Determination of brassinosteroids in the sub-femtomolar range using dansyl-3-aminophenylboronate derivatization and electrospray mass spectrometry

A Fast and Selective Approach for Profiling Vicinal Diols Using Liquid Chromatography-Post Column Derivatization-Double Precursor Ion Scanning Mass Spectrometry

Vicinal diols are important signaling metabolites of various inflammatory diseases, and some of them are potential biomarkers for some diseases. Utilizing the rapid reaction between diol and 6-bromo-3-pyridinylboronic acid (BPBA), a selective and sensitive approach was established to profile these vicinal diols using liquid chromatography-post column derivatization coupled with double precursor ion scan-mass spectrometry (LC-PCD-DPIS-MS). After derivatization, all BPBA-vicinal-diol esters gave a pair of characteristic isotope ions resulting from ⁷⁹Br and ⁸¹Br. The unique isotope pattern generated two characteristic fragment ions of m/z 200 and 202. Compared to a traditional offline derivatization technique, the new LC-PCD-DPIS-MS method retained the capacity of LC separation. In addition, it is more sensitive and selective than a full scan MS method. As an application, an in vitro study of the metabolism of epoxy fatty acids by human soluble epoxide hydrolase was tested. These vicinal-diol metabolites of individual regioisomers from different types of polyunsaturated fatty acids were easily identified. The limit of detection (LOD) reached as low as 25 nM. The newly developed LC-PCD-DPIS-MS method shows significant advantages in improving the selectivity and therefore can be employed as a powerful tool for profiling vicinal-diol compounds from biological matrices.

A new boronic acid reagent for the simultaneous determination of C27-, C28-, and C29-brassinosteroids in plant tissues by chemical labeling-assisted liquid chromatography-mass spectrometry

Brassinosteroids (BRs) are endogenous plant growth-promoting hormones affecting growth and development during the entire life cycle of plants. Naturally occurring BRs can be classified into C₂₇-, C₂₈-, or C₂₉-BRs based on the nature of the alkyl groups occupying the C-24 position in the side chain of the 5a-cholestane carbon skeleton. However, while C₂₇-BRs exhibit similar bioactivities to C₂₈- and C₂₉-BRs, the biosynthetic pathways of C₂₇-BRs in plants have not yet been clearly characterized. In addition to a lack of biochemical and enzymatic evidence regarding the biosynthetic pathways of C₂₇-BRs, even most of the intermediate compounds on their pathways have not been explored and identified due to the lower endogenous levels of C₂₇-BRs. Therefore, the development of highly sensitive analytical methods is essential for studying the biosynthetic pathways and physiological functions of C₂₇-BRs. Accordingly, this study establishes qualitative and quantitative methods for identifying and detecting C₂₇-, C₂₈-, and C₂₉-BRs using a newly synthesized boronic acid reagent denoted as 2-methyl-4-phenylaminomethylphenylboronic acid (2-methyl-4-PAMBA) in conjunction with liquid chromatography-mass spectrometry (LC-MS). Labeling with 2-methyl-4-PAMBA provides derivatives with excellent stability, and the detection sensitivities of BRs, particularly for C₂₇-BRs, are dramatically improved. The limits of detection (with a signal-to-noise ratio of 3) for six BRs, including 2 C₂₇-BRs (28-norCS and 28-norBL), 3 C₂₈-BRs (CS, BL, and TY), and a single C₂₉-BR (28-homoBL), are found to be 0.10-1.68 pg/mL after labeling with 2-methyl-4-PAMBA. Finally, the proposed analytical method is successfully applied for the detection of endogenous BRs in small mass samples of Oryza sativa seedlings, Rape flowers, Arabidopsis shoots, and Arabidopsis flowers. In addition, a method for profiling potential BRs in plants is also developed using LC-MS in multiple reaction monitoring scan mode assisted by 2-methyl-4-PAMBA and 2-methyl-4-PAMBA-d₅ labeling. The developed method is able to identify 10 potential BRs in a Rape flower extract. The proposed quantitative and qualitative methods established by 2-methyl-4-PAMBA labeling are helpful for facilitating an understanding of the physiological functions and biosynthetic pathways of BRs, particularly for C₂₇-BRs. Graphical abstract.

