Enhanced Resolution of Chiral Amino Acids with Capillary Electrophoresis for Biosignature Detection in Extraterrestrial Samples

Development of capillary electrophoresis methods for the detection of microbial metabolites on potential future spaceflight missions

The search for chemical indicators of life is a fundamental component of potential future spaceflight missions to ocean worlds. Capillary electrophoresis (CE) is a useful separation method for the determination of the small organic molecules, such as amino acids and nucleobases, that could be used to help determine whether or not life is present in a sample collected during such missions. CE is under development for spaceflight applications using multiple detection systems, such as laser induced fluorescence (LIF) and mass spectrometry (MS). Here we report CE-based methods for separation and detection of major polar metabolites in cells, such as amino acids, nucleobases/sides, and oxidized and reduced glutathione using detectors that are less expensive alternatives to LIF and MS. Direct UV detection, indirect UV detection, and capacitvely coupled contactless conductivity detection (C4D) were tested with CE, and a combination of direct UV and C4D allowed the detection of the widest variety of metabolites. The optimized method was used to profile metabolites found in samples of Escherichia coli and Pseudoalteromonas haloplanktis and showed distinct differences between the species.

Development of a capillary temperature control system for capillary electrophoresis instruments designed for spaceflight applications

Capillary temperature control during capillary electrophoresis (CE) separations is key for achieving accurate and reproducible results with a broad array of potential methods. However, the difficulty of enabling typical fluid temperature control loops on portable instruments has meant that active capillary temperature control of in situ CE systems has frequently been overlooked. This work describes construction and test of a solid-state device for capillary temperature control that is suitable for inclusion with in situ instruments, including those designed for space missions. Two test articles were built, a thermal mass model (TMM) and a functional model (FM). The TMM demonstrated that temperature gradients could be limited using the proposed control scheme, and that our thermal modeling of the system can be relied on for future adaptations of physical geometries of the system. The FM demonstrated CE analytical performance while under active temperature control and that the device was compatible with the harsh thermal-vacuum environments that might be encountered during space flight.

Enantiomeric purity of synthetic therapeutic peptides: A review

Synthetic therapeutic peptides are a complex and popular class of pharmaceuticals. In recent years, peptides with proven therapeutic activity have gained significant interest in the market. The determination of synthetic peptide enantiomeric purity plays a critical role in the evaluation of the quality of the medicine. Since racemization is one of the most common side reactions occurring in AAs or peptides, enantiomeric impurities such as D-isomers can form during the peptide synthesis or can be introduced from the starting materials (e.g., AAs). The therapeutic effect of a synthetic or semi-synthetic bioactive peptide molecule depends on its AA enantiomeric purity and secondary/tertiary structure. Therefore, the enantiomeric purity determination for synthetic peptides is supportive for interpreting unwanted therapeutic effects and determining the quality of synthetic peptide therapeutics. However, enantiomeric purity analysis encounters formidable analytical challenges during chromatographic separation, as D/L isomers have identical physical-chemical properties except stereochemical configuration. To ensure peptides AA stereochemical configuration whether in the free or bound state, sensitive and reproducible quantitative analytical method is mandatory. In this regard, numerous analytical techniques were emerged for the quantification of D-isomeric impurities in synthetic peptides, but still, very few reports are available in the literature. Thus, the purpose of this paper is to provide an overview of the importance, regulatory requirements, and various analytical methods used for peptide enantiomeric purity determination. In addition, we discussed the available literature in terms of enantiomeric impurity detection, common hydrolysis procedural aspects, and different analytical strategies used for sample preparation.

Chapter 8: Searching for Life Beyond Earth

The search for life beyond Earth necessitates a rigorous and comprehensive examination of biosignatures, the types of observable imprints that life produces. These imprints and our ability to detect them with advanced instrumentation hold the key to our understanding of the presence and abundance of life in the universe. Biosignatures are the chemical or physical features associated with past or present life and may include the distribution of elements and molecules, alone or in combination, as well as changes in structural components or physical processes that would be distinct from an abiotic background. The scientific and technical strategies used to search for life on other planets include those that can be conducted in situ to planetary bodies and those that could be observed remotely. This chapter discusses numerous strategies that can be employed to look for biosignatures directly on other planetary bodies using robotic exploration including those that have been deployed to other planetary bodies, are currently being developed for flight, or will become a critical technology on future missions. Search strategies for remote observations using current and planned ground-based and space-based telescopes are also described. Evidence from spectral absorption, emission, or transmission features can be used to search for remote biosignatures and technosignatures. Improving our understanding of biosignatures, their production, transformation, and preservation on Earth can enhance our search efforts to detect life on other planets.

A Robust Capillary Electrophoresis with Laser-Induced Fluorescence Detection (CE-LIF) Method for Quantitative Compositional Analysis of Trace Amino Acids in Hypersaline Samples

The search for life in our solar system can be enabled by the characterization of extreme environments representing conditions expected on other planets within our solar system. Molecular abundances observed in these environments help establish instrument design requirements, including limits of detection and pH/salt tolerance, and may be used for validation of proposed planetary science instrumentation. Here, we optimize capillary electrophoresis with laser-induced fluorescence detection (CE-LIF) separations for low limit of detection quantitative compositional analysis of amino acids in hypersaline samples using carboxyfluorescein succinimidyl ester (CFSE) as the amine-reactive fluorescent probe. Two methods were optimized for identification and quantification of proteinogenic amino acids, those with and those without acidic side chains, with limits of detection as low as 250 pM, improving on previous CFSE-amino acid CE-LIF methods by an order of magnitude. The resilience of the method to samples with high concentrations of Mg2+ (>4 M diluted to >0.4 M for analysis) is demonstrated on a sample collected from the salt harvesting facility South Bay Salt Works in San Diego, CA, demonstrating the highest Mg2+ tolerance for CE-LIF methods used in amino acid analyses to date. This advancement enables the rapid and robust analysis of trace amino acids and the search for biosignatures in hypersaline systems.

