Chiral secondary amino acids, their importance, and methods of analysis

Underlying Mechanisms of Chromatographic H/D, H/F, cis/trans and Isomerism Effects in GC-MS

Charge-free gaseous molecules labeled with deuterium 2H (D) atoms elute earlier than their protium-analogs 1H (H) from most stationary GC phases. This effect is known as the chromatographic H/D isotope effect (hdIEC) and can be calculated by dividing the retention times (tR) of the protiated (tR(H) ) to those of the deuterated (tR(D)) analytes: hdIEC = tR(H)/tR(D). Analytes labeled with 13C, 15N or 18O have almost identical retention times and lack a chromatographic isotope effect. Derivatives of cis- and trans-analytes such as cis- and trans-fatty acids also differ in their retention times. Analytes that contain trans-C=C-double bonds elute earlier in gas chromatography-mass spectrometry (GC-MS) than their cis-C=C-double bonds containing congeners. The chromatographic cis/trans-effect (ctEC) can be calculated by dividing the retention times of the cis- by those of the trans-analytes: ctEC = tR(c)/tR(t). In the present work, the hdIEC and ctEC values of endogenous and exogenous substances were calculated from previously reported GC-MS analyses and found to range each between 1.0009 and 1.0400. The examination suggests that the H/D-isotope effects and the cis/trans-effects observed in GC-MS are based on differences in the inter-molecular interaction strengths of the analyte derivatives with the stationary phase of GC columns. The deuterium atoms, being larger than the H atoms of the analytes, attenuate the interaction of the skeleton of the molecules with the GC stationary phase. The angulation of trans-analytes decreases the interaction of the skeleton of the molecules with the GC stationary phase, as only parts of the molecules are close enough to the GC stationary phase to interact. Other chromatographic effects caused by hydrogen (H) and fluorine (F) atoms and by stereo-isomerism are considered to be based on a similar mechanism due to the different orientation of the side chains.

Discovery of nostatin A, an azole-containing proteusin with prominent cytostatic and pro-apoptotic activity

Ribosomally synthesized and post-translationally modified peptides (RiPPs) are intriguing compounds with potential pharmacological applications. While many RiPPs are known as antimicrobial agents, a limited number of RiPPs with anti-proliferative effects in cancer cells are available. Here we report the discovery of nostatin A (NosA), a highly modified RiPP belonging among nitrile hydratase-like leader peptide RiPPs (proteusins), isolated from a terrestrial cyanobacterium Nostoc sp. Its structure was established based on the core peptide sequence encoded in the biosynthetic gene cluster recovered from the producing strain and subsequent detailed nuclear magnetic resonance and high-resolution mass spectrometry analyses. NosA, composed of a 30 amino-acid peptide core, features a unique combination of moieties previously not reported in RiPPs: the simultaneous presence of oxazole/thiazole heterocycles, dehydrobutyrine/dehydroalanine residues, and a sactionine bond. NosA includes an isobutyl-modified proline residue, highly unusual in natural products. NosA inhibits proliferation of multiple cancer cell lines at low nanomolar concentration while showing no hemolysis. It induces cell cycle arrest in S-phase followed by mitochondrial apoptosis employing a mechanism different from known tubulin binding and DNA damaging compounds. NosA also inhibits Staphylococcus strains while it exhibits no effect in other tested bacteria or yeasts. Due to its novel structure and selective bioactivity, NosA represents an excellent candidate for combinatorial chemistry approaches leading to development of novel NosA-based lead compounds.