Pursuing extreme sensitivity for determination of endogenous brassinosteroids through direct fishing from plant matrices and eliminating most interferences with boronate affinity magnetic nanoparticles

Brassinosteroids (BRs) are important plant hormones regulating plant growth and development. High-performance analytical methods for quantifying endogenous BRs are important for studying the molecular mechanisms of BR action. Herein we developed a high-performance sample pretreatment method based on boronate affinity magnetic nanoparticles (BAMNPs). The high specificity of boronate affinity enables direct fishing of BRs from plant matrices. The strong binding energy makes it possible to remove most contaminants in plant matrices with a small loss of target BRs. Besides these advantages, the novel two-step oxidation-hydrolysis elution system raised BR recoveries to 70.5%-98.2%, which was much higher than other boronate affinity applications. The high cleanliness of the final eluents lowered the matrix effects to 85.2%-92.4%. As a result, this method enables simultaneously good recoveries of endogenous BRs and thorough removal of matrix interferences, which greatly improves the sensitivity of BR analysis and reduces the use of plant materials for routine analysis to <10 mg. In addition, the sample handling time can be shortened to <3 h due to the operating convenience of BAMNPs and their easy separation from plant powders. Based on these advantages of BAMNP solid phase extraction, the organ-specific BR distribution analysis in Arabidopsis and rice tissues demonstrates excellent sensitivity, good reproducibility and high throughput of the method. Graphical abstract A high-sensitivity and time-saving UPLC-MS/MS-based quantification method for brassinosteroids (BRs) was developed through directly fishing BRs from plant matrices and eliminating most matrix interferences with as-prepared boronate affinity magnetic nanoparticles (BAMNPs).

Profiling of potential brassinosteroids in different tissues of rape flower by stable isotope labeling - liquid chromatography/mass spectrometry analysis

Brassinosteroids (BRs) play crucial roles in a variety of physiological processes in plants. The full elucidation of the functions of RBs relies on sensitive detection and accurate measurement of BRs in plants. However, the identification and quantification of BRs are challenging due to their low abundance as well as poor ionization efficiencies during mass spectrometry-based analysis. Herein, we developed a highly sensitive and selective strategy for profiling potential BRs in plants by stable isotope labeling liquid chromatography multiple reaction monitoring scan mass spectrometry (SIL-LC-MRM-MS) analysis. In the strategy, we used a pair of stable isotope labeling reagents 4-phenylaminomethyl-benzeneboronic acid (4-PAMBA) and d₅-4-phenylaminomethyl-benzeneboronic acid (4-PAMBA-d₅) that can react with C22-C23 cis-diol on BRs for profiling potential BRs in plant tissues. The 4-PAMBA and 4-PAMBA-d₅ labeled BRs could generate two characteristic neutral loss under collision induced dissociation (CID), respectively, which is used to establish the MRM-based detection and screening. The precursor ions of BRs labeled with 4-PAMBA and 4-PAMBA-d₅ were set according to the reported structures of BRs, and the corresponding product ions were predicted by subtracting the lost neutral loss. In this respect, corresponding precursor ions and product ions in MRM transitions are formed. The peak pairs with a fixed mass difference, similar retention times and intensities were assigned as potential BRs. Using the developed SIL-LC-MRM-MS strategy, we successfully found 13 potential BR in different tissues of rape flower. Taken together, the SIL-LC-MRM-MS analytical strategy is promising for profiling potential BRs as well as other compounds that have the same functional moiety from complex biological samples.

