Separation and determination of cysteine enantiomers in plasma after derivatization with 4-fluoro-7-nitrobenzofurazan

Relative quantitation of chiral thiol compounds labeled based on isotope novel mass spectrometry probes: Monitoring of the dynamic changes of chiral thiol compounds in human urine during normal, exercise, and rest recovery states

Monitoring changes in the content of chiral thiol compounds in the human body is crucial for the early diagnosis of oxidative stress-related diseases and the exploration of their pathogenesis. To address this, we synthesized a novel isotope mass spectrometry (MS) probe, denoted as (R)-(5-(3-isothiocyanato (13C) pyrrolidin-1-yl)-5-oxopentyl) triphenylphosphonium (N13CS-OTPP), with triphenylphosphine as its parent structure. In this study, we established a new ultra-high-performance liquid chromatography high-resolution mass spectrometry (UHPLCHRMS) relative quantitative method to monitor chiral thiol compounds in human urine under varying oxidative stress conditions. This method relies on the ratio of 12C/13C isotope-labeled peak areas. To assess the chiral separation efficiency of N13CS-OTPP, we employed three types of thiol compounds (D/L-GSH, D/L-Cys, and D/L-Hcy) and observed separation degrees (Rs) ranging from 1.82 to 1.89. We further validated the accuracy and feasibility of our relative quantitative methods using D/L-Cys-as a model compound. N12C/13CS-OTPP-Cys-exhibited excellent linearity (R2 = 0.9993-0.9994) across different molar ratios (D/L-Cys = 10:1, 4:1, 2:1, 1:1, 1:2, 1:4, 1:10) and achieved a low limit of detection (LOD) of 2.5 fmol. Additionally, we monitored the dynamic changes in urine D/L-Cys-and D/L-Hcy ratios in 12 healthy volunteers (six males and six females) under various oxidative stress states. We generated fitting curves and investigated the trends in chiral thiol compounds in vivo. This study introduces a novel method for the relative quantitative monitoring of chiral thiol compounds in different oxidative stress states within the human body. It also presents a new strategy for understanding the pathogenesis of related diseases resulting from the abnormal metabolism of thiol compounds.

Metal-assisted core-shell plasmonic nanoparticles for small molecule biothiol analysis and enantioselective recognition

Cysteine (Cys) enantiomorphs, important small-molecule biothiols, participate in various antioxidative, flavoring, and poison-removing processes in the food industry. Current cysteine enantiomorph analysis methods require effective strategies for distinguishing them due to their similar structures and reactivity. Herein, we present a metal ion-assisted enantiomorph-selective surface-enhanced Raman scattering (SERS) biosensor based on an amphiphilic polymer matrix (APM), which can promote cysteine enantiomorph (L/D-Cys) identification. The highly selective molecular orientation is perhaps caused by the intermolecular hydrogen bonding with chiral isomers (metal centers). The experimental results show that the SERS biosensor has a sensitivity-distincting factor toward L-Cys and D-Cys. The linear range is from 1 mmol L-1 to 1 nmol L-1, along with a low limit of detection of 0.77 pmol L-1. Moreover, the fabricated Cu-APM biosensor exhibits remarkable stability and high repeatability, with an RSD of 3.7%. Real food cysteine enantiomorph detection was performed with L-Cys-containing samples of onion, cauliflower, garlic, and apple, and D-Cys-containing samples of vinegar, black garlic, cheese, and beer. The results show that the Cu-APM biosensor can be utilized as a powerful tool for real-time determination of Cys enantiomorphs in different food samples. Thus, the metal-ion-assisted enantiomorph-selective SERS biosensor has potential as an adaptable tool for enantiomorph detection and food sample analysis.

