Rapid Immunochemical Methods for Anatoxin-a Monitoring in Environmental Water Samples.

Algal blooms that contaminate freshwater resources with cyanotoxins constitute, nowadays, a global concern. To deal with this problem, a variety of analytical methods, including immunochemical assays, are available for the main algal toxins, for example, microcystins, nodularins, and saxitoxins, with the remarkable exception of anatoxin-a. Now, for the first time, highly sensitive, enantioselective immunoassays for anatoxin-a have been validated using homemade monoclonal antibodies. Two competitive enzyme-linked immunosorbent assays were developed in different formats, with detection limits for (+)-anatoxin-a of 0.1 ng/mL. Excellent recovery values between 82 and 117%, and coefficients of variation below 20%, were observed using environmental water samples fortified between 0.5 and 500 ng/mL. In addition, a lateral-flow immunochromatographic assay was optimized for visual and instrumental reading of results. This test showed a visual detection limit for (+)-anatoxin-a of 4 ng/mL. Performance with a reader was validated in accordance with the European guidelines for semiquantitative rapid methods for small chemical contaminants. Thus, at a screening target concentration of 2 ng/mL, the probability of a blank sample to be classified as "suspect" was as low as 0.2%. Finally, the optimized direct enzyme immunoassay was validated by comparison with high-performance liquid chromatography-tandem mass spectroscopy data and showed a good correlation (r = 0.995) with a slope of 0.94. Moreover, environmental water samples containing more than 2 ng/mL of anatoxin-a were detected by the developed dipstick assay. These results provide supplementary and complementary strategies for monitoring the presence of anatoxin-a in water.

Click Chemistry-Assisted Bioconjugates for Hapten Immunodiagnostics.

Bioorthogonal reactions have revolutionized the way low-molecular-weight compounds are coupled to biomolecules. Organic chemistry, polymer science, and chemical biology are among the disciplines that have benefited the most from this breakthrough. Despite the reliability of the click chemistry concept for the efficient and chemoselective functionalization of biomacromolecules with haptens at preferred positions, the fact that azide-alkyne cycloaddition reactions originate new chemical moieties as part of the linker may have delayed their application in the immunodiagnostic field. Using the mycotoxin ochratoxin A as a model compound, we herein demonstrate for the first time that bioconjugates arising from the ligation between an azido-bearing hapten and an alkyne-modified carrier protein are able to elicit the generation of high-affinity monoclonal antibodies suitable for the development of rapid methods for the immunodetection of small organic molecules.

Synthetic Haptens and Monoclonal Antibodies to the Cyanotoxin Anatoxin-a.

Early warning systems for monitoring toxic events may benefit from the availability of monoclonal antibodies enabling the sensitive and specific detection of anatoxin-a, a cyanotoxin involved in numerous cases of animal poisoning resulting from toxic algal blooms in freshwaters. Through the synthesis of three functionalized derivatives of anatoxin-a, we have succeeded in generating the first-ever reported immunoreagents (bioconjugates and antibodies) suitable for the development of immunoanalytical approaches aimed at rapid and onsite detection of this harmful cyanotoxin.

Rationally designed haptens for highly sensitive monoclonal antibody-based immunoanalysis of fenhexamid.

Immunochemical methods have been consolidated during the last few years as complementary analytical strategies for chemical contaminant and residue determination. However, generation of suitable immunoreagents for small organic molecules demands adequate hapten design. In this study, fenhexamid was considered as a model compound and novel haptens were designed and synthesized in order to evaluate the influence of the linker tethering site on antibody binding properties and immunoassay parameters. Haptens were conceived with the spacer arm at different positions, while the more antigenic aromatic moiety was kept free. The synthesis of these functionalized compounds was accomplished by total construction of the molecule through several steps. This strategy afforded very high-affinity monoclonal antibodies specific of fenhexamid, with IC50 values around or below 0.1 nM. Using these novel immunoreagents, a direct competitive enzyme-linked immunosorbent assay with a remarkably low limit of detection (4 ng L-1) was developed for the determination of fenhexamid residues. The selected immunoassay was investigated in terms of trueness, precision, repeatability, and robustness. The QuEChERS extraction methodology was applied to fortified samples and recoveries between 83% and 113%, with relative standard deviations below 20%, were observed. Moreover, contaminated and blind spiked samples were measured by the developed immunoassay and by ultra-performance liquid chromatography coupled to tandem mass spectrometry, showing statistically comparable results.

Combined heterologies for monoclonal antibody-based immunoanalysis of fluxapyroxad.

