High-performance liquid chromatographic resolution of amino acid enantiomers derivatized with fluorescent chiral Edman reagents

Determination of sequence and absolute configuration of peptide amino acids by HPLC-MS/CD-based detection of liberated N-terminus phenylthiohydantoin amino acids

We report a method for the simultaneous determination of the sequence and absolute configuration of peptide amino acids using a combination of Edman degradation and HPLC–MS/CD. Phenylthiohydantoin (PTH) derivatives of 20 pairs of standard d- and l-amino acids were synthesized by the Edman reaction. The CD spectra of the derivatives revealed that each pair of the PTH derivatives exhibited the absorption with opposite signs at around 270 nm. These standard PTH derivatives showed well-resolved resolution without interference from byproducts in the ion chromatogram and clear positive/negative CD absorptions when subjected on a reversed phase HPLC–MS system coupled with a CD-2095 HPLC detector. This method was applied for the detection of a synthetic pentapeptide and a natural depsipeptide (halicylindramide C). The sequence and configuration of the pentapeptide and up to eight residues of halicylindramide C were successfully analyzed by this method. The amino acid configuration of the pentapeptide was also determined successfully by subjecting its acid hydrolysates to the Edman reaction followed by HPLC–MS/CD.

Chiral derivatizations applied for the separation of unusual amino acid enantiomers by liquid chromatography and related techniques

Amino acids are essential for life, and have many functions in metabolism. One particularly important function is to serve as the building blocks of peptides and proteins, giving rise complex three dimensional structures through disulfide bonds or crosslinked amino acids. Peptides are frequently cyclic and contain proteinogenic as well as nonproteinogenic amino acids in many instances. Since most of the proteinogenic α-amino acids contain at least one stereogenic center (with the exception of glycine), the stereoisomers of all these amino acids and the peptides in which they are to be found may possess differences in biological activity in living systems. The impetus for advances in chiral separation has been highest in the past 25 years and this still continues to be an area of high focus. The important analytical task of the separation of isomers is achieved mainly by chromatographic and electrophoretic methods. This paper reviews indirect separation approaches, i.e. derivatization reactions aimed at creating the basis for the chromatographic resolution of biologically and pharmaceutically important enantiomers of unusual amino acids and related compounds, with emphasis on the literature published from 1980s. The main aspects of the chiral derivatization of amino acids are discussed, i.e. derivatization on the amino group, transforming the molecules into covalently bonded diastereomeric derivatives through the use of homochiral derivatizing agents. The diastereomers formed (amides, urethanes, urea and thiourea derivatives, etc.) can be separated on achiral stationary phases. The applications are considered, and in some cases different derivatizing agents for the resolution of complex mixtures of proteinogenic d,l-amino acids, non-proteinogenic amino acids and peptides/amino acids from peptide syntheses or microorganisms are compared.

Chiral benzofurazan-derived derivatization reagents for indirect enantioseparations by HPLC

The separation of enantiomers of biologically important molecules, such as chiral pharmaceuticals and amino acids, is an important issue because a significant difference in the activity of the enantiomers is usually observed in biological systems. Chiral separations can be carried out by so-called direct methods or by indirect methods following derivatization with a chiral reagent. Many such chiral labeling reagents have been developed for various functional groups, such as amino groups, carboxyl groups, thiols, and hydroxyl groups. This chapter describes methodologies for the indirect HPLC determination of chiral molecules, based upon diastereomer formation. The derivatization, separation, and detection procedures with the chiral benzofurazan-bearing reagents, i.e., 4-(3-aminopyrrolidin-1-yl)-7-(N,N-dimethylaminosulfonyl)-2,1,3-benzoxadiazole (DBD-APy) and 4-(3-isothiocyanatopyrrolidin-1-yl)-7-(N,N-dimethylaminosulfonyl)-2,1,3-benzoxadiazole (DBD-PyNCS), are described as representative examples.