Solid-State Single-Molecule Sensing with the Electronic Life-Detection Instrument for Enceladus/Europa (ELIE)

Growing evidence of the potential habitability of Ocean Worlds across our solar system is motivating the advancement of technologies capable of detecting life as we know it-sharing a common ancestry or physicochemical origin with life on Earth-or don't know it, representing a distinct emergence of life different than our one known example. Here, we propose the Electronic Life-detection Instrument for Enceladus/Europa (ELIE), a solid-state single-molecule instrument payload that aims to search for life based on the detection of amino acids and informational polymers (IPs) at the parts per billion to trillion level. As a first proof-of-principle in a laboratory environment, we demonstrate the single-molecule detection of the amino acid L-proline at a 10 μM concentration in a compact system. Based on ELIE's solid-state quantum electronic tunneling sensing mechanism, we further propose the quantum property of the HOMO-LUMO gap (energy difference between a molecule's highest energy-occupied molecular orbital and lowest energy-unoccupied molecular orbital) as a novel metric to assess amino acid complexity. Finally, we assess the potential of ELIE to discriminate between abiotically and biotically derived α-amino acid abundance distributions to reduce the false positive risk for life detection. Nanogap technology can also be applied to the detection of nucleobases and short sequences of IPs such as, but not limited to, RNA and DNA. Future missions may utilize ELIE to target preserved biosignatures on the surface of Mars, extant life in its deep subsurface, or life or its biosignatures in a plume, surface, or subsurface of ice moons such as Enceladus or Europa. One-Sentence Summary: A solid-state nanogap can determine the abundance distribution of amino acids, detect nucleic acids, and shows potential for detecting life as we know it and life as we don't know it.

Detection and analysis of chiral molecules as disease biomarkers

The chirality of small metabolic molecules is important in controlling physiological processes and indicating the health status of humans. Abnormal enantiomeric ratios of chiral molecules in biofluids and tissues occur in many diseases, including cancers and kidney and brain diseases. Thus, chiral small molecules are promising biomarkers for disease diagnosis, prognosis, adverse drug-effect monitoring, pharmacodynamic studies and personalized medicine. However, it remains difficult to achieve cost-effective and reliable analysis of small chiral molecules in clinical procedures, in part owing to their large variety and low concentration. In this Review, we describe current and emerging techniques that detect and quantify small-molecule enantiomers and their biological importance.

Optimized derivatization of primary amines with the fluorogenic reagent naphthalene-2,3-dicarboxaldehyde toward reproducible quantitative analysis in biological systems

Primary amines are the target of many bioanalytical analyses, as they are ubiquitous in biological systems and responsible for numerous important processes including neurotransmission and cell signaling. Primary amines can be sensitively detected via fluorescence after their reaction with the fluorogenic reagent naphthalene-2,3-dicarboxaldehyde (NDA) in the presence of cyanide through the formation of fluorescent N-substituted 1-cyanobenz[f]isoindole (CBI) derivatives. While fluorogenic reagents such as NDA can be advantageous for sensitive detection, improvements in both long-term stability and speed of reaction are necessary to enable practical and reproducible quantitative analysis. In this work, various CBI-amines were interrogated for their fluorescence characteristics over time under previously reported conditions (75:25 aqueous buffer:acetonitrile). An amine-specific decline in fluorescence and delay to reach maximum fluorescence were observed. Based on structural characteristics, we hypothesized that these effects were due to the solvents employed enabling analyte intermolecular interactions that resulted in fluorescence quenching over time. To mitigate fluorescence-quenching intermolecular interactions, we developed two strategies to improve the fluorescence of the CBI-product over long time periods: (1) the addition of the complexation reagent β-cyclodextrin to the reaction matrix and (2) the substitution of acetonitrile with dimethyl sulfoxide. Both strategies improved fluorescence stability over time, and the incorporation of dimethyl sulfoxide also enabled more rapid attainment of maximum fluorescence and a higher absolute fluorescence when compared to initial conditions. When employed in combination, these two approaches further improve fluorescence stability over time for the most hydrophobic analytes. In the future, these strategies can be employed for the practical and reproducible quantitative analysis of primary amines in biological systems, including those related to neurological disorders and disease states.

A voltage trade study for the design of capillary electrophoresis instruments for spaceflight

Capillary electrophoresis (CE) systems have undergone extensive development for spaceflight applications. A flight-compatible high voltage power supply and the necessary voltage isolation for other energized components can be large contributors to both the volume and mass of a CE system, especially if typical high voltage levels of 25-30 kV are used. Here, we took advantage of our custom CE hardware to perform a trade study for simultaneous optimization of capillary length, high voltage level, and separation time, without sacrificing method performance. A capillary electrophoresis with capacitively coupled contactless conductivity detection (CE-C4 D) method recently developed by our group to target inorganic cations and amino acids relevant to astrobiology was used as a test case. The results indicate that a 50 cm long capillary with 15 kV applied voltage (half of that used in the original method) can be used to achieve measurement goals while minimizing instrument size.

The Impact of Extreme Low Temperatures on Raman Spectra of Amino Acids Relevant for the Search for Life on Europa

Raman spectroscopy, an emerging technology for in situ space exploration, has been suggested for life detection for the Europa Lander Mission. However, obtaining spectra of samples from the europan icy shell requires measurements at temperatures down to -233°C, which will affect the Raman spectra of any potential biosignatures. In this study, we obtained Raman spectra of amino acids using a 785 nm Raman system at temperatures ranging down to -196°C, analogous to Europa's surface and near subsurface. Significant Raman band width narrowing and decreasing variance were observed at lower temperatures leading to higher-precision Raman measurements, which required higher spectral resolution that could be as high as 2 cm^-1 for full identification of amino acids. Such spectral resolution is much higher than the resolution of contemporary Raman instruments for planetary exploration and may be particularly problematic for miniaturized instruments. Shifting of Raman bands to both higher and lower frequencies by as much as ∼25 cm^-1 together with changes in the Raman band intensity were recorded. The emergence of new bands and diminishing of the original bands also occurred for some amino acids. A significantly increased fluorescence background was observed in spectra of fluorescent molecules (i.e., tryptophan). A link between the type of vibrational modes associated with Raman bands and the change in their Raman shift at extreme low temperatures was identified and described. This link offers an exciting new method of molecule identification solely based on the comparison of spectra collected at two different temperatures and could greatly improve the identification capabilities in Raman spectroscopy for a wide array of applications.

