed HRE@SiO₂ stationary phase was characterized by Fourier-transform infrared spectroscopy, elemental analysis, thermogravimetric analysis, scanning electron microscopy, laser diffraction granulometry, and nitrogen gas adsorption. The HRE@SiO₂ column exhibited excellent chromatographic performance, satisfactory performance reproducibility, and typical reversed-phase chromatographic behavior. An HRE@SiO₂ column was used to separate PTX and its analogs, achieving resolutions exceeding 7.43 for consecutively eluted species. Stoichiometric displacement theory for retention (SDT-R), the van Deemter equation, and van 't Hoff plots were used to analyze the separation mechanism and properties of the HRE@SiO₂ column. The results showed that hydrophobic interactions determine the analyte retention and the separation of PTX and its analogs on an HRE@SiO₂ column is an exothermic process driven by enthalpy. Furthermore, an HRE@SiO₂ column was employed to separate and purify PTX from crude yew-bark extract, increasing PTX purity from 6% to 82%. The findings of this study provide insights for developing rosin-based stationary phases for the separation of natural products.

Retention modeling and adsorption mechanisms in reversed-phase liquid chromatography

To interpret the dependence of solute retention behavior on modifier content in reversed-phase liquid chromatography, a theoretical framework, based on the concentration dependence of solvophobic forces imposed on solutes and the competitive adsorptions of solutes and solvent modifiers, was proposed. The generality of the developed model was demonstrated by comparing the model with conventional retention models. The linear dependence of the Gibbs energy change of solute adsorption with respect to the modifier concentration was assumed, and the model was fitted to the experimental results, with good agreement demonstrated between the experimental data and the model. Retention behaviors were inferred to be determined by two key dimensionless groups that represented the reductions in the retention factors resulting from a weakened solvophobic interaction and modifier competitive adsorption. The retention behaviors were successfully deconvoluted for each contribution as a function of the modifier concentration by using the fitted parameters. The effects of both contributions on the retention behaviors were enhanced for the solutes with aromatic groups. The standard Gibbs energy change _SL^o of benzene adsorption was found to depend linearly on the number of modifier molecules present but independent of modifier identity. For the solutes associated with hydrogen-bonding groups, the degree of reduction in the solvophobic interactions was considerably reduced. Hence, the relative contributions of both mechanisms to solute retention depend greatly on the solute structure. Perturbation method was performed to investigate the modifier adsorption mechanisms. The results show that the standard Gibbs energy change _SL^o for the first-layer adsorption of modifiers changed linearly with the carbon number of modifier molecule. These results demonstrated that the proposed model can offer a physically consistent quantitative description of retention when solvent composition is varied.

pH-dependent selective separation of acidic and basic proteins using quaternary ammoniation functionalized cysteine-zwitterionic stationary phase with RPLC/IEC mixed-mode chromatography

In this paper, a cysteine-functionalized zwitterionic stationary phase (Cys-silica) was prepared based on the "thiol-ene" click chemistry between cysteine and vinyl-functionalized silica, and was further modified with bromoethane, 1-bromooctane and 1-bromooctadecane, respectively, to obtain a series of quaternary ammoniation-functionalized stationary phases (Cys-silica-C_n, n = 2, 8 and 18). These zwitterionic stationary phases were regarded as reversed-phase/ion-exchange (RP/IEC) mixed-mode chromatography (MMC) stationary phases for protein separation. The retention behaviors of proteins on these zwitterionic stationary phases were carefully investigated. The results indicated that the retentions of acidic and basic proteins on these zwitterinonic stationary phases were significantly influenced by the acetonitrile and salt concentrations, pH of mobile phase as well as the hydrophobicity of the ligand. The separation selectivity of proteins on these zwitterionic stationary phases strongly depended on the pH value of mobile phase. The baseline separation of 6 kinds of basic proteins can be achieved at pH 8.0 using Cys-silica-C2 or Cys-silica-C8 column, and 5 kinds of acidic proteins can also be separated completely at pH 4.0 with Cys-silica-C2 column. Moreover, owing to the quaternary ammoniation-functionalization on Cys-silica by using appropriately hydrophobic bromoalkanes, the selectivity and separation efficiency of proteins can be enhanced greatly. As a result, the acidic and basic proteins can be separated completely step by step from the complex sample by adjusting pH of mobile phase using a single Cys-silica-C2 column, which illustrates that the cysteine-functionalized zwitterionic stationary phase has a great potential for protein separation.