High stability of thermoresponsive polymer-brush-grafted silica beads as chromatography matrices

Thermo-responsive chromatography matrices with three types of graft architecture were prepared, and their separation performance and stability for continuous use were investigated. Poly(N-isopropylacrylamide)(PIPAAm) hydrogel-modified silica beads were prepared by a radical polymerization through modified 4,4'-azobis(4-cyanovaleric acid) and N,N'-methylenebisacrylamide. Dense PIPAAm brush-grafted silica beads and dense poly(N-tert-Butylacrylamide (tBAAm)-b-IPAAm) brush-grafted silica beads were prepared through a surface-initiated atom transfer radical polymerization (ATRP) using CuCl/CuCl(2)/ Tris(2-(N,N-dimethylamino)ethyl)amine (Me(6)TREN) as an ATRP catalytic system and 2-propanol as a reaction solvent. Dense PIPAAm brush-grafted silica beads exhibited the highest separation performance because of their strong hydrophobic interaction between the densely grafted well-defined PIPAAm brush on silica-bead surfaces and analytes. Using an alkaline mobile phase, dense themoresponsive polymer brushes, especially having a hydrophobic basal layer, exhibited a high stability for continuous use, because polymer brush on the silica bead surfaces prevented the access of water to silica surface, leading to the hydrolysis of silica and cleavage of grafted polymers. Thus, the precisely modulating graft configuration of thermoresponsive polymers provided chromatography matrices with a high separation efficiency and stability for continuous use, resulting in elongating the longevity of chromatographic column.

Thermoresponsive polymer brush on monolithic-silica-rod for the high-speed separation of bioactive compounds

Poly(N-isopropylacrylamide), one of the most utilized thermoresponsive polymers, brush-grafted monolithic-silica columns were prepared through surface-initiated atom transfer radical polymerization (ATRP) for effective thermoresponsive-chromatography matrices. ATRP initiator was grafted on monolithic silica-rod surfaces by flowing a toluene solution containing ATRP initiator into monolithic silica-rod columns. N-Isopropylacrylamide (IPAAm) monomer and CuCl/CuCl(2)/Me(6)TREN, an ATRP catalytic system, were dissolved in 2-propanol, and the reaction solution was pumped into the preprepared initiator-modified columns at 25 °C for 16 h. The constructed PIPAAm-brush structure on the monolithic silica-rod surface was confirmed by XPS, elemental analysis, SEM observation, and GPC measurement of grafted PIPAAm. The prepared monolithic silica-rod columns were also characterized by chromatographic analysis. PIPAAm-brush-modified monolithic silica-rod columns were able to separate hydrophobic steroids with a short analysis time (10 min), compared to PIPAAm-brush-modified silica-beads-packed columns, because of the horizontally limited diffusion path length of monolithic supporting materials. Additionally, diluted PIPAAm-brush monolithic silica-rod column gave a further shorting analysis time (5 min). These results indicated (1) surface-initiated ATRP constructed PIPAAm-brush structures on monolithic silica-rod surfaces and (2) PIPAAm-brush grafted monolithic silica-rod column prepared by ATRP was a promising tool for analyzing hydrophobic-bioactive compounds with a short analysis time.

Chiral separations

The main goal of this review is to provide a brief overview of chiral separations to researchers who are versed in the area of analytical separations but unfamiliar with chiral separations. To researchers who are not familiar with this area, there is currently a bewildering array of commercially available chiral columns, chiral derivatizing reagents, and chiral selectors for approaches that span the range of analytical separation platforms (e.g., high-performance liquid chromatography, gas chromatography, supercritical-fluid chromatography, and capillary electrophoresis). This review begins with a brief discussion of chirality before examining the general strategies and commonalities among all of the chiral separation techniques. Rather than exhaustively listing all the chiral selectors and applications, this review highlights significant issues and differences between chiral and achiral separations, providing salient examples from specific classes of chiral selectors where appropriate.