Three-Minute Enantioselective Amino Acid Analysis by Ultra-High-Performance Liquid Chromatography Drift Tube Ion Mobility-Mass Spectrometry Using a Chiral Core-Shell Tandem Column Approach

Fast liquid chromatography (LC) amino acid enantiomer separation of 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) derivatives using a chiral core-shell particle tandem column with weak anion exchange and zwitterionic-type quinine carbamate selectors in less than 3 min was achieved. Enantiomers of all AQC-derivatized proteinogenic amino acids and some isomeric ones (24 in total plus achiral glycine) were baseline separated (Rs > 1.5 except for glutamic acid with Rs = 1.3), while peaks of distinct amino acids and structural isomers (constitutional isomers and diastereomers of leucine and threonine) of the same configuration overlapped to various degrees. For this reason, drift tube ion mobility-mass spectrometry was added (i.e., LC-IM-MS) as an additional selectivity filter without extending run time. The IM separation dimension in combination with high-resolution demultiplexing enabled confirmation of threonine isomers (threonine, allo-threonine, homoserine), while leucine, isoleucine, and allo-isoleucine have almost identical collisional cross-section (DTCCSN2) values and added no selectivity to the partial LC separation. Density functional theory (DFT) calculations show that IM separation of threonine isomers was possible due to conformational stabilization by hydrogen bond formation between the hydroxyl side chain and the urea group. Generally, the CCSN2 of protonated ions increased uniformly with addition of the AQC label, while outliers could be explained by consideration of intramolecular interactions and additional structural analysis. Preliminary validation of the enantioselective LC-IM-MS method for quantitative analysis showed compliance of accuracy and precision with common limits in bioanalytical methods, and applicability to a natural lipopeptide and a therapeutic synthetic peptide could be demonstrated.

Mammalian D-Cysteine: A new addition to the growing family of biologically relevant D-amino acids

Mammalian D-Cysteine is racemized from L-cysteine by serine racemase, a pyridoxal phosphate (PLP)-dependent enzyme. Endogenous D-Cysteine plays a role in neural development by inhibiting proliferation of neural progenitor cells (NPCs) via protein kinase B (AKT) signaling mediated by the FoxO family of transcription factors. D-Cysteine binds to Myristoylated Alanine Rich C Kinase Substrate (MARCKS) and alters phosphorylation at Ser 159/163 and its translocation from the membrane. By racemizing serine and cysteine, mammalian serine racemase may play important roles in neural development highlighting its importance in psychiatric disorders.

Comparative and Selective Interaction of Amino Acid d-Cysteine with Colloidal Gold Nanoparticles in the Presence of a Fluorescent Probe in Aqueous Medium

In this communication, we report the comparative and selective interaction of amino acid d-cysteine (d-Cys) with citrate caped gold nanoparticles (Au NPs) in the presence of a fluorescent dye, rhodamine B (RhB), in aqueous solution. Au NPs of size 27.5 nm could almost fully quench the steady-state fluorescence emission of RhB at their optimum concentrations in the mixed solution. The interactions of d-Cys, l-Cys, all other relevant d- and l-amino acids, neurotransmitters, and other relevant biological compounds with the Au NPs/RhB mixed solution have been explored by monitoring the fluorescence recovery efficiencies from the almost fully quenched state of RhB fluorescence via a simple steady-state spectrofluorometric method. The higher fluorescence recovery for the interaction of d-Cys with the Au NPs/RhB mixed system is accompanied by a distinct color change (red-wine to bluish-black) of the assay medium after the reaction compared to that of all other interfering compounds considered in this work. The sensitivity of this fluorometric response lies in a broad linear range of concentrations of d-Cys and the limit of detection (LOD) is found to be 4.2 nM, which is low compared to many other methods available in the literature. The different degrees of interaction of d-Cys and l-Cys with the Au NPs/RhB mixed sample have been further explored by circular dichroism (CD) spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The selective interaction of d-Cys with the proposed Au NPs/RhB mixed system is also found to be correlated with interparticle cross-linking and aggregations of nanoparticles by the analysis of ζ potential and dynamic light scattering (DLS) study, transmission electron microscopy (TEM), atomic force microscopy (AFM), UV-vis absorption spectroscopy etc. The proposed interaction mechanism is further studied with a normal human urine sample to elucidate that the optimized combination of Au NPs and RhB may be realized as an efficient platform for detection of the amino acid d-Cys in a real biosample via a simple fluorometric approach.