Nowadays, instrumental methodologies and rapid bioanalytical techniques complement each other for the analysis of toxic chemical compounds. Fluxapyroxad was commercialized a few years ago as a fungicide and today it is being used worldwide to control a variety of pests. In the present study, the development of monoclonal antibody-based immunochemical methods for the analysis of this chemical in food samples was evaluated for the first time. Novel haptens were synthesized and protein bioconjugates were prepared. High-affinity and specific monoclonal antibodies to fluxapyroxad were generated from two haptens with alternative linker tethering sites. Haptens with linker site heterology and a structurally heterologous hapten with a minor modification of the molecule conformation and volume but with a significant alteration of the electronic density of the pyrazole moiety were evaluated for immunoassay development. Direct and indirect competitive immunoassays were characterized and optimized, showing IC50 values for fluxapyroxad of 0.14 and 0.05 ng mL-1, respectively. The combination of two heterologies was particularly adequate in the indirect format. The two developed immunoassays showed excellent recoveries and coefficients of variation in fluxapyroxad-fortified plums and four varieties of grapes. Finally, a good correlation was found between the indirect immunoassay and UPLC-MS/MS when fruit samples with incurred residues of fluxapyroxad were analyzed. These monoclonal antibody-based immunochemical methods hold great promise for fluxapyroxad monitoring.

Protein-Free Hapten-Carbon Nanotube Constructs Induce the Secondary Immune Response.

Carbon nanotubes are novel technological tools with multiple applications. The interaction between such nanoparticles and living organisms is nowadays a matter of keen research by academic and private institutions. In this study, carbon nanotube constructs were investigated as delivery vehicles for immunostimulation and induction of the secondary immune response to a small organic molecule, namely, a hapten. Two types of nanoconstructs were prepared: on one hand, carbon nanotubes carrying a protein bioconjugate of a hapten covalently linked to the carbon surface, and on the other hand, covalent carbon nanotube constructs of the same model chemical compound without the carrier protein. Nanotube vehicles carrying a hapten-protein bioconjugate were demonstrated to stimulate the immune system and to induce a strong primary immune response against the hapten with as low as 0.1 µg of the model chemical. The influence of the different elements of those nanoconstructs over the immune response was investigated to better understand the molecular mechanisms that are involved. As expected, the presence of the carrier protein was shown to be necessary in order to trigger the immune response. Interestingly, we found that a remarkable secondary immune response to the model organic compound occurred in the absence of a carrier protein. Additionally, a satisfactory adjuvant effect of carbon nanotubes was observed and a potent immune response was elicited without employing an oil-based adjuvant.

Determination of succinate-dehydrogenase-inhibitor fungicide residues in fruits and vegetables by liquid chromatography-tandem mass spectrometry.

In recent years, a second generation of succinate-dehydrogenase-inhibitor (SDHI) fungicides has been introduced into the market for effective treatment of fruit and vegetable crops, with fluxapyroxad, boscalid, fluopyram, penflufen, bixafen, penthiopyrad, and isopyrazam being some of the members of this new class of agrochemical. We herein report the development of an analytical procedure for the determination of residues of these SDHI fungicides in food samples, based on a modification of the QuEChERS extraction method followed by ultra-performance liquid chromatography coupled to tandem-mass-spectrometry determination. The proposed method reached limits of detection from 0.8 to 2.0 µg L(-1). Apple, strawberry, tomato, and spinach samples were used as model samples. Spiked samples, from 10 to 1000 µg kg(-1), were analysed by the proposed method and quantitative recoveries were obtained (from 81 to 115 % for apples, from 84 to 136 % for strawberries, from 84 to 135 % for tomatoes, and from 80 to 136 % for spinach), with precision better than 20 % in all cases. Thus, the proposed method can be used for the analysis of SDHI fungicide residues to efficiently ensure that marketed fruits and vegetables comply with the maximum residue levels established by competent authorities.

Design and development of heterologous competitive immunoassays for the determination of boscalid residues.

Boscalid is a modern agrochemical belonging to the so-called chemical class of succinate dehydrogenase inhibitor fungicides. With the aim of developing rapid analytical screening methods for this relevant compound, we herein report the synthesis of new boscalid mimics and the study of their suitability for the production of polyclonal antibodies. Aliphatic spacer arms equivalent in length and composition were tethered at two different aromatic rings of the target molecular structure. These haptens, besides being used for immunization, were employed in the development of heterologous competitive enzyme-linked immunosorbent assays (cELISAs) in order to improve assay detectability. Direct and indirect immunoassays were tailored and applied to the determination of samples with incurred boscalid residues. The assays were characterized in terms of sensitivity, specificity, trueness, and precision. Limit of quantification was established at 5 µg kg(-1), coefficients of variation were lower than 20%, and recoveries from spiked samples ranged from 90 to 137%. Finally, ELISA performance was evaluated by Deming regression analysis with tomato and cucumber samples, selecting ultra-performance liquid chromatography-mass spectrometry as the reference method. The results showed that the proposed cELISAs are useful for the routine determination of boscalid fungicides in foods with high-sample throughput and affordable cost.