Structure-activity relationship of brassinosteroids and their agricultural practical usages

Brassinosteroids (BRs) control several important agronomic traits, such as strengthening resistance to diverse adversity, improving the quality, and increasing crop yield. Their chemical structures and varieties, specific methods for the evaluation of bioactivities, structure-activity relationships, potential novel compounds, and practical agricultural uses were summarized. The findings allow the examination of brassinosteroids in two important issues: 1) Do the results of different bioevaluation protocols provide similar activities for BRs? and 2) which bioevaluated compounds would proof to have a greater potential for application in agricultural usages?

Immunoaffinity chromatography combined with tandem mass spectrometry: A new tool for the selective capture and analysis of brassinosteroid plant hormones

Brassinosteroids (BRs) are plant-specific steroid hormones that play essential roles in the regulation of many important physiological processes in plant life. Their extremely low concentrations (~pmoles/g FW) in plant tissue and huge differences in polarity of individual members within the BR family hamper their detection and quantification. To address this problem, an immunoaffinity sorbent with broad specificity and high capacity for different BR metabolites containing a monoclonal antibody (mAb) against a BR spacer (20S)-2α,3α-dihydroxy-7-oxa-7α-homo-5α-pregnane-6-one-20 carboxylic acid (BR4812) was used for the rapid and highly selective isolation of endogenous BRs containing a 2α,3α-diol in ring A from minute plant samples. This enrichment procedure was successfully applied as a sample preparation method prior to quantitative analysis of BRs in real plant tissues by ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS). Use of immunoaffinity chromatography (IAC) increased the sensitivity of the UHPLC-MS/MS analysis owing to improvements in the BR signal-to-noise ratio (S/N) and matrix factor (MF). Although MF values of BRs analyzed in classical samples ranged from 8.9% to 47.4%, MF values for the IAC purified samples reached 44.5-96.6%. Thus, the developed IAC-UHPLC-MS/MS approach was shown to be a simple, robust, effective and extremely fast procedure requiring minute amounts of plant samples suitable for the quantitative profiling of many BR metabolites, helping to overcome the major problems associated with their determination in very complex plant matrices.

The determination of 22 natural brassinosteroids in a minute sample of plant tissue by UHPLC-ESI-MS/MS

The triterpenoid plant hormones brassinosteroids (BRs) are believed to influence almost every aspect of plant growth and development. We have developed a sensitive mass spectrometry-based method for the simultaneous profiling of twenty-two naturally occurring brassinosteroids including biosynthetic precursors and the majority of biologically active metabolites. Using ultra-high performance liquid chromatographic (UHPLC) analysis, the run time was reduced up to three times (to 9 min) in comparison to standard HPLC BRs analyses, the retention time stability was improved to 0.1-0.2 % RSD and the injection accuracy was increased to 1.1-4.9 % RSD. The procedures for extraction and for two-step purification based on solid-phase extraction (SPE) were optimised in combination with subsequent UHPLC analysis coupled to electrospray ionisation tandem mass spectrometry (ESI-MS/MS) using Brassica flowers and Arabidopsis plant tissue extracts. In multiple reaction monitoring (MRM) mode, the average detection limit for BRs analysed was close to 7 pg, and the linear range covered up to 3 orders of magnitude. The low detection limits for this broad range of BR metabolites enabled as little as 50 mg of plant tissue to be used for quantitative analyses. The results of determinations exploiting internal standards showed that this approach provides a high level of practicality, reproducibility and recovery. The method we have established will enable researchers to gain a better understanding of the dynamics of the biosynthesis and metabolism of brassinosteroids and their modes of action in plant growth and development.