Synthesis of Novel Planar-Chiral Charge-Compensated nido-Carborane-Based Amino Acid

Amino acids with unusual types of chirality and their derivatives have recently attracted attention as precursors in the synthesis of chiral catalysts and peptide analogues with unique properties. In this study, we have synthesized a new nido-carborane-based planar-chiral amino acid, in the molecule of which the amino group is directly bonded to the B(3) atom, and the carboxyl group is attached to the B(9) atom through the CH2S+(Me) fragment. 3-Amino-9-dimethylsulfonio-nido-carborane, prepared in three steps from 3-amino-closo-carborane in a high yield, was a key intermediate in the synthesis of the target planar-chiral amino acid. The carboxymethyl group at the sulfur atom was introduced by the demethylation reaction of the dimethylsulfonio derivative, followed by S-alkylation. The structure of new 3,9-disubstituted nido-carboranes was studied for the first time using NMR spectroscopy. The resonances of all boron atoms in the 11B NMR spectrum of 3-amino-9-dimethylsulfonio-nido-carborane were assigned based on the 2D NMR correlation experiments. The nido-carborane-based planar-chiral amino acid and related compounds are of interest as a basis for peptide-like compounds and chiral ligands.

Oxidation Products of Tryptophan and Proline in Adipokinetic Hormones-Artifacts or Post-Translational Modifications?

BACKGROUND

Adipokinetic hormones (AKHs) regulate important physiological processes in insects. AKHs are short peptides with blocked termini and Trp in position 8. Often, proline occupies position 6. Few post-translational modifications have been found, including hydroxyproline ([Hyp6]) and kynurenine. Our recent data suggest that the Hyp- and Kyn-containing AKHs occur more often than originally thought and we here investigate if they are natural or artifactual.

METHODS

From crude extracts of the corpora cardiaca (CC) of various insect species, AKHs were analyzed using liquid chromatography coupled to high-resolution mass spectrometry (LC-MS). Synthetic [Hyp6]-AKHs were tested in an in vivo metabolic assay. Freshly dissected Periplaneta americana and Blaberus atropos CCs (with precautions taken against oxidation) were analyzed. B. atropos CC were placed into a depolarizing saline and the released AKHs were measured.

RESULTS

Hyp was detected in several decapeptides from cockroaches. The modified form accompanied the AKH at concentrations below 7%. The [Hyp6]-AKHs of B. atropos were present in fresh CC preparations and were shown to be releasable from the CC ex vivo. Synthetic [Hyp6]-containing peptides tested positively in a hypertrehalosemic bioassay. Hydroxyprolination was also detected for Manto-CC from the termite Kalotermes flavicollis and for Tetsu-AKH of the grasshopper, Tetrix subulata. Oxidized Trp-containing forms of Nicve-AKH were found in species of the burying beetle genus Nicrophorus.

CONCLUSIONS

Trp oxidation is known to occur easily during sample handling and is likely the reason for the present findings. For hydroxyprolination, however, the experimental evidence suggests endogenous processes.

Enantioseparation of a-substituted proline analogs with macrocyclic glycopeptide-based chiral stationary phases immobilized on superficially porous particles of silica applying liquid chromatography with ultraviolet and mass spectrometric detection

In this study, the liquid chromatography-based direct enantioseparation of the stereoisomers of α-substituted proline analogs has been investigated utilizing chiral stationary phases with UV and/or mass spectrometric (MS) detection. Macrocyclic antibiotics, such as vancomycin, teicoplanin, modified teicoplanin, and teicoplanin aglycone, all covalently immobilized to 2.7 μm superficially porous silica particles have been applied as stationary phases. Mobile phases utilizing mixtures of methanol and acetonitrile with different additives (polar-ionic mode) were optimized during method development. Best separations were achieved with mobile phases of 100% MeOH containing either 20 mM acetic acid or 20 mM triethylammonium acetate. Special attention was given to the applicability of MS-compatible mobile phases. Acetic acid was found to be advantageous as a mobile phase additive for MS detection. Enantioselective chromatographic behaviors are interpreted based on the explored correlations between the analytes' structural features and those of the applied chiral stationary phases. For the thermodynamic characterization, separations were studied in the temperature range of 5-50 °C. Generally, retention and selectivity decreased with increasing temperature, and in most cases, enthalpy-driven enantiorecognition was observed, but entropic contributions also were present. Unexpectedly, unusual shapes for the van Deemter curves were registered in the kinetic evaluations. General trends could be observed in the enantiomeric elution orders: S < R on VancoShell and NicoShell, and opposite R < S on TeicoShell and TagShell columns.