Chromatographic analysis of biomolecules with pressurized carbon dioxide mobile phases - A review

Fundamental investigation of impact of water and TFA additions in peptide sub/supercritical fluid separations

The retention of three peptides was studied under analytical and overloaded conditions at different concentrations of trifluoroacetic acid (TFA) and water added to the co-solvent methanol (MeOH). Four columns with different stationary phase properties, i.e., silica, diol, 2-ethylpyridine and cyanopropyl (CN) columns, were evaluated in this investigation. The overall aim was to get a deeper understanding on how column chemistry as well as water and TFA in the co-solvent affect the analytical and overloaded elution profiles using multivariate design of experiments and adsorption measurements of co-solvent components. Multivariate experimental design modeling indicated that water had on average around five times higher effect on the retention than the addition of TFA. The results also showed that the retention increases with the addition of TFA and water to the co-solvent on all columns except the CN column, on which the retention decreased. When examining the effect of adding water to the co-solvent, evidence of a hydrophilic interaction liquid chromatography (HILIC)-like retention mechanism was found on the three other columns with more polar stationary phases. However, on the CN column water acted as an additive, decreasing the retention due to competition with the peptide for available adsorption surface. Adsorption isotherm measurements of the polar solvent MeOH showed that MeOH adsorbs much weaker on the CN column than on the other columns. Addition of TFA and water to the co-solvent substantially sharpened the elution profiles under both overloaded and analytical conditions. Adding a small amount of TFA (from 0 % to 0.05 %) to the co-solvent substantially improved the peak shape of the elution profiles, while further addition (from 0.05 % to 0.15 %) had only a minor effect on the elution profile shape. The reduced retention on the CN column could not be explained by TFA adsorption, which was very weak on all studied columns (retention factor, 0.05-0.15). One could therefore speculate that the ion-pairing complex formed between the peptide and TFA in the mobile phase, reduce the retention due to its reduced polarity. On the other columns displaying HILIC-like properties, the TFA probably just decreased the pH of the mobile phase, thereby promoting the partitioning of the peptide into the water-rich layer. Finally, peak deformation due to diluent-eluent mismatch was observed under overloaded conditions. This was most severe in the cases where MeOH adsorption to the stationary phase was strong and the peptides were only mildly retained. Adding 1,4-dioxan to the diluent resolved this issue.

Large-scale purification of a deprotected macrocyclic peptide by supercritical fluid chromatography (SFC) integrated with liquid chromatography in discovery chemistry

A macrocyclic peptide A was successfully purified in large quantities (∼30 g) in >95 % purity by an integrated two-step orthogonal purification process combining supercritical fluid chromatography (SFC) with medium-pressure reverse-phase liquid chromatography (MP-RPLC). MP-RPLC was used to fractionate the crude peptide A, remove unwanted trifluoroacetic acid (TFA) originating from the peptide A cleavage off the resin, and convert the peptide A into ammonium acetate salt form, prior to the final purification by SFC. A co-solvent of methanol/acetonitrile containing ammonium acetate and water in CO2 was developed on a Waters BEH 2-Ethylpyridine column. The developed SFC method was readily scaled up onto a 5 cm diameter column to process multi-gram quantities of the MP-RPLC fraction to reach > 95 % purity with a throughput/productivity of 0.96 g/h. The incorporation of SFC with MP-RPLC has been demonstrated to have a broader application in other large-scale polypeptide purifications.

Comprehensive Single-Platform Lipidomic/Metabolomic Analysis Using Supercritical Fluid Chromatography-Mass Spectrometry

Supercritical fluid chromatography (SFC) is a rapidly expanding technique in the analysis of nonpolar to moderately polar substances and, more recently, also in the analysis of compounds with higher polarity. Herein, we demonstrate a proof of concept for the application of a commercial SFC instrument with electrospray ionization-mass spectrometry (MS) detection as a platform for the comprehensive analysis of metabolites with the full range of polarities, from nonpolar lipids up to highly polar metabolites. The developed single-platform SFC-MS lipidomic/metabolomic method is based on two consecutive injections of lipid and polar metabolite extracts from biphase methyl tert-butyl ether extraction using a diol column and two different gradient programs of methanol-water-ammonium formate modifier. Detailed development of the method focused mainly on the pressure limits of the system, the long-term repeatability of results, and the chromatographic performance, including optimization of the flow rate program, modifier composition and gradient, and injection solvent selection. The developed method enabled fast and comprehensive analysis of lipids and polar metabolites from plasma within a 24 min cycle with two injections using a simple analytical platform based on a single instrument, column, and mobile phase. Finally, the results from SFC-MS analysis of polar metabolites were compared with widely established liquid chromatography MS analysis in metabolomics. The comparison showed different separation selectivity of metabolites using both methods and overall lower sensitivity of the SFC-MS due to the higher flow rate and worse chromatographic performance.