Profiling of aldehyde-containing compounds by stable isotope labelling-assisted mass spectrometry analysis

We developed a strategy for non-targeted profiling of aldehyde-containing compounds by stable isotope labelling in combination with liquid chromatography-double neutral loss scan-mass spectrometry (SIL-LC-DNLS-MS) analysis. A pair of stable isotope labelling reagents (4-(2-(trimethylammonio)ethoxy)benzenaminium halide, 4-APC and d4-4-(2-(trimethylammonio)ethoxy)benzenaminium halide, 4-APC-d4) that can selectively label aldehyde-containing compounds were synthesized. The 4-APC and 4-APC-d4 labelled compounds were capable of generating two characteristic neutral fragments of 87 Da and 91 Da, respectively, under collision induced dissociation (CID). Therefore, double neutral loss scans were carried out simultaneously to record the signals of the potential aldehyde-containing compounds. In this respect, the aldehyde-containing compounds from two samples labelled with 4-APC and 4-APC-d4 were ionized at the same time but recorded separately by mass spectrometry. The peak pairs with characteristic mass differences (n × 4 Da) can be readily extracted from the DNLS spectra and assigned as potential aldehyde-containing candidates, which facilitates the identification of the target aldehydes. 4-APC and 4-APC-d4 labelling also dramatically increased detection sensitivities of the derivatives. Using the SIL-LC-DNLS-MS strategy, we successfully profiled the aldehyde-containing compounds in human urine and white wine. Our results showed that 16 and 19 potential aldehyde-containing compounds were discovered in human urine and white wine, respectively. In addition, 5 and 4 aldehyde-containing compounds in human urine and white wine were further identified by comparison with aldehyde standards. Altogether, SIL-LC-DNLS-MS demonstrated to be a promising approach in the identification and relative quantification of aldehyde-containing compounds from complex samples.

A new ELISA for quantification of brassinosteroids in plants

Starting from (22R,23R)-2α,3α,22,23,26-pentahydroxy-5α-cholestan-6-one 26-hemisuccinate, conjugates of 28-norcastasterone with horse radish peroxidase and bovine serum albumin were prepared. The latter conjugate was injected into rabbits; produced polyclonal antibodies were used to quantitate 6-keto-brassinosteroids. The newly developed analytical system was used in combination with two other immunoenzymatic assays for brassinosteroids to determine individual compounds of this series. In addition, a direct method of brassinosteroid analysis was proposed. It has the advantage of requiring no sample pretreatment steps such as extraction with organic solvents and chromatography.

Quo vadis plant hormone analysis?

Plant hormones act as chemical messengers in the regulation of myriads of physiological processes that occur in plants. To date, nine groups of plant hormones have been identified and more will probably be discovered. Furthermore, members of each group may participate in the regulation of physiological responses in planta both alone and in concert with members of either the same group or other groups. The ideal way to study biochemical processes involving these signalling molecules is 'hormone profiling', i.e. quantification of not only the hormones themselves, but also their biosynthetic precursors and metabolites in plant tissues. However, this is highly challenging since trace amounts of all of these substances are present in highly complex plant matrices. Here, we review advances, current trends and future perspectives in the analysis of all currently known plant hormones and the associated problems of extracting them from plant tissues and separating them from the numerous potentially interfering compounds.

An improved simplified high-sensitivity quantification method for determining brassinosteroids in different tissues of rice and Arabidopsis

Quantification of brassinosteroids is essential and extremely important to study the molecular mechanisms of their physiological roles in plant growth and development. Herein, we present a simple, material and cost-saving high-performance method for determining endogenous brassinosteroids (BRs) in model plants. This new method enables simultaneous enrichment of a wide range of bioactive BRs such as brassinolide, castasterone, teasterone, and typhasterol with ion exchange solid-phase extraction and high-sensitivity quantitation of these BRs based on isotope dilution combined with internal standard approach. For routine analysis, the consumption of plant materials was reduced to one-twentieth of previously reported and the overall process could be completed within 1 day compared with previous 3 to 4 days. The strategy was validated by profiling BRs in different ecotypes and mutants of rice (Oryza sativa) and Arabidopsis (Arabidopsis thaliana), and the BR distributions in different model plants tissues were determined with the new method. The method allows plant physiologists to monitor the dynamics and distributions of BRs with 1 gram fresh weight of model plant tissues, which will speed up the process for the molecular mechanism research of BRs with these model plants in future work.

