Preparative separation of components of the color additive D&C Red No. 28 (phloxine B) by pH-zone-refining counter-current chromatography

Preparative separation and identification of novel subsidiary colors of the color additive D&C Red No. 33 (Acid Red 33) using spiral high-speed counter-current chromatography

Three low-level subsidiary color impurities (A, B, and C) often present in batches of the color additive D&C Red No. 33 (R33, Acid Red 33, Colour Index No. 17200) were separated from a portion of R33 by spiral high-speed counter-current chromatography (HSCCC). The separation involved use of a very polar solvent system, 1-BuOH/5mM aq. (NH4)2SO4. Addition of ammonium sulfate to the lower phase forced partition of the components into the upper phase, thereby eliminating the need to add a hydrophobic counterion as was previously required for separations of components from sulfonated dyes. The very polar solvent system used would not have been retained in a conventional multi-layer coil HSCCC instrument, but the spiral configuration enabled retention of the stationary phase, and thus, the separation was possible. A 1g portion of R33 enriched in A, B, and C was separated using the upper phase of the solvent system as the mobile phase. The retention of the stationary phase was 38.1%, and the separation resulted in 4.8 mg of A of >90% purity, 18.3mg of B of >85% purity, and 91 mg of C of 65-72% purity. A second separation of a portion of the C mixture resulted in 7 mg of C of >94% purity. The separated impurities were identified by high-resolution mass spectrometry and NMR spectroscopic techniques as follows: 5-amino-3-biphenyl-3-ylazo-4-hydroxy-naphthalene-2,7-disulfonic acid, A; 5-amino-4-hydroxy-6-phenyl-3-phenylazo-naphthalene-2,7-disulfonic acid, B; and 5-amino-4-hydroxy-3,6-bis-phenylazo-naphthalene-2,7-disulfonic acid, C. The isomers A and B are compounds reported for the first time. Application of the spiral HSCCC method resulted in the additional benefit of yielding 930 mg of the main component of R33, 5-amino-4-hydroxy-3-phenylazo-naphthalene-2,7-disulfonic acid, of >97% purity.

Preparative separation of six rhynchophylla alkaloids from Uncaria macrophylla wall by pH-zone refining counter-current chromatography

pH-Zone refining counter-current chromatography was successfully applied to the preparative isolation and purification of six alkaloids from the ethanol extracts of Uncaria macrophylla Wall. Because of the low content of alkaloids (about 0.2%, w/w) in U. macrophylla Wall, the target compounds were enriched by pH-zone refining counter-current chromatography using a two-phase solvent system composed of petroleum ether–ethyl acetate–isopropanol–water (2:6:3:9, v/v), adding 10 mM triethylamine in organic stationary phase and 5 mM hydrochloric acid in aqueous mobile phase. Then pH-zone refining counter-current chromatography using the other two-phase solvent system was used for final purification. Six target compounds were finally isolated and purified by following two-phase solvent system composed of methyl tert-butyl ether (MTBE)–acetonitrile–water (4:0.5:5, v/v), adding triethylamine (TEA) (10 mM) to the organic phase and HCl (5 mM) to aqueous mobile phase. The separation of 2.8 g enriched total alkaloids yielded 36 mg hirsutine, 48 mg hirsuteine, 82 mg uncarine C, 73 mg uncarine E, 163 mg rhynchophylline, and 149 mg corynoxeine, all with purities above 96% as verified by HPLC Their structures were identified by electrospray ionization-mass spectrometry (ESI-MS) and ¹H-NMR spectroscopy.

Preparation of two novel monobrominated 2-(2',4'-dihydroxybenzoyl)-3,4,5,6-tetrachlorobenzoic acids and their separation from crude synthetic mixtures using vortex counter-current chromatography

