Determination of the absolute configuration of alpha-hydroxyglycine derivatives by enzymatic conversion and chiral high-performance liquid chromatography

Kinetics and stereochemistry of hydrolysis of an N-(phenylacetyl)-α-hydroxyglycine ester catalyzed by serine β-lactamases and DD-peptidases

The α-hydroxydepsipeptide 3-carboxyphenyl N-(phenylacetyl)-α-hydroxyglycinate (5) is a quite effective substrate of serine β-lactamases and low molecular mass DD-peptidases. The class C P99 and ampC β-lactamases catalyze the hydrolysis of both enantiomers of 5, although they show a strong preference for one of them. The class A TEM-2 and class D OXA-1 β-lactamases and the Streptomyces R61 and Actinomadura R39 DD-peptidases catalyze hydrolysis of only one enantiomer of at any significant rate. Experiments show that all of the above enzymes strongly prefer the same enantiomer, a surprising result since β-lactamases usually prefer L(S) enantiomers and DD-peptidases D(R). Product analysis, employing peptidylglycine α-amidating lyase, showed that the preferred enantiomer is D(R). Thus, it is the β-lactamases that have switched preference rather than the DD-peptidases. Molecular modeling of the P99 β-lactamase active site suggests that the α-hydroxyl 5 of may interact with conserved Asn and Lys residues. Both α-hydroxy and α-amido substituents on a glycine ester substrate can therefore enhance its productive interaction with the β-lactamase active site, although their effects are not additive; this may also be true for inhibitors.

Diastereomeric and enantiomeric high-performance liquid chromatographic separation of synthetic anisodamine

In order to investigate the enantiomeric pharmacokinetics and biotransformation of synthetic anisodamine (654-2), a cholinoceptor antagonist widely used in clinic in China, it has been preparatively separated into two racemates (I and II) by using ZORBAX Eclipse XDB-C18 column. The diastereo- and/or enantioseparations of 654-2, I and II were carried out by HPLC using CHIRALPAK AD-H as chiral stationary phase (CSP) and acetonitrile-2-propanol-DEA 97:3:0.1 (v/v/v) as mobile phase. The methods were optimized by studying mobile phase modifiers, concentration of modifier and column temperature. The HPLC method for the simultaneous separation of two pairs of enantiomers of 654-2 has been validated.

Enantiomeric separation of mineralocorticoid receptor (hMR) antagonists using the Chiralcel® OJ-H HPLC column with novel polar cosolvent eluent systems

This study demonstrates the increased versatility of the Chiralcel OJ-H stationary phase when using various alcohol/acetonitrile mobile phases. This chiral stationary phase has traditionally been employed in the normal phase mode and more recently with neat alcohols as eluents. Selected isomeric human mineralocorticoid receptor (hMR) antagonist pharmaceutical candidates and synthetic intermediates were separated using the Chiralcel OJ-H HPLC column with novel polar cosolvent eluent systems. The capacity factors, resolution, and selectivity of the chiral separations were assessed while varying the alcohol/acetonitrile composition and alcohol identity. The mixed polar eluents provide separations that are nearly always superior to both the traditional hexane-rich and single-alcohol "polar organic" eluents for the compounds tested in this article.

Determination of absolute configuration via vibrational circular dichroism

Vibrational circular dichroism (VCD) provides a growing and promising technology for the determination of the absolute configuration of molecules in solution, including drug molecules. The practical application of VCD spectroscopy consists of the experimental determination and comparison to quantum chemically calculated data. The key features of the VCD technology are presented and an example of an application of the technique is discussed.:

Chiral liquid chromatography contribution to the determination of the absolute configuration of enantiomers

The review covers examples in which chiral HPLC, as a source of pure enantiomers, has been combined with classical methods (X-ray, vibrational circular dichroism (VCD), enzymatic resolutions, nuclear magnetic resonance (NMR) techniques, optical rotation, circular dichroism (CD)) for the on- or off-line determination of absolute configuration of enantiomers. Furthermore, it is outlined that chiral HPLC, which associates enantioseparation process and classical purification process, opens new perspectives in the classical determination of absolute configuration by chemical correlation or chemical interconversion methods. The review also contains a discussion about the various approaches to predict the absolute configuration from the retention behavior of the enantiomers on chiral stationary phases (CSPs). Some examples illustrate the advantages and limitations of molecular modeling methods and the use of chiral recognition models. The assumptions underlying some of these methods are critically analyzed and some possible emerging new strategies are outlined.