A selective pretreatment method for determination of endogenous active brassinosteroids in plant tissues: double layered solid phase extraction combined with boronate affinity polymer monolith microextraction

BACKGROUND

Brassinosteriods (BRs), a group of important phytohormones, have various effects on plant growth and development. However, their physiological functions in plants have not been fully understood to date. Endogenous BRs in plant tissue are extremely low and the elucidation of BRs functions relies on sensitive detection method. Reported methods for the determination of BRs required large amount of plant tissue, tedious pretreatment process, and were lack of selectivity. Therefore, development of a simple and selective method for the sensitive quantification of BRs is highly needed.

RESULTS

We established a pretreatment method of BRs in plant tissues by employing double layered solid phase extraction (DL/SPE) combined with boronate affinity polymer monolith microextraction (BA/PMME). After the initial depigmentation with DL/SPE cartridge, BA/PMME was employed to selectively extract BRs from sample matrix. Uniquely, most sample matrix was successfully removed by BA monolith purification. Using this method, BRs was determined by liquid chromatography-mass spectrometry (LC-MS). Endogenous active BRs could be detected in only 1 g fresh weigh (FW) leaves or 0.5 g FW flower tissues.

CONCLUSION

A DL/SPE-BA/PMME pretreatment method for the determination of endogenous brassinosteroids in plant tissues was developed and validated. The proposed method was sensitive and selective. Besides, it may be further developed for the determination of other BRs including their precursors and conjugates.

[Progress in the analysis of brassinosteroids]

Brassinosteroids have been considered as a class of plant hormones with high activity. However, the complex matrix of the plant samples and the ultra-trace level of naturally occurring brassinosteroids make their separation and determination very difficult. This review summarizes the progress in the development of sample pretreatment and determination of brassinosteroids.

Profiling of hormones and related metabolites in seed dormancy and germination studies

Seed dormancy and germination are regulated by several plant hormones, such as abscisic acid, gibberellin, auxin (indole-3-acetic acid), ethylene, and brassinosteroid. Endogenous concentrations of a hormone are determined by the balance between biosynthesis and deactivation, and contribute to the regulation of physiological responses. Therefore, profiling of all hormones and their metabolites (hormonome) is a powerful approach to elucidate the regulatory networks of hormone metabolism. The methods involved in the use of liquid chromatography-electrospray ionization-tandem mass spectrometry to develop a high-sensitive and high-throughput hormonome platform are described in this chapter.

Fragmentation investigation of brassinosteroid compounds by ion trap and quadrupole time-of-flight mass spectrometry

The fragmentation mechanisms of three types of brassinosteroids (BRs), 23,24-tris-epicastasterone, epicastasterone, tris-epicastasterone, 24-epibrassinolide and 6-deoxo-24-epicastasterone, have been extensively investigated by tandem mass spectrometry (MS(n), n = 1, 2, 3, 4, 5) with the assistance of high mass accuracy quadrupole time-of-flight mass spectrometry (QToF MS). The electrospray ionization (ESI) mass spectrometric fragmentation pathways of these five BRs were comprehensively elucidated for the first time. Cleavages of side chains, neutral losses of water or other molecules and opening of a ring induce the main fragmentation patterns. The results from the present study can potentially afford important guidance for the structural elucidation of different BRs and provide some fundamental data for metabolomic analysis of BRs.