Autonomous CE Mass-Spectra Examination for the Ocean Worlds Life Surveyor

Ocean worlds such as Europa and Enceladus are high priority targets in the search for past or extant life beyond Earth. Evidence of life may be preserved in samples of surface ice by processes such as deposition from active plumes, hydrofracturing, or thermal convection. Terrestrial life produces unique distributions of organic molecules that translate into recognizable biosignatures. Identification and quantification of these organic compounds can be achieved by separation science such as capillary electrophoresis coupled to mass spectrometry (CE-MS). However, the data generated by such an instrument can be multiple orders of magnitude larger than what can be transmitted back to Earth during an ocean world's mission. This requires onboard science data analysis capabilities that summarize and prioritize CE-MS observations with limited computational resources. In response, the autonomous capillary electrophoresis mass-spectra examination (ACME) onboard science autonomy system was created for application to the ocean world's life surveyor (OWLS) instrument suite. ACME is able to compress raw mass spectra by two to three orders of magnitude while preserving most of its scientifically relevant information content. This summarization is achieved by the extraction of raw data surrounding autonomously identified ion peaks and the detection and parameterization of unique background regions. Prioritization of the summarized observations is then enabled by providing estimates of scientific utility, including presence of key target compounds, and the uniqueness of an observation relative to previous observations.

Microfluidic Chromatography for Enhanced Amino Acid Detection at Ocean Worlds

Increasing interest in the detection of biogenic signatures, such as amino acids, on icy moons and bodies within our solar system has led to the development of compact in situ instruments. Given the expected dilute biosignatures and high salinities of these extreme environments, purification of icy samples before analysis enables increased detection sensitivity. Herein, we outline a novel compact cation exchange method to desalinate proteinogenic amino acids in solution, independent of the type and concentration of salts in the sample. Using a modular microfluidic device, initial experiments explored operational limits of binding capacity with phenylalanine and three model cations, Na⁺, Mg²⁺, and Ca²⁺. Phenylalanine recovery (94-17%) with reduced conductivity (30-200 times) was seen at high salt-to-amino-acid ratios between 25:1 and 500:1. Later experiments tested competition between mixtures of 17 amino acids and other chemistries present in a terrestrial ocean sample. Recoveries ranged from 11% to 85% depending on side chain chemistry and cation competition, with concentration shown for select high affinity amino acids. This work outlines a nondestructive amino acid purification device capable of coupling to multiple downstream analytical techniques for improved characterization of icy samples at remote ocean worlds.

Comparison of the Performance of Different Bile Salts in Enantioselective Separation of Palonosetron Stereoisomers by Micellar Electrokinetic Chromatography

Bile salts are a category of natural chiral surfactants which have ever been used as the surfactant and chiral selector for the separation of many chiral compounds by micellar electrokinetic chromatography (MEKC). In our previous works, the application of sodium cholate (SC) in the separation of four stereoisomers of palonosetron (PALO) by MEKC has been studied systematically. In this work, the parameters of other bile salts, including sodium taurocholate (STC), sodium deoxycholate (SDC), and sodium taurodeoxycholate (STDC) in the separation of PALO stereoisomers by MEKC were measured and compared with SC. It was found that all of four bile salts provide chiral recognition for both pairs of enantiomers, as well as achiral selectivity for diastereomers of different degrees. The structure of steroidal ring of bile salts has a greater impact on the separation than the structure of the side chain. The varying separation results by different bile salts were elucidated based on the measured parameters. A model to describe the contributions of the mobility difference of solutes in the aqueous phase and the selectivity of micelles to the chiral and achiral separation of stereoisomers was introduced. Additionally, a new approach to measure the mobility of micelles without enough solubility for hydrophobic markers was proposed, which is necessary for the calculation of separation parameters in MEKC. Under the guidance of derived equations, the separation by SDC and STDC was significantly improved by using lower surfactant concentrations. The complete separation of four stereoisomers was achieved in less than 3.5 min by using 4.0 mM of SDC. In addition, 30.0 mM of STC also provided the complete resolution of four stereoisomers due to the balance of different separation mechanisms. Its applicability for the analysis of a small amount of enantiomeric impurities in the presence of a high concentration of the effective ingredient was validated by a real sample.

Biological Homochirality and the Search for Extraterrestrial Biosignatures

Most amino acids and sugar molecules occur in mirror, or chiral, images of each other, knowns as enantiomers. However, life on Earth is mostly homochiral: proteins contain almost exclusively L-amino acids, while only D-sugars appear in RNA and DNA. The mechanism behind this fundamental asymmetry of life remains unknown, despite much progress in the theoretical and experimental understanding of homochirality in the past decades. We review three potential mechanisms for the emergence of biological homochirality on primal Earth and explore their implications for astrobiology: the first, that biological homochirality is a stochastic process driven by local environmental fluctuations; the second, that it is driven by circularly-polarized ultraviolet radiation in star-forming regions; and the third, that it is driven by parity violation at the elementary particle level. We argue that each of these mechanisms leads to different observational consequences for the existence of enantiomeric excesses in our solar system and in exoplanets, pointing to the possibility that the search for life elsewhere will help elucidate the origins of homochirality on Earth.

Detection of Biosignatures by Capillary Electrophoresis Mass Spectrometry in the Presence of Salts Relevant to Ocean Worlds Missions

Capillary electrophoresis (CE) is a promising liquid-based technique for in situ chemical analysis on ocean worlds that allows the detection of a wide range of organic molecules relevant to the search for life. CE coupled with mass spectrometry (MS) is particularly valuable as it also enables the discovery of unknown compounds. Here we demonstrate that CE coupled to MS via electrospray ionization (ESI) can readily analyze samples containing up to half the saturation levels of salts relevant to ocean worlds when using 5 M acetic acid as the separation media. A mixture containing amino acids, peptides, nucleobases, and nucleosides was analyzed in the presence of two salts, NaCl and MgSO₄, based on their relevance to Europa and Enceladus. We demonstrate here CE-MS limits of detection for these organics ranging from 0.05 to 1 μM (8 to 89 ppb) in the absence of salts. More importantly, we demonstrate here for the first time that organics in the low micromolar range (1-50 μM) are detected by CE-MS in the presence of 3 M NaCl without desalting, preconcentration, or derivatization. This demonstration highlights how CE-MS is uniquely suited for organic analysis on future missions to ocean worlds.