In silico method development for the reversed-phase liquid chromatography separation of proteins using chaotropic mobile phase modifiers

Recent advances in biomedical and pharmaceutical processes has enabled a notable increase of protein- and peptide-based drug therapies and vaccines that often contain a higher-order structure critical to their efficacy. Hyphenation of chromatographic and spectrometric techniques is at the center of all facets of biopharmaceutical analysis, purification and chemical characterization. Although computer-assisted chromatographic modeling of small molecules has reached a mature stage across the pharmaceutical industry, software-based method optimization approaches for large molecules has yet to see the same revitalization. Conformational changes of biomolecules under chromatographic conditions have been identified as the major culprit in terms of sub-optimal modeling outcomes. In order to circumvent these challenges, we herein investigate the outcomes generated via computer-assisted modeling from using different chaotropic and denaturing mobile phases (trifluoroacetic acid, sodium perchlorate and guanidine hydrochloride in acetonitrile/water-based eluents). Linear and polynomial regression retention models using ACD/Labs software were built as a function of gradient slope, column temperature and mobile phase buffer for eight different model proteins ranging from 12 to 670 kDa (holo-transferrin, cytochrome C, apomyoglobin, ribonuclease A, ribonuclease A type I-A, albumin, y-globulin and thyroglobulin bovine). Correlation between experimental and modeled outputs was substantially improved by using strong chaotropic and denaturing modifiers in the mobile phase, even when using linear regression modeling as typically observed for small molecules. On the contrary, the use of conventional TFA buffer concentrations at low column temperatures required the used of polynomial regression modeling indicating potential conformational structure changes of proteins upon chromatographic conditions. In addition, we illustrate the power of modern computer-assisted chromatography modeling combined with chaotropic agents in the developing of new RPLC assays for protein-based therapeutics and vaccines.

Cell membrane chromatography for the analysis of the interaction between chloroquine and hydroxychloroquine with ACE2 receptors

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CMC was used to study the interaction of CQ and HCQ with ACE2 receptors.

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The article provides new insights on the use of CQ and HCQ in treating with COVID-19.

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ACE2/CMC can be used to separate bioactive ingredients active towards ACE2 receptor.

The recent emergence of the novel pathogenic coronavirus disease 2019 (COVID-19) is responsible for a worldwide pandemic. In sight of this, there has been growing interest in the use of chloroquine (CQ) and hydroxychloroquine (HCQ) as potential treatments. In this study, we use angiotensin converting enzyme 2 (ACE2) over-expressed cell membrane chromatography (CMC) to study the interaction of CQ and HCQ with ACE2 receptor. Both CQ and HCQ were retained on the ACE2/CMC column. Then we analyzed the binding character of CQ and HCQ to ACE2 by CMC frontal analysis, ionic force investigation and competitive binding experiment. Results showed that CQ and HCQ KD values obtained from the CMC frontal analysis method were 8.22(±0.61) × 10⁻⁷ M and 11.70(±2.44) × 10⁻⁷ M. Compare to CQ, HCQ has the weaker affinity with ACE2. The action force of CQ, HCQ and ACE2 is mainly ionic force. CQ and HCQ have different degrees of competitive binding relationship with ACE2. Our study revealed the interaction of CQ and HCQ with ACE2 receptor, which provides new insights for the use of CQ and HCQ in the treatment of COVID-19. Moreover, this biomimetic drug screening method is expected to open the door for rapid targeting and separating bioactive ingredients active towards ACE2 receptor.

Effect of difluoroacetic acid and biological matrices on the development of a liquid chromatography-triple quadrupole mass spectrometry method for determination of intact growth factor proteins

In biological systems, variable protein expression is a crucial marker for numerous diseases, including cancer. The vast majority of liquid chromatography-triple quadrupole mass spectrometry-based quantitative protein assays use bottom-up methodologies, where proteins are subjected to proteolytic cleavage prior to analysis. Here, the effect of difluoroacetic acid and biological matrices on the developement of a multiple reaction monitoring based top-down reversed-phase liquid chromatography-triple quadrupole mass spectrometry method for analysis of cancer-related intact proteins was evaluated. Seven growth factors (5.5-26.5 kDa; isoelectric points: 4.6-9.9) were analyzed on a wide-pore C4 column. The optimized method was performed at 30°C, using a 0.2 mL/min flow rate, a 10 %B/min gradient slope, and 0.05% v/v difluoroacetic acid as a mobile phase modifier. The increase of mass spectrometry sensitivity due to the difluoroacetic acid (estimated limits of detection in biological matrices 1-500 ng/mL) significantly varied for proteins with lower and higher charge state distributions. Matrix effects, as well as the specificity of the method were assessed for variable biological samples and pretreatment methods. This work demonstrates method development to improve the ability to target intact proteins directly by more affordable triple quadrupole mass spectrometry instrumentation, which could be beneficial in many application fields.