Metabolite annotations based on the integration of mass spectral information

A large number of metabolites are found in each plant, most of which have not yet been identified. Development of a methodology is required to deal systematically with unknown metabolites, and to elucidate their biological roles in an integrated 'omics' framework. Here we report the development of a 'metabolite annotation' procedure. The metabolite annotation is a process by which structures and functions are inferred for metabolites. Tomato (Solanum lycopersicum cv. Micro-Tom) was used as a model for this study using LC-FTICR-MS. Collected mass spectral features, together with predicted molecular formulae and putative structures, were provided as metabolite annotations for 869 metabolites. Comparison with public databases suggests that 494 metabolites are novel. A grading system was introduced to describe the evidence supporting the annotations. Based on the comprehensive characterization of tomato fruit metabolites, we identified chemical building blocks that are frequently found in tomato fruit tissues, and predicted novel metabolic pathways for flavonoids and glycoalkaloids. These results demonstrate that metabolite annotation facilitates the systematic analysis of unknown metabolites and biological interpretation of their relationships, which provide a basis for integrating metabolite information into the system-level study of plant biology.

A New Type of Modified Brassinosteroids for Enzyme-linked Immunosorbent Assay

New modified brassinosteroids (BS) with a full set of specific functional groups and with an additional one at C-26 have been synthesized and used as haptens for enzyme-linked immunosorbent assay. The haptens were conjugated to bovine serum albumin (BSA) and the obtained immunoconjugates were applied for production of polyclonal antibodies. A conjugation of the haptens to horseradish peroxidase (HRP) gave the corresponding enzyme-labeled BS – another principal component for immuno-competitive analysis. The steroids were linked to proteins through spacers at the terminal side chain carbon atom (C-26) that made both the cyclic part and the side chain exposed for recognition by antibodies. It allowed maximization of their sensitivity and for the first time made possible immunochemical distinguishing of the most important group of natural BS, B-lactones, from B-ketones and 6-deoxo derivatives. The synthesis of haptens based on a sequence of transformations starting from stigmasterol included Claisen rearrangement of 22-allylic ethers as a key reaction in the side chain construction.

Labeled brassinosteroids for biochemical studies

The present paper describes the results of our studies on the synthesis of brassinolide biosynthetic precursors as tools for investigations of new biosynthetic routes leading to brassinosteroids. The corresponding labeled compounds containing three or six deuterium atoms at terminal methyl group(s) of the side chain (in a position ensuring lack of isotopic exchange) were prepared starting from stigmasterol or bisnorcholenic acid. Two strategies for the construction of the carbon skeleton of the side chain were applied in this study: Claisen rearrangement of allylic alcohols and convergent synthesis based on the coupling of 22-aldehydes with appropriate chiral sulfone. More than 20 brassinolide precursors (actual or suspected) have been prepared for metabolic studies that enabled identification of new brassinosteroids and biosynthetic subpathways to brassinolide in Secale cereale and Arabidopsis thaliana.

Enzyme immunoassay of (24R)-brassinosteroids

Brassinosteroids are a new group of phytohormones that are widely distributed in plants and play an important role in the processes of plant growth and development. Physiological concentrations of brassinosteroids in plants are extremely low, and their analysis in organs and tissues is very difficult. This study is devoted to the chemical aspects of elaboration and to bioanalytical parameters of an immunoenzymatic system for quantitative determination of the phytohormones 24-epicastasterone and 24-epibrassinolide.

Derivatization in mass spectrometry--8. Soft ionization mass spectrometry of small molecules

This is the first of two reviews devoted to derivatization approaches for "soft" ionization mass spectrometry (FAB, MALDI, ESI, APCI) and deals, in particular, with small molecules. The principles of the main "soft" ionization mass spectrometric methods as well as the reasons for derivatizing small molecules are briefly described. Derivatization methods for modification of amines, carboxylic acids, amino acids, alcohols, carbonyl compounds, monosaccharides, thiols, unsaturated and aromatic compounds etc. to improve their ionizability and to enhance structure information content are discussed. The use of "fixed"-charge bearing derivatization reagents is especially emphasized. Chemical aspects of derivatization and "soft" ionization mass spectrometric properties of derivatives are considered.