Science Objectives for Flagship-Class Mission Concepts for the Search for Evidence of Life at Enceladus

Cassini revealed that Saturn's Moon Enceladus hosts a subsurface ocean that meets the accepted criteria for habitability with bio-essential elements and compounds, liquid water, and energy sources available in the environment. Whether these conditions are sufficiently abundant and collocated to support life remains unknown and cannot be determined from Cassini data. However, thanks to the plume of oceanic material emanating from Enceladus' south pole, a new mission to Enceladus could search for evidence of life without having to descend through kilometers of ice. In this article, we outline the science motivations for such a successor to Cassini, choosing the primary science goal to be determining whether Enceladus is inhabited and assuming a resource level equivalent to NASA's Flagship-class missions. We selected a set of potential biosignature measurements that are complementary and orthogonal to build a robust case for any life detection result. This result would be further informed by quantifications of the habitability of the environment through geochemical and geophysical investigations into the ocean and ice shell crust. This study demonstrates that Enceladus' plume offers an unparalleled opportunity for in situ exploration of an Ocean World and that the planetary science and astrobiology community is well equipped to take full advantage of it in the coming decades.

Determining the "Biosignature Threshold" for Life Detection on Biotic, Abiotic, or Prebiotic Worlds

The field of prebiotic chemistry has demonstrated that complex organic chemical systems that exhibit various life-like properties can be produced abiotically in the laboratory. Understanding these chemical systems is important for astrobiology and life detection since we do not know the extent to which prebiotic chemistry might exist or have existed on other worlds. Nor do we know what signatures are diagnostic of an extant or "failed" prebiotic system. On Earth, biology has suppressed most abiotic organic chemistry and overprints geologic records of prebiotic chemistry; therefore, it is difficult to validate whether chemical signatures from future planetary missions are remnant or extant prebiotic systems. The "biosignature threshold" between whether a chemical signature is more likely to be produced by abiotic versus biotic chemistry on a given world could vary significantly, depending on the particular environment, and could change over time, especially if life were to emerge and diversify on that world. To interpret organic signatures detected during a planetary mission, we advocate for (1) gaining a more complete understanding of prebiotic/abiotic chemical possibilities in diverse planetary environments and (2) involving experimental prebiotic samples as analogues when generating comparison libraries for "life-detection" mission instruments.

The chirality determination of amino acids by forming complexes with cyclodextrins and metal ions using ion mobility spectrometry, and a DFT calculation

Chiral recognition is of highly interest in the areas of chemistry, pharmaceuticals, and bioscience. An effective strategy of enantiomeric determination of amino acids (AAs) was developed in this work. All 19 natural AAs enantiomers can be easily distinguished by ion mobility-mass spectrometry of the non-covalent complexes of AAs with cyclodextrins (α-CD, β-CD and γ-CD) and Mg²⁺ without any chemical derivatization. Differences of the mobilities between the enantiomers' complexes is from 0.006 to 0.058 V s/cm². In addition, the complex of [β-CD + Phe + Mg]²⁺ was selected as an example to study the relative quantification by measuring L/D-Phe at different molar ratio of 10:1 to 1:10 in the μM range, resulting in a good linearity (R² > 0.99) and high sensitivity at 2 μM. A DFT calculation was also performed to illustrate the detailed molecular structure of the complexes of CDs, Mg²⁺ and D- or L-Phe. Both experiment and theoretical calculation showed that Mg²⁺ plays an important role in host/guest interactions, which changed the molecular conformations by non-covalent interaction between Mg²⁺ and CDs, and resulted in the different collision cross-sections of the complex ions of CDs, Mg²⁺ and D- or L-AAs in the gas phase. This effective and convenient strategy could potentially be utilized in scientific research and industry for routine enantiomeric determination of natural AAs, peptides and some other small chiral biomolecules such as non-natural AAs and carboxylic acid-containing drugs.

Chirality in Organic and Mineral Systems: A Review of Reactivity and Alteration Processes Relevant to Prebiotic Chemistry and Life Detection Missions

Chirality is a central feature in the evolution of biological systems, but the reason for biology’s strong preference for specific chiralities of amino acids, sugars, and other molecules remains a controversial and unanswered question in origins of life research. Biological polymers tend toward homochiral systems, which favor the incorporation of a single enantiomer (molecules with a specific chiral configuration) over the other. There have been numerous investigations into the processes that preferentially enrich one enantiomer to understand the evolution of an early, racemic, prebiotic organic world. Chirality can also be a property of minerals; their interaction with chiral organics is important for assessing how post-depositional alteration processes could affect the stereochemical configuration of simple and complex organic molecules. In this paper, we review the properties of organic compounds and minerals as well as the physical, chemical, and geological processes that affect organic and mineral chirality during the preservation and detection of organic compounds. We provide perspectives and discussions on the reactions and analytical techniques that can be performed in the laboratory, and comment on the state of knowledge of flight-capable technologies in current and future planetary missions, with a focus on organics analysis and life detection.

Solid phase extraction on reverse phase chromatographic media subjected to stresses expected for extraterrestrial implementation

Reverse phase solid phase extraction matrices were tested using standard analytes then exposed to space flight stresses to ensure functionality when deployed for liquid sample analysis on remote bodies.

Optimization of Fluorescence Labeling of Trace Analytes: Application to Amino Acid Biosignature Detection with Pacific Blue

Fluorescence labeling of biomolecules and fluorescence detection platforms provide a powerful approach to high-sensitivity bioanalysis. Reactive probes can be chosen to target specific functional groups to enable selective analysis of a chosen class of analytes. Particularly, when targeting trace levels of analytes, it is important to optimize the reaction chemistry to maximize the labeling efficiency and minimize the background. Here, we develop methods to optimize the labeling and detection of Pacific Blue (PB)-tagged amino acids. A model is developed to quantitate labeling kinetics and completeness in the circumstance where analyte labeling and reactive probe hydrolysis are in competition. The rates of PB hydrolysis and amino acid labeling are determined as a function of pH. Labeling kinetics and completeness as a function of PB concentration are found to depend on the ratio of the hydrolysis time to the initial labeling time, which depends on the initial PB concentration. Finally, the optimized labeling and detection conditions are used to perform capillary electrophoresis analysis demonstrating 100 pM sensitivities and high-efficiency separations of an 11 amino acid test set. This work provides a quantitative optimization model that is applicable to a variety of reactive probes and targets. Our approach is particularly useful for the analysis of trace amine and amino acid biosignatures in extraterrestrial samples. For illustration, our optimized conditions (reaction at 4 °C in a pH 8.5 buffer) are used to detect trace amino acid analytes at the 100 pM level in an Antarctic ice core sample.