Synthesis of ?uorescent label, DBD-?-proline, and the resolution ef?ciency for chiral amines by reversed-phase chromatography

DBD-d(and l)-beta-proline, new fluorescent chiral derivatization reagents, were synthesized from the reaction of 4-(N,N-dimethylaminosulfonyl)-7- fl uoro-2,1,3-benzoxadiazole (DBD-F) with beta-proline. The racemic mixture synthesized was separated by a chiral stationary phase (CSP) column, Chiralpak AD-H, with n-hexane-EtOH-TFA-diethylamine (70:30:0.1:0.1) as the mobile phase. The dl-forms were decided according to the results obtained from a circular dichroism (CD) detector after separation by the CSP column. The fractionated enantiomers reacted with chiral amine to produce a couple of diastereomers. The labeling proceeded in the presence of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and pyridine as the activation reagents. The reaction conditions were mild and no racemization occurred during the diastereomer formation. The resulting diastereomers fluoresced at around 570 nm (excitation at around 460 nm). Good linearity of the calibration curves was obtained in the range 1-75 pmol and the detection limits on chromatogram were less than 1 pmol. The separability of the diastereomers was compared with the diastereomers derived from DBD-d(or l)-proline. The resolution values (Rs) obtained from the diastereomers of three chiral amines with DBD-d(or l)-beta-proline were higher than those derived from DBD-d(or l)-proline, e.g. dl-phenylalanine methylester (dl-PAME), 2.23 vs 1.37; (R)(S)-1-phenylethylamine [(R)(S)-PEA], 2.09 vs 1.13; and (R)(S)-1-(1-naphthyl)ethylamines [(R)(S)-NEA], 5.19 vs 1.23. The results suggest that the position of COOH group on pyrrolidine moiety in the structures is one of the important factors for the efficient separation of a couple of the diastereomers.

Resolution of chiral drugs by liquid chromatography based upon diastereomer formation with chiral derivatization reagents

Chiral derivatization reagents for resolution of biologically important compounds, such as chiral drugs by high-performance liquid chromatography (HPLC), based upon pre-column derivatization and diastereomer formation, are reviewed. The derivatization reagents for various functional groups, i.e., amine, carboxyl, carbonyl, hydroxyl and thiol, are evaluated in terms of reactivity, stability, wavelength, handling, versatility, sensitivity, and selectivity. The applicability of the reagents to the analyses of drugs and bioactive compounds are included in the text.

Amino acid sequence and D/L-configuration determination methods for D-amino acid-containing peptides in living organisms

D-amino acid-containing peptides with biological activities have been isolated from invertebrates and amphibians, and partial racemization of amino acid residues in mammalian peptides associated with aging and diseases have been discussed. Here, we review the amino acid configuration determination methods in these peptides and recent progress of simultaneous determination method for sequence and configuration of amino acid residues. The applicability of C-terminus sequence analysis and mass spectrometry to configuration determination of amino acids is also discussed.

Fluorogenic and fluorescent labeling reagents with a benzofurazan skeleton

Fluorogenic and fluorescent labeling reagents having a benzofurazan (2,1,3-benzoxadiazole) skeleton such as 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F), 4-N,N-dimethylaminosulfonyl-7-fluoro-2,1,3-benzoxadiazole (DBD-F), 4-aminosulfonyl-7-fluoro-2,1,3-benzoxadiazole (ABD-F), ammonium 7-fluoro-2,1,3-benzoxadiazole-4-sulfonate (SBD-F), 4-hydrazino-7-nitro-2,1,3-benzoxadiazole (NBD-H), 4-N,N-dimethylaminosulfonyl-7-hydrazino-2,1,3-benzoxadiazole (DBD-H), 4-nitro-7-N-piperazino-2,1,3-benzoxadiazole (NBD-PZ), 4-N,N-dimethylaminosulfonyl-7-N-piperazino-2,1,3-benzoxadiazole (DBD-PZ), 4-(N-chloroformylmethyl-N-methyl)amino-7-N,N-dimethylaminosulfonyl-2,1,3-benzoxadiazole (DBD-COCl) and 7-N,N-dimethylaminosulfonyl-4-(2,1,3-benzoxadiazolyl) isothiocyanate (DBD-NCS) are reviewed in terms of synthetic method, reactivity, fluorescence characteristics, sensitivity and application to analytes.