The present work describes the preparation of two compounds considered to be likely precursors of an impurity present in samples of the color additives D&C Red No. 27 (Color Index 45410:1) and D&C Red No. 28 (Color Index 45410, phloxine B) submitted to the U.S. Food and Drug Administration for batch certification. The two compounds, 2-(2',4'-dihydroxy-3'-bromobenzoyl)-3,4,5,6-tetrachlorobenzoic acid (3BrHBBA) and its 5'-brominated positional isomer (5BrHBBA), both not reported previously, were separated from synthetic mixtures by vortex counter-current chromatography (VCCC). 3BrHBBA was prepared by chemoselective ortho-bromination of the dihydroxybenzoyl moiety. Two portions of the obtained synthetic mixture, 200mg and 210 mg, respectively, were separated by VCCC using two two-phase solvent systems that consisted of hexane-ethyl acetate-methanol-aqueous 0.2% trifluoroacetic acid (TFA) in the volume ratios of 8:2:5:5 and 7:3:5:5, respectively. These separations produced 35 mg and 78 mg of 3BrHBBA, respectively, each product of over 98% purity by HPLC at 254 nm. 5BrHBBA was prepared by monobromination of the dihydroxybenzoyl moiety in the presence of glacial acetic acid. To separate the obtained synthetic mixture, VCCC was performed in the pH-zone-refining mode with a solvent system consisting of hexane-ethyl acetate-methanol-water (6:4:5:5, v/v) and with TFA used as the retainer acid and aqueous ammonia as the eluent base. Separation of a 1-g mixture under these conditions resulted in 142 mg of 5BrHBBA of ∼ 99% purity by HPLC at 254 nm. The isolated compounds were characterized by high-resolution mass spectrometry and proton nuclear magnetic resonance spectroscopy.

Separation of pyridine derivatives from synthetic mixtures by pH-zone-refining counter-current chromatography

Three novel pyridine derivatives were successfully separated from their partially purified synthetic mixtures by pH-zone-refining counter-current chromatography using a multilayer coil planet centrifuge designed in our laboratory. After equilibration of a two-phase solvent system consisting of methyl tert-butyl ether-tetrahydrofuran-water at an optimized volume ratio of 4:6:7, triethylamine (10 mM) was added to the organic phase as a retainer and hydrochloric acid (10 mM) to the aqueous phase as an eluter. Separation runs of 1.20, 1.16, and 1.15 g of three samples yielded three pyridine derivatives in weights of 306, 255, and 314 mg at a high purity of over 98.5, 99.0, and 98.2% (determined by HPLC), respectively. The structures were identified by electron impact mass spectrometry and (1)H NMR.

Preparative separation of isomeric and stereoisomeric dicarboxylic acids by pH-zone-refining counter-current chromatography

This work involves the preparative separation of some isomeric dicarboxylic acids using pH-zone-refining counter-current chromatography (CCC), a relatively new preparative technique for the separation of ionizable compounds. The paper concentrates especially on the separation of a synthetic mixture of closely related cis and trans pairs of 1-methyl- and 1,3-dimethyl-1,3-cyclohexanedicarboxylic acids. The elution sequence of the isomers is discussed in terms of their relative acidities (pK(a) values) in solution and gas phase, hydrophobicities, and steric configuration. Two possible explanations are suggested for the mechanism of separation. They both involve the amount of retainer acid used, as it affects the separation and plays a role in the chemohydrodynamic equilibrium of the dicarboxylic acids in the column.

Identification of the decarboxylated analog of tetrabromotetrachloro-fluorescein and its quantification in the color additives D&C Red Nos. 27 and 28 (phloxine B) using high-performance liquid chromatography

The present work describes (a) the identification and characterization of an impurity, 2,4,5,7-tetrabromo-6-hydroxy-9-(2,3,4,5-tetrachlorophenyl)-3H-xanthen-3-one (BCPX), in the color additives D&C Red Nos. 27 and 28 (phloxine B) and (b) the determination of the extent and level of BCPX contamination in certified lots of these colors. For these purposes, BCPX (a compound not previously reported in the literature) was synthetically prepared. Test portions from 42 certified lots of D&C Red Nos. 27, 28 and 27 lakes were analyzed for BCPX using an HPLC method that included gradient elution and UV-vis photodiode array detection. Those lots were submitted for certification by both domestic (six) and foreign (six) manufacturers during the past 4 years. Of the test portions analyzed, 32 (76.2%) contained BCPX in amounts ranging from 0.01 to 3.21%. The remaining 10 test portions (23.8%) contained no detectable BCPX or less than 0.008%, which is the limit of quantification for the present method. The analyses revealed substantial differences in the level of BCPX across different manufacturers. The wide range of BCPX levels found in the analyzed lots suggests that the presence of BCPX in D&C Red Nos. 27 and 28 may be avoided or significantly reduced during the manufacturing process.