Powerful Steroid-Based Chiral Selector for High-Throughput Enantiomeric Separation of α-Amino Acids Utilizing Ion Mobility-Mass Spectrometry

Stereospecific recognition of amino acids (AAs) plays a crucial role in chiral biomarker-based diagnosis and prognosis. Separation of AA enantiomers is a long and tedious task due to the requirement of AA derivatization prior to the chromatographic or electrophoretic steps which are also time-consuming. Here, a mass-tagged chiral selector named [d₀]/[d₅]-estradiol-3-benzoate-17β-chloroformate ([d₀]/[d₅]-17β-EBC) with high reactivity and good enantiomeric resolution in regard to AAs was developed. After a quick and easy chemical derivatization step of AAs using 17β-EBC as the single chiral selector before ion mobility-mass spectrometry analysis, good enantiomer separation was achieved for 19 chiral proteinogenic AAs in a single analytical run (∼2 s). A linear calibration curve of enantiomeric excess was also established using [d₀]/[d₅]-17β-EBC. It was demonstrated to be capable of determining enantiomeric ratios down to 0.5% in the nanomolar range. 17β-EBC was successfully applied to investigate the absolute configuration of AAs among peptide drugs and detect trace levels of d-AAs in complex biological samples. These results indicated that [d₀]/[d₅]-17β-EBC may contribute to entail a valuable step forward in peptide drug quality control and discovering chiral disease biomarkers.

Bile Salts in Chiral Micellar Electrokinetic Chromatography: 2000-2020

Bile salts are naturally occurring chiral surfactants that are able to solubilize hydrophobic compounds. Because of this ability, bile salts were exploited as chiral selectors added to the background solution (BGS) in the chiral micellar electrokinetic chromatography (MEKC) of various small molecules. In this review, we aimed to examine the developments in research on chiral MEKC using bile salts as chiral selectors over the past 20 years. The review begins with a discussion of the aggregation of bile salts in chiral recognition and separation, followed by the use of single bile salts and bile salts with other chiral selectors (i.e., cyclodextrins, proteins and single-stranded DNA aptamers). Advanced techniques such as partial-filling MEKC, stacking and single-drop microextraction were considered. Potential applications to real samples, including enantiomeric impurity analysis, were also discussed.

Fabrication of Homochiral Metal-Organic Frameworks in TiO2 Nanochannels for In Situ Identification of 3,4-Dihydroxyphenylalanine Enantiomers

Enantioselective identification of chiral molecules is important for biomedical and pharmaceutical research. However, owing to identical molecular formulas and chemical properties of enantiomers, signal transduction and amplification are still the two major challenges in chiral sensing. In this study, we developed an enantioselective membrane by integrating homochiral metal-organic frameworks (MOFs) with nanochannels for the sensitive identification and quantification of chiral compounds. The membrane was designed using a TiO₂ nanochannel membrane (TiNM) as the metal ion precursor of MOFs (using MIL-125(Ti)) and incorporating l-glutamine (l-Glu) into the framework of MIL-125(Ti). Using 3,4-dihydroxyphenylalanine (DOPA) as the model analyte, the as-prepared homochiral l-Glu/MIL-125(Ti)/TiNM exhibits a remarkable chiral recognition to d-DOPA than l-DOPA. More importantly, benefiting from the highly enlarged surface area and confinement effect provided by the MOFs-in-nanochannel architecture, the discrimination for chiral recognition is largely amplified through the chelation interaction of Fenton-like activity of Fe³⁺ onto DOPA. Using 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) as the substrate, the positively charged ABTS^•+ product via Fenton-like reaction induces significant ionic transport changes in nanochannels, which in turn provides information about chiral recognition. This innovative signal amplification strategy on homochiral nanochannels might pave a new way for sensitive monitoring and chiral recognition.

Capillary electrophoresis method for analysis of inorganic and organic anions related to habitability and the search for life

In situ missions of exploration require analytical methods that are capable of detecting a wide range of molecular targets in complex matrices without a priori assumptions of sample composition. Furthermore, these methods should minimize the number of reagents needed and any sample preparation steps. We have developed a method for the detection of metabolically relevant inorganic and organic anions that is suitable for implementation on in situ spaceflight missions. Using 55 mM acetic acid, 50 mM triethylamine, and 5% glycerol, more than 21 relevant anions are separated in less than 20 min. The method is robust to sample ionic strength, tolerating high concentrations of background salts (up to 900 mM NaCl and 300 mM MgSO₄ ). This is an important feature for future missions to ocean worlds. The method was validated using a culture of Escherichia coli and with high salinity natural samples collected from Mono Lake, California.

Electrochemical Polymerization Induced Chirality Fixation of Crystalline Pillararene-Based Polymer and Its Application in Interfacial Chiral Sensing

A new strategy has been developed for the direct chirality fixation, which is induced by electrochemical polymerization, of macrocyclic hosts pillar[5]arene. Taking advantage of electrochemical polymerization, thiophene-modified pillar[5]arene monomers (Th-P[5]A) have been regularly arranged under the action of an electric field to form chiral nanofiber-like crystalline pillar[5]arene-based polymers (poly-Th-P[5]A), showing a significant circular dichroism (CD) signal. With the active photochemical properties, poly-Th-P[5]A is first used as a photoelectrochemical (PEC) chiral sensor for the identification and determination of l- and d-ascorbic acid (l-AA, d-AA) without adding any extra photoactive probes. Importantly, the chiral recognition between poly-Th-P[5]A and l-AA also triggers a polarity conversion for the photocurrent of the polymer, and it greatly results in a broad chiral detection range for l-AA, crossing 6 orders of magnitude. This work provides a promotional strategy for building a PEC chiral recognition platform based on pillararenes.