Chiral derivatization reagents for drug enantioseparation by high-performance liquid chromatography based upon pre-column derivatization and formation of diastereomers: enantioselectivity and related structure

Structures and related enantioselectivities of the respective chiral derivatization reagents (CDRs) for drug enantioseparation by high-performance chromatography based upon pre-column derivatization and diastereomeric formation are reviewed. The elution order of diastereomers caused reaction of some CDRs with enantiomeric amino acids and carboxylic acids. The development of new CDRs available for indirect HPLC methods is also discussed. (c) 2001 John Wiley & Sons, Ltd.

Determination of D-amino acids labeled with fluorescent chiral reagents, R(-)- and S(+)-4-(3-isothiocyanatopyrrolidin-1-yl)-7-(N, N-dimethylaminosulfonyl)-2,1,3-benzoxadiazoles, in biological and food samples by liquid chromatography

D-Amino acids in food and biological samples labeled with R(-)- and S(+)-4-(3-isothiocyanatopyrrolidin-1-yl)-7-(N, N-dimethylaminosulfonyl)-2,1,3-benzoxadiazoles (DBD-PyNCS) were separated by reversed-phase chromatography and detected fluorometrically at 550 nm (excitation at 460 nm). DL-Amino acids were efficiently labeled at 55 degrees C for 20 min in basic medium. The resulting thiocarbamoyl-amino acids were resolved by an isocratic elution using water:30% methanol in acetonitrile (72:28) containing 0.1% trifluoracetic acid as mobile phase for hydrophilic amino acids and gradient elutions using sodium acetate buffer (pH 5. 2)/acetonitrile as gradient solvent mixture for hydrophobic amino acids, respectively. The detection limits (S/N = 3) of DL-amino acids tested were in the range of 0.16-0.75 pmol. The proposed method was applied to determine the D-amino acid(s) in milk, cream, fermented dairy products (yogurt and yakult), tomato products (juice, puree, and catchup), fermented beverages (beer and red wine), and human urine. The existence of D-amino acid(s) was demonstrated in all the samples tested. Furthermore, the identification of the D-amino acid(s) was performed using both isomers of DBD-PyNCS and by on-line HPLC-electrospray ionization-MS.

Total resolution of 17 DL-amino acids labelled with a fluorescent chiral reagent, R(-)-4-(3-isothiocyanatopyrrolidin-1-y1)-7-(N,N-dimethylaminosulfonyl)- 2,1,3-benzoxadiazole, by high-performance liquid chromatography

Total resolution of 17 DL-amino acids after derivatization with a fluorescent chiral tagging reagent, 4-(3-isothiocyanatopyrrolidin-1-yl)-7-(N,N-dimethylaminosulfony l)-2,1,3- benzoxadiazole [R(-)-DBD-PyNCS], was studied by reversed-phase liquid chromatography. The reaction of the reagent with amino acids proceeds effectively at 55 degrees C for 20 min in the presence of 1% TEA to produce the corresponding fluorescent diastereomers (excitation at 460 nm, emission at 550 nm). Each pair of the resulting derivatives was efficiently separated with water-acetonitrile containing 1% acetic acid as the mobile phase. Peak resolution was in the range of 0.92 (DL-Arg)-9.8 (DL-Cys). Although mutual separation of some DL-amino acids was possible using the elution solvent, simultaneous resolution of 17 DL-amino acids was difficult with a single chromatographic run, even if some gradient elutions were adopted. Therefore, both gradient and isocratic elution systems were used for total resolution of the DL-amino acids. Thus, 17 DL-amino acids were well resolved by a gradient and an isocratic elution systems. The proposed derivatization and elution methods were applied to the determination of DL-amino acids in yogurt. The results showed that some of the L-amino acids, i.e., Glu, Asp, Ser, Gly, Ala, Thr, Pro, Lys, Phe and Met, were found in the methanol extracts of yogurt. On the other hand, the D-amino acids that were identified in the extracts were D-Glu, D-Asp and D-Ala, and the mean % to each L-amino acid were 11.9% (D-Glu), 27.6% (D-Asp) and 56.7% (D-Ala), respectively.