Controlling the elution order of insulin and its analogs in sub-/supercritical fluid chromatography using methanesulfonic acid and 18-crown-6 as mobile phase additives

The purity analysis of therapeutic peptides can often be challenging, demanding the application of more than a single analytical technique. Supercritical fluid chromatography nowadays is a promising alternative to reversed-phase liquid chromatography, providing orthogonal and complementary information. This study investigated its applicability for the separation of human insulin, its analogs and degradation products. A previously published method development protocol for peptides up to 2000 Da was successfully applied to the higher molecular weight insulins (6 kDa). A single gradient method was optimized for all insulins using a Torus DEA column (100 × 3.0 mm, 1.7 μm), carbon dioxide and a modifier consisting of methanol/acetonitrile/water/methanesulfonic acid (65:35:2:0.1, v/v/v/v). Consecutively, the crown ether 18-crown-6, which is well known to complex charged lysine sidechains and other amino functionalities, was added to the modifier to evaluate its impact on selectivity. A decreased retention and a shift in the elution order for the insulins were observed. An inverse effect on retention was found when combined with a neutral stationary phase chemistry (Viridis BEH).

Applicability of supercritical fluid chromatography for oligonucleotide analysis: A proof-of-concept study

We evaluated the suitability of supercritical fluid chromatography (SFC) for oligonucleotide analysis using 4-mer oligonucleotides with various phosphorothioate (PS) contents as model compounds. Column screening showed that the diol-modified column was able to separate sequences with different PS contents. Optimization of the column body and additives allowed us to analyze polar oligonucleotides using SFC. Various sequences were also analyzed using the optimized method. A good peak shape was obtained when the guanine plus cytosine content of the analyte was two or less in the 4-mer oligonucleotides. Furthermore, we found that the retention times of the selected sequences were positively correlated with polar surface areas, indicating that oligonucleotides interact with polar stationary phases. In contrast, more hydrophobic full PS sequences were retained more strongly in the diol column than the full phosphodiester (PO) sequences. This suggests that the diol column has unique selectivity for PO and PS linkages. These results indicate that SFC is potentially applicable to oligonucleotide analysis with a separation mechanism that is different from that of ion-pair reversed-phase liquid chromatography.

Ternary eluent compositions in supercritical fluid chromatography improved fingerprinting of therapeutic peptides

Currently, little information has been published on the application of ternary eluent compositions in supercritical fluid chromatography for separating peptides. This work investigates the benefits of adding acetonitrile to methanol as the modifier. Three cyclic antibiotic peptides (bacitracin, colistin, and daptomycin) ranging between 1000 and 2000 Da were chosen as model substances. The ternary mixture of carbon dioxide, methanol, and acetonitrile is optimized to increase the resolution of the peptide's fingerprint. In addition, varying compositions of methanol and acetonitrile were found to change the elution order of the analytes, which is a valuable tool during method development. An individual gradient method using two Torus 2-PIC columns (each 100 × 3.0 mm, 1.7 μm), carbon dioxide, and a modifier consisting of acetonitrile/methanol/water/methanesulfonic acid (60:40:2:0.1, v:v:v:v) was optimized for each of the peptides. Subsequently, a generic method development protocol applicable to polypeptides is proposed.

Clinical application of serum-based proteomics technology in human tumor research

The rapid development of proteomics technology in the past decades has led to further human understanding of tumor research, and in some ways, the technology plays a very important supporting role in the early detection of tumors. Human serum has been shown to contain a variety of proteins closely related to life activities, and the dynamic change in proteins can often reflect the physiological and pathological conditions of the body. Serum has the advantage of easy extraction, so the application of proteomics technology in serum has become a hot spot and frontier area for the study of malignant tumors. However, there are still many difficulties in the standardized use of proteomic technologies, which inevitably limit the clinical application of proteomic technologies due to the heterogeneity of human proteins leading to incomplete whole proteome populations, in addition to most serum protein markers being now not highly specific in aiding the early detection of tumors. Nevertheless, further development of proteomics technologies will greatly increase our understanding of tumor biology and help discover more new tumor biomarkers with specificity that will enable medical technology.

Sub/supercritical Fluid Chromatography Purification and Desalting of a Cyclic Dinucleotide STING Agonist

An efficient and "endotoxin-free" purification of a cyclic dinucleotide (CDN) STING agonist was achieved to produce multigram quantities of pure BMT-390025, an active pharmaceutical ingredient (API), for toxicological studies. A two-step sub/supercritical fluid chromatography (SFC) procedure was developed for the achiral purification and desalting of the polar ionic CDN. A robust SFC process employing methanol-acetonitrile-water with ammonium acetate as co-solvent in CO₂ on BEH 2-ethylpyridine was established and scaled up as the first step to achieve a successful purification. The desalting/salt-switching (i.e. removing acetate and acetamide) was conducted using methanol-water with ammonium hydroxide as co-solvent on the same column in the second step to convert the final API to the ammonium salt. Water with additive was essential to eliminating salt precipitation and improving the peak shape and resolution. Due to the extreme hydrophilicity of BMT-390025, 65% of co-solvent was needed to adequately elute the target in both steps. More than 40 g of crude API was purified and desalted producing >20 g of pure BMT-390025 as the ammonium salt which was obtained with a chemical purity of >98.5% and met the endotoxin requirement of <0.1 EU/mg. In addition, >80 g of its penultimate prior to the deprotection of the silyl group was purified at a high throughput of 6.3 g/h (0.42 g/day/g SP).