Polysaccharides as separation media for the separation of proteins, peptides and stereoisomers of amino acids

Reliable separation of peptides, amino acids and proteins as accurate as possible with the maximum conformation and biological activity is crucial and essential for drug discovery. Polysaccharide, as one of the most abundant natural biopolymers with optical activity on earth, is easy to be functionalized due to lots of hydroxyl groups on glucose units. Over the last few decades, polysaccharide derivatives are gradually employed as effective separation media. The highly-ordered helical structure contributes to complex, diverse molecular recognition ability, allowing polysaccharide derivatives to selectively interact with different analytes. This article reviews the development, application and prospects of polysaccharides as separation media in the separation of proteins, peptides and amino acids in recent years. The chiral molecules mechanism, advantages, limitations, development status and challenges faced by polysaccharides as separation media in molecular recognition are summarized. Meanwhile, the direction of its continued development and future prospects are also discussed.

Automated Capillary Electrophoresis System Compatible with Multiple Detectors for Potential In Situ Spaceflight Missions

The in situ search for chemical signatures of life on extraterrestrial worlds requires automated hardware capable of performing detailed compositional analysis during robotic missions of exploration. The use of electrophoretic separations in this search is particularly powerful, enabling analysis of a wide range of soluble organic compounds potentially indicative of life, as well as inorganic compounds that can serve as indicators of habitability. However, to detect this broad range of compounds with a single electrophoresis instrument, a combination of different detection modes is required. For detection of any ionizable species, including organic compounds that do not participate in terrestrial biology (i.e., "unknown unknowns"), mass spectrometry (MS) is essential. Inorganic ions, or any dissolved charged species present, can be analyzed using capacitively coupled contactless conductivity detection (C⁴D). Additionally, for the trace analysis of compounds of key interest to astrobiology (particularly, amino acids), laser-induced fluorescence (LIF) detection holds unique promise, due to the fact that it has the highest demonstrated sensitivity of any form of detection. Here, we demonstrate a fully automated, portable capillary electrophoresis analyzer that is capable of all these modes of detection. The prototype system developed here addresses the three most significant challenges for doing electrophoretic separations: precise sample injection, HV isolation, and automation of all operational steps. These key challenges were successfully addressed with the use of custom-designed rotor-stator valves with optimized operational sequences incorporating gas purging steps, rinses, and HV application.

Extraction and Separation of Chiral Amino Acids for Life Detection on Ocean Worlds Without Using Organic Solvents or Derivatization

In situ instrumentation that can detect amino acids at parts-per-billion concentration levels and distinguish an enantiomeric excess of either d- or l-amino acids is vital for future robotic life-detection missions to promising targets in our solar system. In this article, a novel chiral amino acid analysis method is described, which reduces the risk of organic contamination and spurious signals from by-products by avoiding organic solvents and organic additives. Online solid-phase extraction, chiral liquid chromatography, and mass spectrometry were used for automated analysis of amino acids from solid and aqueous environmental samples. Carbonated water (pH ∼3, ∼5 wt % CO₂ achieved at 6 MPa) was used as the extraction solvent for solid samples at 150°C and as the mobile phase at ambient temperature for chiral chromatographic separation. Of 18 enantiomeric amino acids, 5 enantiomeric pairs were separated with a chromatographic resolution >1.5 and 12 pairs with a resolution >0.7. The median lower limit of detection of amino acids was 2.5 μg/L, with the lowest experimentally verified as low as 0.25 μg/L. Samples from a geyser site (Great Fountain Geyser) and a geothermal spring site (Lemon Spring) in Yellowstone National Park were analyzed to demonstrate the viability of the method for future in situ missions to Ocean Worlds.

Discriminating Abiotic and Biotic Fingerprints of Amino Acids and Fatty Acids in Ice Grains Relevant to Ocean Worlds

Identifying and distinguishing between abiotic and biotic signatures of organic molecules such as amino acids and fatty acids is key to the search for life on extraterrestrial ocean worlds. Impact ionization mass spectrometers can potentially achieve this by sampling water ice grains formed from ocean water and ejected by moons such as Enceladus and Europa, thereby exploring the habitability of their subsurface oceans in spacecraft flybys. Here, we extend previous high-sensitivity laser-based analog experiments of biomolecules in pure water to investigate the mass spectra of amino acids and fatty acids at simulated abiotic and biotic relative abundances. To account for the complex background matrix expected to emerge from a salty Enceladean ocean that has been in extensive chemical exchange with a carbonaceous rocky core, other organic and inorganic constituents are added to the biosignature mixtures. We find that both amino acids and fatty acids produce sodiated molecular peaks in salty solutions. Under the soft ionization conditions expected for low-velocity (2-6 km/s) encounters of an orbiting spacecraft with ice grains, the unfragmented molecular spectral signatures of amino acids and fatty acids accurately reflect the original relative abundances of the parent molecules within the source solution, enabling characteristic abiotic and biotic relative abundance patterns to be identified. No critical interferences with other abiotic organic compounds were observed. Detection limits of the investigated biosignatures under Enceladus-like conditions are salinity dependent (decreasing sensitivity with increasing salinity), at the μM or nM level. The survivability and ionization efficiency of large organic molecules during impact ionization appear to be significantly improved when they are protected by a frozen water matrix. We infer from our experimental results that encounter velocities of 4-6 km/s are most appropriate for impact ionization mass spectrometers to detect and discriminate between abiotic and biotic signatures.

A modular, easy-to-use microcapillary electrophoresis system with laser-induced fluorescence for quantitative compositional analysis of trace organic molecules

Microcapillary electrophoresis (μCE) enables high-resolution separations in miniaturized, automated microfluidic devices. Pairing this powerful separation technique with laser-induced fluorescence (LIF) enables a highly sensitive, quantitative, and compositional analysis of organic molecule monomers and short polymers, which are essential, ubiquitous components of life on Earth. Improving methods for their detection has applications to multiple scientific fields, particularly those related to medicine, industry, and space science. Here, a modular benchtop system using μCE with LIF detection was constructed and tested by analyzing standard amino acid samples of valine, serine, alanine, glycine, glutamic acid, and aspartic acid in multiple borate buffered solutions of increasing concentrations from 10 mM to 50 mM, all pH 9.5. The 35 mM borate buffer solution generated the highest resolution before Joule heating dominated. The limits of detection of alanine and glycine using 35 mM borate buffer were found to be 2.12 nM and 2.91 nM, respectively, comparable to other state-of-the-art μCE-LIF instruments. This benchtop system is amenable to a variety of detectors, including a photomultiplier tube, a silicon photomultiplier, or a spectrometer, and currently employs a spectrometer for facile multi-wavelength detection. Furthermore, the microdevice is easily exchanged to fit the desired application of the system, and optical components within the central filter cube can be easily replaced to target alternative fluorescent dyes. This work represents a significant step forward for the analysis of small organic molecules and biopolymers using μCE-LIF systems.