Separation of amino acid and peptide stereoisomers by nonionic micelle-mediated capillary electrophoresis after chiral derivatization

Enantiomers of amino acids and peptides were derivatized with a fluorescent chiral reagent, 4-(3-isothiocyanatopyrrolidinl-yl)-7-nitro-2,1,3-benzoxadiazole [R-(-)- or S-(+)-NBD-PyNCS] and the resulting diastereomeric derivatives separated by capillary electrophoresis (CE). The CE running buffer consisted of 25 mM acetate buffer (pH 4) and 10 mM of the nonionic surfactant Triton X-100. The excitation maximum of NBD-PyNCS at 480 nm matches the major Ar-ion emission line at 488 nm allowing sensitive laser-induced fluorescence detection with limits of detection around 50 nM. D-Proline and D-aspartate spiked (at 10(-4) M and 10(-5) M concentrations, respectively) into complex biological matrices (rabbit serum and homogenate of Aplysia californica buccal ganglion) are detected without matrix interferences. This method has also been applied to the determination of D- and L-amino acid residues in peptides after acid hydrolysis. Results from the chiral analysis of the naturally-occurring peptide, gramicidin D, are shown.

Enantioseparation of beta-blockers labelled with a chiral fluorescent reagent, R (-)-DBD-PyNCS, by reversed-phase liquid chromatography

A fluorescent chiral tagging reagent, 4-(3-isothiocyanatopyrrolidin-1-yl)-7-(N,N-dimethylaminosulfony l)-2,1, 3-benzoxadiazole [R(-)-DBD-PyNCS], has been used for the liquid chromatographic resolution of racemic pairs of beta-blockers. The reagents reacts with beta-blockers at 65 degrees C for 90 min in aqueous acetonitrile containing 0.05% triethylamine to produce the corresponding pair of diastereomers. No racemization occurs during the tagging reaction under these conditions. From results of the time-course study of oxprenolol the reactivities of the enantiomers of beta-blockers with R(-)-DBD-PyNCS are comparable. The optimum excitation and emission wavelengths of the resulting derivatives were ca. 460 and 550 nm, respectively. The derivatives of beta-blockers were efficiently resolved by a reversed-phase column with water-acetonitrile containing 0.1% trifluoroacetic acid as the eluent. The resolution (Rs) values of the diastereomers derived from 10 beta-blockers were in the range of 1.54-4.80. The Rs value for timolol was 0.643. The detection limits (signal-to-noise ratio of 2) were one or two orders of magnitude lower with beta-blockers having the iso-propylamino structure (15-300 fmol) than with those having the tert-butylamino structure (1.25-8.00 pmol). The proposed procedure was applied to the determination of R(+)- and S(-)-propranolol in rat plasma and saliva after oral administration of R(+)-propranolol hydrochloride or S(-)-propranolol hydrochloride.

Recent progress in liquid chromatographic enantioseparation based upon diastereomer formation with fluorescent chiral derivatization reagents

Techniques for the resolution by liquid chromatography of racemic compounds based upon diastereomer formation with a fluorescent chiral reagent are outlined in this review. The tagging reagents for various functional groups, i.e. amine, carboxyl, hydroxyl and thiol, are evaluated in terms of optical purity, handling, flexibility, stability, sensitivity and selectivity. The applicabilities of the reagents to drugs and biologically important substances are included in the text. This review is limited to reagents with fluorophores and reagents that exhibit fluorescence.