Online Comprehensive High pH Reversed Phase × Low pH Reversed Phase Approach for Two-Dimensional Separations of Intact Proteins in Top-Down Proteomics

A proof-of-concept study is presented on the use of comprehensive two-dimensional liquid chromatography mass spectrometry (LC × LC-MS) for the separation of intact protein mixtures using a different mobile phase pH in each dimension. This system utilizes mass spectrometry (MS) friendly pH modifiers for the online coupling of high pH reversed phase liquid chromatography (HPH-RPLC) in the first dimension (¹D) followed by low pH reversed phase liquid chromatography (LPH-RPLC) in the second dimension (²D). Owing to the ionic nature of proteins, the use of a different mobile phase pH was successful to provide altered selectivity between the two dimensions, even for closely related protein variants, such as bovine cytochrome c and equine cytochrome c, which differ by only three amino acids. Subminute gradient separation of proteins in the second dimension was successful to minimize analysis time, while maintaining high peak capacity. Unlike peptides, the elution order of studied proteins did not follow their isoelectric points, where acidic proteins would be expected to be more retained at low pH (and basic proteins at high pH). The steep elution isotherms (on-off retention mechanism) of proteins and the very steep gradients utilized in the second-dimension column succeeded in overcoming pH and organic solvent content mismatch. The utility of the system was demonstrated with a mixture of protein standards and an Escherichia coli protein mixture.

Two-Dimensional Separation Using High-pH and Low-pH Reversed Phase Liquid Chromatography for Top-down Proteomics

Advancements in chromatographic separation are critical to in-depth top-down proteomics of complex intact protein samples. Reversed-phase liquid chromatography is the most prevalent technique for top-down proteomics. However, in cases of high complexities and large dynamic ranges, 1D-RPLC may not provide sufficient coverage of the proteome. To address these challenges, orthogonal separation techniques are often combined to improve the coverage and the dynamic range of detection. In this study, a "salt-free" high-pH RPLC was evaluated as an orthogonal dimension of separation to conventional low-pH RPLC with top-down MS. The RPLC separations with low-pH conditions (pH=2) and high-pH conditions (pH=10) were compared to confirm the good orthogonality between high-pH and low-pH RPLC's. The offline 2D RPLC-RPLC-MS/MS analyses of intact E. coli samples were evaluated for the improvement of intact protein identifications as well as intact proteoform characterizations. Compared to the 163 proteins and 328 proteoforms identified using a 1D RPLC-MS approach, 365 proteins and 886 proteoforms were identified using the 2D RPLC-RPLC top-down MS approach. Our results demonstrate that the 2D RPLC-RPLC top-down approach holds great potential for in-depth top-down proteomics studies by utilizing the high resolving power of RPLC separations and by using mass spectrometry compatible buffers for easy sample handling for online MS analysis.

Study of structure-dependent chromatographic behavior of glycopeptides using reversed phase nanoLC

Analysis of glycosylation is challenging due to micro- and macro-heterogeneity of the protein attachment. A combination of LC with MS/MS is one of the most powerful tools for glycopeptide analysis. In this work, we show the effect of various monosaccharide units on the retention time of glycopeptides. Retention behavior of several glycoforms of six peptides obtained from tryptic digest of haptoglobin, hemopexin, and sex hormone-binding globulin was studied on a reversed phase chromatographic column. We observed reduction of the retention time with increasing number of monosaccharide units of glycans attached to the same peptide backbone. Fucosylation of larger glycans provides less significant retention time shift than for smaller ones. Retention times of glycopeptides were expressed as relative retention times. These relative retention times were used for calculation of upper and lower limits of glycopeptide retention time windows under the reversed phase conditions. We then demonstrated on the case of a glycopeptide of haptoglobin that the predicted retention time window boosts confidence of identification and minimizes false-positive identification. Relative retention time, as a qualitative parameter, is expected to improve LC-MS/MS characterization of glycopeptides.

Bioanalysis of bevacizumab and infliximab by high-temperature reversed-phase liquid chromatography with fluorescence detection after immunoaffinity magnetic purification

This study presents two simple and rapid methods for the quantification of therapeutic mAbs based on LC. Two mAbs (bevacizumab and infliximab) in plasma samples were purified using magnetic beads immobilized with a commercially-available idiotype antibody for each mAb. Purified mAbs were separated with HT-RPLC and detected with their native fluorescence. Using immunoaffinity beads, each mAb was selectively purified and detected as a single peak in the chromatogram. The HT-RPLC achieved good separation for the mAbs with sharp peaks within 20 min. The calibration curves of the two mAbs ranged from 1 to 20 μg mL(-1) (bevacizumab) and 1-10 μg mL(-1) (infliximab), and they had strong correlation coefficients (r(2) > 0.998). The LOD of bevacizumab and infliximab was 0.07 and 0.15 μg mL(-1), and the LLOQ of bevacizumab and infliximab was 0.12 and 0.25 μg mL(-1), respectively. Thus, the sensitivities were sufficient for clinical analysis. Immunoaffinity purification with HT-RPLC produced a selective and accurate bioanalysis without an LC-MS/MS instrument. Both methods could become general-purpose analytical methods and complement the results obtained with conventional LBA.