Recent advances in microchip enantioseparation and analysis

This review summarizes recent developments (over the past decade) in the field of microfluidics-based solutions for enantiomeric separation and detection. The progress in various formats of microchip electrodriven separations, such as MCE, microchip electrochromatography, and multidimensional separation techniques, is discussed. Innovations covering chiral stationary phases, surface coatings, and modification strategies to improve resolution, as well as integration with detection systems, are reported. Finally, combinations with other microfluidic functional units are also presented and highlighted.

Label-free test kit for D-amino acid analysis by 1,4-benzenediboronic-acid-induced aggregation of gold nanoparticles

The level of d-amino acids (DAAs) is significantly related to bacterial contamination in health and disease, food science and nutrition, and industrial applications. Most sensing methods for the detection of DAAs need expensive equipment, and skilled operation experience, making the test kit for DAA analysis much more complex and challenging. Toward this end, we exploited a label-free DAA test kit based on 1,4-benzenediboronic acid (BDBA)-induced gold nanoparticles (AuNPs) aggregations. DAAs were first catalyzed by their specific catalase (DAAO, d-amino acid oxidase) and oxidized to produce H2O2. Then, the produced H2O2 inhibited the citrate-capped AuNPs aggregation under BDBA, while the unreacted BDBA could lead to AuNPs aggregation. As a result, the UV/vis absorption spectra and optical photographs of the AuNPs solution are changed in the presence of different DAA target amounts. This method not only can be used for qualitative and semi-quantitative analysis with a naked-eye readout, but also it can quantify DAAs in aqueous solutions with high sensitivity and specificity. Furthermore, the DAAs test kit provided an effective and selective quantification of DAAs with excellent precision and accuracy in bacterial samples. We believe this DAA test kit provides the potential to be further developed for DAA detection for satisfying both lab and practical needs in different fields.

Chiral Micellar Electrokinetic Chromatography

The potential of Micellar Electrokinetic Chromatography to achieve enantiomeric separations is reviewed in this article. The separation principles and the most frequently employed separation strategies to achieve chiral separations by Micellar Electrokinetic Chromatography are described. The use of chiral micellar systems alone or combined with other micellar systems or chiral selectors, as well as of mixtures of achiral micellar systems with chiral selectors is discussed together with the effect of different additives present in the separation medium. Indirect methods based on the derivatization of analytes with chiral derivatizing reagents and the use of achiral micelles are also considered. Preconcentration techniques employed to improve sensitivity and the main approaches developed to facilitate the coupling with Mass Spectrometry are included. The most recent and relevant methodologies developed by chiral Micellar Electrokinetic Chromatography and their applications in different fields are presented.

Ionic Current Rectification Triggered Photoelectrochemical Chiral Sensing Platform for Recognition of Amino Acid Enantiomers on Self-Standing Nanochannel Arrays

Chirality is an intrinsic and essential property of nature, and the enantiomeric discrimination of chiral molecules can provide important information leading to a better understanding of chiral recognition in biological systems and furthering the development of useful molecular devices in biochemical and pharmaceutical studies. Therefore, the exploration of new detection techniques with high accuracy and reliability for high-throughput enantioselective detection is still highly desirable. Herein, a chiral enantiomers selective recognition platform based on self-standing titanium dioxide nanochannel arrays (TiO₂ NCAs) is proposed to implement the ionic current rectification triggered photoelectrochemical (PEC) detection, which provides a novel sensing platform to discriminate chiral amino acid through synchronous output dual response signals of ionic current and photocurrent. The utilization of nanochannel arrays guaranteed the high-throughput detection, and the dual signal model avoided a "false positive" detection. In addition, based on the fabricated detection platform, various chiral substances can be facilely detected through integrating different chiral regulation units, which shed light on the construction of reliable chiral sensors for practical applications in a biological environment.

Analog Experiments for the Identification of Trace Biosignatures in Ice Grains from Extraterrestrial Ocean Worlds

Reliable identification of biosignatures, such as amino acids, fatty acids, and peptides, on extraterrestrial ocean worlds is a key prerequisite for space missions that search for life or its emergence on these worlds. One promising approach is the use of high-performance in situ impact ionization mass spectrometers to sample water ice grains emerging from ocean-bearing moons such as Europa or Enceladus. A predecessor of such detectors, the Cosmic Dust Analyzer on board the Cassini spacecraft, has proven to be very successful in analyzing inorganic and organic ocean constituents and with that characterizing the habitability of Enceladus ocean. However, biosignatures have not been definitively identified in extraterrestrial ocean environments so far. Here, we investigate with an analog experiment the spectral appearance of amino acids, fatty acids, and peptides in water ice grains, together with their detection limits, as applicable to spaceborne mass spectrometers. We employ a laboratory-based laser induced liquid beam ion desorption technique, proven to simulate accurately the impact ionization mass spectra of water ice grains over a wide range of impact speeds. The investigated organics produce characteristic mass spectra, with molecular peaks as well as clearly identifiable, distinctive fragments. We find the detection limits of these key biosignatures to be at the μM or nM level, depending on the molecular species and instrument polarity, and infer that impact ionization mass spectrometers are most sensitive to the molecular peaks of these biosignatures at encounter velocities of 4-6 km/s.

Experimental and Simulation Efforts in the Astrobiological Exploration of Exooceans

The icy satellites of Jupiter and Saturn are perhaps the most promising places in the Solar System regarding habitability. However, the potential habitable environments are hidden underneath km-thick ice shells. The discovery of Enceladus' plume by the Cassini mission has provided vital clues in our understanding of the processes occurring within the interior of exooceans. To interpret these data and to help configure instruments for future missions, controlled laboratory experiments and simulations are needed. This review aims to bring together studies and experimental designs from various scientific fields currently investigating the icy moons, including planetary sciences, chemistry, (micro-)biology, geology, glaciology, etc. This chapter provides an overview of successful in situ, in silico, and in vitro experiments, which explore different regions of interest on icy moons, i.e. a potential plume, surface, icy shell, water and brines, hydrothermal vents, and the rocky core.