Catalysis, stereochemistry, and inhibition of ureidoglycolate lyase

Ureidoglycolate lyase (UGL, EC 4.3.2.3) catalyzes the breakdown of ureidoglycolate to glyoxylate and urea, which is the final step in the catabolic pathway leading from purines to urea. Although the sequence of enzymatic steps was worked out nearly 40 years ago, the stereochemistry of the uric acid degradation pathway and the catalytic properties of UGL have remained very poorly described. We now report the first direct investigation of the absolute stereochemistry of UGL catalysis. Using chiral chromatographic analyses with substrate enantiomers, we demonstrate that UGL catalysis is stereospecific for substrates with the (S)-hydroxyglycine configuration. The first potent competitive inhibitors for UGL are reported here. These inhibitors are compounds which contain a 2,4-dioxocarboxylate moiety, designed to mimic transient species produced during lyase catalysis. The most potent inhibitor, 2,4-dioxo-4-phenylbutanoic acid, exhibits a KI value of 2.2 nM and is therefore among the most potent competitive inhibitors ever reported for a lyase enzyme. New synthetic alternate substrates for UGL, which are acyl-alpha-hydroxyglycine compounds, are described. Based on these alternate substrates, we introduce the first assay method for monitoring UGL activity directly. Finally, we report the first putative primary nucleotide and derived peptide sequence for UGL. This sequence exhibits a high level of similarity to the fumarylacetoacetate hydrolase family of proteins. Close mechanistic similarities can be visualized between the chemistries of ureidoglycolate lyase and fumarylacetoacetate hydrolase catalysis.

Simultaneous analysis of enantiomeric composition of amino acids and N-acetyl-amino acids by enantioselective chromatography

Among the three chiral columns, CHIROBIOTIC T, CHIRLPAK WH, and CHIRALCEL OD-R, tested for the separation of racemic amino acids and N-acetyl-amino acids, only CHIROBIOTIC T chiral column which is based on covalently bonded amphoteric glycopeptide, teicoplanin, as the stationary phase ligand could be successfully developed to enantiomerically separate racemic amino acids and N-acetyl amino acids simultaneously. This method can be used to determine the enantiomeric composition of amino acids and N-acetyl-amino acids in the catalysis of D-aminoacylase or L-aminoacylase and the conversion rate of N-acylamino acid racemases.

Reversed-phase liquid chromatographic separation of enantiomers on polysaccharide type chiral stationary phases

The direct chiral separation by chiral stationary phases (CSPs) is one of the most important techniques to analyze enantiomeric purity as well as to get enantiomerically pure material quickly. Among various types of CSPs, polysaccharide type CSPs are well known by their versatility and durability. They are not only effective under normal-phase conditions, but also under reversed-phase conditions. In order to get a good separation under the reversed-phase conditions, it is the key to choose an appropriate mobile phase. For example, a simple mixture of water/acetonitrile or water/methanol are sufficient for a neutral analyte, while it is necessary to use an acidic solution instead of water for an acidic analyte and a solution of a chaotropic salt (or a basic solution) for a basic analyte, respectively. The paper also presents lists of more than 350 separation examples that include 22 validated methods for drug analyses from serum, plasma, or urine samples on polysaccharide type CSPs under reversed-phase conditions.

High-performance liquid chromatographic enantiomeric separation of an enzyme inhibitor which possesses both a chiral center and tautomeric moieties

The enantiomeric separation using high-performance liquid chromatography (HPLC) on chiral stationary phases (CSPs) of a chiral compound which exists in solution in several tautomeric forms is described. 2,4-Dioxo-5-acetamido-6-phenylhexanoic acid is the most potent inhibitor known for peptidylamidoglycolate lyase (PGL, EC 4.3.2.5), an enzyme which plays an essential role in carboxyl-terminal amidation of many biological peptides. Synthesis of this inhibitor entails an alkaline hydrolysis step, under which condition the compound is racemized; thus, HPLC with a CSP was employed to obtain the individual enantiomers of this inhibitor. Since 2,4-dioxo-5-acetamido-6-phenylhexanoic acid exists in solution in several tautomeric forms, the strategy of first converting this compound from its multiple enol forms into a single diketo tautomer, which was then applied to various CSPs, was employed. Successful preparative scale enantiomeric separation of this compound was achieved using a Chiralpak AD CSP. Enantiomeric separation was also accomplished on a D-penicillamine column, but this CSP was found to be less satisfactory for preparative purposes.