Two variables dominating the retention of intact proteins under gradient elution with simultaneous ultrafast high-resolution separation by hydrophobic interaction chromatography

The retention of proteins under gradient elution in HIC is dominated by two variables of steady and migration regions.

Recovery of Proteins Affected by Mobile Phase Trifluoroacetic Acid Concentration in Reversed-Phase Chromatography

It was found that recoveries of proteins depend on trifluoroacetic acid concentration in the mobile phase and showed maximum in the range of 0.01-0.1 v/v%. Transferrin and lysozyme were used to evaluate the recoveries of proteins from dedicated reversed-phase columns. Different types of reversed-phase columns were evaluated, such as core shell type materials (Aeris Widepore with C4, C8 and C18 modification) as well as fully porous hybrid particles (Waters BEH, modified with C4 and C18 alkyl chains). Recoveries ranged between 60.7-95.2% for transferrin and 72.1-99.8% for lysozyme. Based on the data presented, at least two different adsorption effects, the well-known hydrophobic and silanophilic/polar interaction might influence the recovery. In addition to this, conformational effects due to ion pairing with the acidic mobile phase additive might change them.

Protein purification by aminosquarylium cyanine dye-affinity chromatography

The most selective purification method for proteins and other biomolecules is affinity chromatography. This method is based on the unique biological-based specificity of the biomolecule-ligand interaction and commonly uses biological ligands. However, these ligands may present some drawbacks, mainly because of their cost and lability. Dye-affinity chromatography overcomes the limitations of biological ligands and is widely used owing to the low cost of synthetic dyes and to their resistance to biological and chemical degradation. In this work, immobilized aminosquarylium cyanine dyes are used in order to exploit affinity interactions with standard proteins such as lysozyme, α-chymotrypsin and trypsin. These studies evaluate the affinity interactions occurring between the immobilized ligand and the different proteins, as a reflection of the sum of several molecular interactions, namely ionic, hydrophobic and van der Waals, spread throughout the structure, in a defined spatial manner. The results show the possibility of using an aminosquarylium cyanine dye bearing a N-hexyl pendant chain, with a ligand density of 1.8 × 10(-2) mmol of dye/g of chromatographic support, to isolate lysozyme, α-chymotrypsin and trypsin from a mixture. The application of a decreasing ammonium sulfate gradient resulted in the recovery of lysozyme in the flowthrough. On the other hand, α-chymotrypsin and trypsin were retained, involving different interactions with the ligand. In conclusion, this study demonstrates the potential applicability of ligands such as aminosquarylium cyanine dyes for the separation and purification of proteins by affinity chromatography.

Peptide-column interactions and their influence on back exchange rates in hydrogen/deuterium exchange-MS

Hydrogen/deuterium exchange (HDX) methods generate useful information on protein structure and dynamics, ideally at the individual residue level. Most MS-based HDX methods involve a rapid proteolytic digestion followed by LC/MS analysis, with exchange kinetics monitored at the peptide level. Localizing specific sites of HDX is usually restricted to a resolution the size of the host peptide because gas-phase processes can scramble deuterium throughout the peptide. Subtractive methods may improve resolution, where deuterium levels of overlapping and nested peptides are used in a subtractive manner to localize exchange to smaller segments. In this study, we explore the underlying assumption of the subtractive method, namely, that the measured back exchange kinetics of a given residue is independent of its host peptide. Using a series of deuterated peptides, we show that secondary structure can be partially retained under quenched conditions, and that interactions between peptides and reversed-phase LC columns may both accelerate and decelerate residue HDX, depending upon peptide sequence and length. Secondary structure is induced through column interactions in peptides with a solution-phase propensity for structure, which has the effect of slowing HDX rates relative to predicted random coil values. Conversely, column interactions can orient random-coil peptide conformers to accelerate HDX, the degree to which correlates with peptide charge in solution, and which can be reversed by using stronger ion pairing reagents. The dependency of these effects on sequence and length suggest that subtractive methods for improving structural resolution in HDX-MS will not offer a straightforward solution for increasing exchange site resolution.