β-cyclodextrin modified quantum dots as pseudo-stationary phase for direct enantioseparation based on capillary electrophoresis with laser-induced fluorescence detection

In recent years, quantum dots (QDs) have attracted a tremendous amount of attention due to their compelling features. In this work, a kind of composite QDs based on β-cyclodextrin (β-CD) and its derivatives modification was prepared, and for the first time utilized to separate and determine enantiomers in the combined system of capillary electrophoresis with laser-induced fluorescence detection (CE-LIF). By taking advantages of the inclusion complexation of β-CD and the fluorescence property of QD core, the composite QDs were added into the running buffer as pseudo-stationary phase. The resultant CE-LIF method accomplished enantioseparation for six groups of model analytes without need of capillary preparation and analyte derivatization. The effects of composite QDs concentration, the pH value and concentration of the running buffer on resolution have been investigated individually. The RSDs of interday and intraday repeatability were in the range of 2.7-8.1%, 0.7-3.9%, and 1.5-3.8% for the peak area, migration time and resolution, respectively. The theoretical calculation results of the binding energies and binding constant further validated the interaction mechanism of composite QDs and target analytes. Furthermore, this developed method was successfully applied to the analysis of the active components (catechin and epicatechin) in Chinese herb Catechu, and the recoveries were in the range of 92.2-108%. The experimental results suggested that the preparation strategy of the composite QDs is appropriate for enantioseparation of more enantiomers by adjusting the modifiers on the surface of QDs, which is particularly promising for electrophoretic enantioseparation based on fluorescence detection, especially for those analytes lacking proper derivative functional groups.

Long-term thermal stability of fluorescent dye used for chiral amino acid analysis on future spaceflight missions

Future spaceflight missions focused on life detection will carry with them new, state-of-the-art instrumentation capable of highly selective and sensitive organic analysis. CE-LIF is an ideal candidate for such a mission due to its high separation efficiency and low LODs. One perceived risk of utilizing this technique on a future mission is the stability of the chemical reagents in the spaceflight environment. Here, we present an investigation of the thermal stability of the fluorescent dye (5-carboxyfluorescein succinimidyl ester) used for amino acid analysis. The dye was stored at 4, 25, and 60°C for 1 month, 6 months, 1 year, and 2 years. When stored at 4°C for 2 years, 25°C for 6 months, or 60°C for 1 month there was no effect on CE-LIF assay performance due to dye degradation. Beyond these time points, while the dye degradation begins to interfere with the analysis, it is still possible to perform the analysis and achieve the majority of amino acid biosignature science goals described in the science definition team report for the potential Europa Lander mission. This work indicates that thermal control of the dye at ≤4°C will be needed during transit on future spaceflight missions to maintain dye stability.

Chemical Ionization Mass Spectrometry: Applications for the In Situ Measurement of Nonvolatile Organics at Ocean Worlds

A new technique that has applications for the detection of nonvolatile organics on Ocean Worlds has been developed. Here, liquid mixtures of fatty acids (FAs) and/or amino acids (AAs) are introduced directly into a miniature quadrupole ion trap mass spectrometer (QITMS) developed at Jet Propulsion Laboratory and analyzed. Two ionization methods, electron impact and chemical ionization (EI and CI, respectively), are compared and contrasted. Further, multiple CI reagents are tested to explore their potential to "soften" ionization of FAs and AAs. Both EI and CI yield mass spectra that bear signatures of FAs or AAs; however, soft CI yields significantly cleaner mass spectra that are easier to interpret. The combination of soft CI with tandem mass spectrometry (MS/MS) has also been demonstrated for AAs, generating "fingerprint" mass spectra of fragments from protonated parent ions. To mimic potential Ocean World conditions, water is used as the primary collision gas in MS/MS experiments. This technique has the potential for the in situ analysis of molecules in the cryogenic plumes of Ocean Worlds (e.g., Enceladus) and comets with the ultimate goal of detecting potential biosignatures.

Methodologies for Analyzing Soluble Organic Compounds in Extraterrestrial Samples: Amino Acids, Amines, Monocarboxylic Acids, Aldehydes, and Ketones

Soluble organic compositions of extraterrestrial samples offer valuable insights into the prebiotic organic chemistry of the solar system. This review provides a summary of the techniques commonly used for analyzing amino acids, amines, monocarboxylic acids, aldehydes, and ketones in extraterrestrial samples. Here, we discuss possible effects of various experimental factors (e.g., extraction protocols, derivatization methods, and chromatographic techniques) in order to highlight potential influences on the results obtained from different methodologies. This detailed summary and assessment of current techniques is intended to serve as a basic guide for selecting methodologies for soluble organic analyses and to emphasize some key considerations for future method development.

New Advances in Amino Acid Profiling in Biological Samples by Capillary Electrophoresis-Mass Spectrometry

Capillary electrophoresis-mass spectrometry (CE-MS) offers a high efficiency microseparation platform for amino acid profiling when analyzing volume-restricted biological samples, such as a dried blood spot punch. Direct analysis of amino acids and their analogs is routinely achieved using strongly acidic buffer conditions under positive-ion mode detection with a coaxial sheath liquid interface for electrospray ionization (ESI). New advances in online sample preconcentration, pre-column chemical derivatization, and/or low flow/sheathless CE-MS interface designs can further improve sensitivity while allowing for resolution of amino acid stereoisomers and labile aminothiols with low nanomolar detection limits. Additionally, multiplexed separations in CE-MS based on serial injection of seven or more samples within a single run greatly boosts sample throughput (<2-3 min/sample) without added infrastructure costs while allowing for stringent quality control and signal batch correction. Accurate prediction of the electromigration behavior of amino acids and their analogs offers a convenient approach for structural elucidation that is complementary to high-resolution MS and MS/MS. Simultaneous analysis of amino acids together with other classes of ionic metabolites by CE-MS allows for comprehensive metabolomic screening as required for new advances in clinical medicine, nutritional sciences, and population health.