In vitro antibacterial activities of phloxine B and other halogenated fluoresceins against methicillin-resistant Staphylococcus aureus

Fluorescein dyes in which the benzoic acid moiety has been tetrachlorinated (50 to 100 micro g/ml) inhibit in vitro Staphylococcus aureus growth (MIC, 25 micro g/ml). Specifically, under standard room illumination, phloxine B at a concentration of 100 micro g/ml killed 99% of the cultures (mid-log phase). It also reduced S. aureus CFU by 10(4). Structure-activity analysis revealed that the activity against S. aureus increases with the increase in the number of the substituting halogens in the hydroxyxanthene moiety.

Lack of mutagenicity of a red food colour impurity (1-carboxyl-5,7-dibromo-6-hydroxy-2,3,4-trichloroxanthone) in phloxine B

To evaluate the mutagenicity of 1-carboxyl-5,7-dibromo-6-hydroxy-2,3,4-trichloroxanthone (HXCA), which is an impurity present in Food Red No. 104 (FR104, Phloxine B, the Japanese counterpart of D&C Red No. 28), HXCA was isolatedfrom FR104 using pH-zone-refining counter-current chromatography and preparative HPLC. A large amount of HXCA was synthesized to perform the Ames test, and its identity was confirmed by comparison of its HPLC retention time, UV-Vis, MS and NMR spectra with those of HXCA isolated from FR104. The results of the Ames test using synthetic HXCA showed that it did not possess mutagenic activity. The results of the mutagenicity test for HXCA will provide useful information for the establishment of an upper limit for HXCA in FR104 (or D&C Red No. 28) for use in regulatory considerations.

Purification of Food Color Red No. 106 (acid red) using pH-zone-refining counter-current chromatography

pH-Zone-refining counter-current chromatography was successfully applied to the separation of the main components of Food Color Red No. 106 (R-106, acid red, Color Index No. 45100). A 300-mg quantity of sample was separated using the following two-phase solvent system: n-butanol-water, 40 mM sulfuric acid in organic stationary phase and 30 mM ammonia in aqueous mobile phase. The obtained fractions were analyzed by high-performance liquid chromatography and fast atom bombardment mass spectrometry. The separation yielded 261.9 mg of main component of acid red with purity of 99.9%.

Preparative separation of isomeric 2-(2-quinolinyl)-1H-indene-1,3(2H)-dione monosulfonic acids of the color additive D&C Yellow No. 10 (quinoline yellow) by pH-zone-refining counter-current chromatography

The main components of the color additive D&C Yellow No. 10 (Quinoline Yellow, Color Index No. 47005), 2-(2-quinolinyl)-1H-indene-1,3(2H)-dione-6'-sulfonic acid (6SA) and 2-(2-quinolinyl)-1H-indene-1,3(2H)-dione-8'-sulfonic acid (8SA), were isolated from the dye mixture by pH-zone-refining counter-current chromatography (CCC) in the ion-exchange mode. These positional isomers were separated from a portion of dye using sulfuric acid as the retainer acid and dodecylamine as the ligand (ion exchanger). The added ligand enhanced the partitioning of the hydrophilic components in the organic stationary phase of the two-phase solvent system that consisted of isoamyl alcohol-methyl tert.-butyl ether-acetonitrile-water (3:1:1:5). Thus, separation of 1.8 g of D&C Yellow No. 10 using the above method resulted in 0.6 g of 6SA and 0.18 g of 8SA of over 99% purity. The isolated compounds were characterized by mass spectrometry and proton nuclear magnetic resonance with correlated spectroscopy assignments. The study exemplifies a new field of applications for pH-zone-refining CCC, to the separation of positional isomers of strongly hydrophylic compounds containing sulfonic acid groups.

PH-Zone-refining counter-current chromatography of lappaconitine from Aconitum sinomontanum nakai I. Separation of prepurified extract

pH-Zone-refining counter-current chromatography was applied to the separation of diterpenoid alkaloids from a crude sample from a crude prepurified sample containing lappaconitine at about 90% purity using a multilayer coil planet centrifuge. The experiment was performed with a two-phase solvent system composed of methyl tert.-butyl ether-tetrahydrofuran-distilled water (2:2:3, v/v) where triethylamine (10 mM) was added to the upper organic stationary phase as a retainer and hydrochloric acid (10 mM) to the aqueous mobile phase as an eluter. The separation of 10.5 g of the sample yielded 9.0 g of lappaconitine at a high purity of over 99% as determined by HPLC.

Rapid quantification of hexachlorobenzene in the color additives D&C Red Nos. 27 and 28 (phloxine B) using solid-phase microextraction and gas chromatography-mass spectrometry

The present paper describes the development of a method for the quantification of hexachlorobenzene (HCB) in the color additives D&C Red Nos. 27 and 28 (phloxine B) using solid-phase microextraction followed by gas chromatography-mass spectrometry (GC-MS) analysis. The method is simple and fast (1 h for each analysis), generates little solvent waste, and does not involve a solid matrix, thus permitting a more efficient extraction than does a previously developed Soxhlet extraction-GC-MS method. Test portions from 30 batches of US-certified color additives D&C Red Nos. 27 and 28 were analyzed for HCB using the new method. Those batches represent domestic (five) and foreign (one) manufacturers that requested certification for the colors during the past four years. All the samples contained HCB, ranging from 0.2 ppm to 244.3 ppm. The analyses revealed significant differences in the levels of HCB across batches from the same manufacturer as well as among different manufacturers. The range of HCB levels found in the analyzed batches (0.2-244.3 ppm) suggest that the contamination with HCB may be decreased by avoiding use of starting material (tetrachlorophthalic anhydride) heavily contaminated with HCB.

Preparative separation of alkaloids from the root of Sophora flavescens Ait by pH-zone-refining counter-current chromatography

pH-Zone-refining counter-current chromatography was applied to the separation of alkaloids from a crude extract of the root of Sophora flavescens Ait using a multilayer coil planet centrifuge. After methyl tert.-butyl ether and water were equilibrated, triethylamine (10 mM) was added to the organic phase as a retainer and hydrochloric acid (5-10 mM) to the aqueous phase as an eluter. The separation was performed by eluting the aqueous phase while the organic phase was used as the stationary phase. From 1.0 g of the crude extract, sophocarpine (170 mg) and matrine (600 mg) were separated within 4.5 h at high purity of over 98%.

Peptide separation by pH-zone-refining countercurrent chromatography

Peptides without protecting groups have been successfully separated by pH-zone-refining countercurrent chromatography (CCC) using an ion-pair reagent, di-(2-ethylhexyl)phosphoric acid (DEHPA), as a modifier in the stationary phase. Preliminary studies indicated that two parameters, i.e., the DEHPA concentration in the stationary phase and hydrophobicity of the solvent system should be adjusted according to the hydrophobicity of the analytes. Hydrophobic and hydrophilic groups of dipeptides were each separated under optimized conditions. The method was successfully applied to gram-quantity separations of bacitracin complex and bovine insulin.

pH-zone-refining countercurrent chromatography

A recently developed preparative technique, pH-zone-refining countercurrent chromatography (CCC), separates organic acids and bases according to their pKa and hydrophobicity. The hydrodynamic mechanism of pH-zone-refining CCC is illustrated in two elution modes along with a simple mathematical analysis. Separations include acidic and basic derivatives of amino acids and peptides, hydroxyxanthene dyes, alkaloids, indole auxins, and structural and geometrical isomers. In addition, recently developed affinity ligand separations of enantiomers, polar catecholamines, and free peptides are also described. Technical guidance is provided for interested users so that they can conduct a systematic search for the optimum solvent system and experimental conditions. Advantages as well as limitations of the present technique are discussed.

Isolation of 4'-bromo-4,5,6,7-tetrachlorofluorescein from a synthetic mixture by pH-zone-refining counter-current chromatography with continuous pH monitoring

A synthetic mixture containing mainly 4'-bromo-4,5,6,7-tetrachlorofluorescein (4'BrTCF, ca. 25% by high-performance liquid chromatography) was prepared by direct bromination of tetrachlorofluorescein in ethanol. The 4'BrTCF was isolated and purified by pH-zone-refining counter-current chromatography (CCC), using two different two-phase solvent systems: diethyl ether-acetonitrile-water (4:1:5,v/v) and ter.-butyl methyl ether-water (1:1,v/v). For each system, the upper organic phase was acidified and used as the stationary phase and the lower aqueous phase was made basic and used as the mobile phase. pH-Zone-refining CCC of two 5-g batches of the crude mixture yielded 1.65 g of 4'BrTCF that was ca. 95% pure by HPLC. For these separations, a commercial counter-current chromatograph was fitted with a pH electrode in a flow cell to record the pH of the effluent "on line,' thereby replacing the tedious and time-consuming manual measurement of the pH of each fraction. Additionally, UV spectra of the effluent were continuously monitored and stored by using a computerized scanning UV-Vis detector equipped with an adjustable short-pathlength flow cell.

pH-Zone refining counter-current chromatography of polar catecholamines using di-(2-ethylhexyl)phosphoric acid as a ligand

The use of di-(2-ethylhexyl)phosphoric acid (DEHPA) as a ligand in the stationary phase effectively increased the partition coefficient of polar catecholamines. pH-Zone refining counter-current chromatography of six components, i.e. five catecholamines and one amino acid (DOPA), was successfully performed using a two-phase solvent system composed of methyl tert.-butyl ether and water by adding DEHPA (20%) and ammonium acetate (200 mM) to the organic phase and HCl (50 mM) to the aqueous mobile phase. DOPA was eluted first as a normal peak followed by the five catecholamines which formed a succession of highly concentrated rectangular peaks associated with sharp impurity peaks at their borders (UV tracing at 280 nm). Both pH and standard partition coefficient of collected fractions indicated minimum overlap between the main peaks. Each component was identified by NMR analysis.

Resolution of gram quantities of racemates by high-speed counter-current chromatography

Gram quantities of (+/-)-dinitrobenzoyl amino acids were separated by high-speed counter-current chromatography (CCC) using N-dodecanoyl-L-proline-3,5-dimethylanilide as a chiral selector (CS). Standard and pH-zone-refining CCC techniques were compared. By using the standard technique, 10 mg to a maximum of 1 g of samples was resolved in 2-9 h simply by increasing the concentration of the CS in the stationary phase. By using pH-zone-refining CCC, even more sample (2 g) was efficiently separated in less time (3 h). In both techniques, leakage of CS from the column was negligible. The method requires no solid support and the same column can be used repeatedly to separate a variety of enantiomers by dissolving appropriate chiral selectors in the stationary phase.

Separation of peptide derivatives by pH-zone-refining counter-current chromatography

pH-Zone-refining counter-current chromatography was applied to the preparative separation of oligopeptide derivatives containing up to three amino acids. Both acidic benzyloxycarbonyl peptides and basic peptides-beta-naphthylamide were successfully separated with two-phase solvent systems composed of methyl tert.-butyl ether, 1-butanol, acetonitrile and water using a set of suitable retainer and eluent reagents for each group. The preparative ability of the method was demonstrated in the separation of multigram quantities of analyte with a standard separation column with a total capacity of 325 ml. The effects of important factors such as polarity of the two-phase solvent system, concentrations of the eluent base and the retainer acid were discussed.

Preparative separation of stereoisomeric 1-methyl-4-methoxymethylcyclohexanecarboxylic acids by pH-zone-refining counter-current chromatography

The application of pH-zone-refining counter-current chromatography (CCC) to the preparative separation of stereoisomeric acids is described. The separation was accomplished on the basis of the difference in acidity of the two stereoisomers. pH-Zone-refining CCC of 400 mg of a crude synthetic mixture of stereoisomeric 1-methyl-4-methoxymethylcyclohexanecarboxylic acids yielded 49.5 and 40 mg of the pure Z- and E-stereoisomers respectively. The two-phase solvent system consisted of hexane-ethyl acetate-methanol-water (1:1:1:1). Trifluoro acetic and octanoic acids were used as retainer acids. The eluent base was aqueous ammonia. The eluted fraction were monitored by gas chromatography-mass spectrometry.