Haptens, bioconjugates, and antibodies for penthiopyrad immunosensing.

Haptens, bioconjugates, and antibodies for highly sensitive immunochemical analysis of the new-generation fungicide penthiopyrad are described. Two haptens with equivalent carboxylated linkers were prepared, and the purified active esters were efficiently coupled to proteins. The results revealed slightly different antibody-eliciting capacities for the two synthetic derivatives. All of the produced antibodies were specific for penthiopyrad, and showed affinity values in the nanomolar range.

Mepanipyrim haptens and antibodies with nanomolar affinity.

Mepanipyrim is an anilinopyrimidine fungicide used worldwide for crop protection. With the aim of developing useful immunoreagents for mepanipyrim immunoanalysis, two new functionalized derivatives were prepared and antibodies were generated. Affinity and specificity were assessed by direct and indirect competitive ELISA using homologous and heterologous conjugates. Although all antibodies were selective for the target analyte, the immunizing hapten structure was revealed as a determinant for high-affinity antibody production (IC(50) = 3 nM).

Structure-immunogenicity relationship of kresoxim-methyl regioisomeric haptens.

Kresoxim-methyl was one of the two first strobilurins to be discovered, and nowadays it is widely used as an antifungal agent in crop protection. Because of its low molecular weight and negligible structural complexity, the generation of antibodies to kresoxim-methyl noticeably requires the preparation of functionalized haptens. In this study, the introduction of a hydrocarbon spacer arm at the aromatic moieties of the target molecule was carried out by a convergent strategy based on the Sonogashira cross-coupling reaction, and functionalized linkers of the same length were also tethered to the aliphatic toxophore group by the O-alkylation reaction. Evaluation of the immune response, in terms of antibody affinity, showed a differential behavior among five synthesized haptens whose sole dissimilarity was the derivatization site. The characteristic (methoxyimino)acetate moiety of strobilurins was revealed as the optimum linker position for high-affinity polyclonal and monoclonal antibody production. However, good monoclonal antibodies were isolated from mice immunized with a hapten carrying the linker at an opposite site, which otherwise generated a poor polyclonal response in rabbits. Site-heterology was confirmed as a feasible approach for the improvement of the apparent affinity, particularly with polyclonal antibodies. Several of the monoclonal antibodies generated in the context of this project could be proper binders for kresoxim-methyl immunosensing over different analytical platforms.

Correction: Addante-Moya et al. Assessment of the Optimum Linker Tethering Site of Alternariol Haptens for Antibody Generation and Immunoassay Development. Toxins 2021, 13, 883.

In the original publication [...].

General diastereoselective synthetic approach toward isospongian diterpenes. Synthesis of (-)-marginatafuran, (-)-marginatone, and (-)-20-acetoxymarginatone.

This work describes a synthetic approach to the carbocyclic skeleton of isospongian diterpenes that uses the commercially available monoterpene (S)-carvone as a C-ring synthon, which is incorporated into the tetracyclic isospongian framework via a C→ABC→ABCD ring annulation strategy using intramolecular Diels-Alder and ring-closing metathesis reactions. This approach has been successfully used to prepare both the title natural isospongians and several nonnatural oxygenated analogues. A preliminary evaluation of the inhibitory activity of the small collection of synthesized isospongians on the mammalian mitochondrial respiratory chain revealed that most were able to inhibit the integrated electron transfer chain (NADH oxidase activity) in the micromolar range.

Antibody generation and immunoassay development in diverse formats for pyrimethanil specific and sensitive analysis.

Immunochemical techniques are complementary tools to modern analytical requirements. These methods rely on the production of immunoreagents with adequate binding properties. In the present study, a rationally designed and functionalized derivative of pyrimethanil--a modern anilinopyrimidine fungicide--was synthesized in order to generate for the first time high-affinity and selective antibodies to this xenobiotic. A single coupling procedure--based on hapten activation using N,N'-disuccinimidyl carbonate and purification of the active ester--was followed to prepare both immunizing and assay conjugates. Polyclonal antibodies were produced and characterized by enzyme-linked immunosorbent assay (ELISA) in four alternative formats: one indirect and three direct competitive procedures. The selected immunoassay displayed a limit of detection of 0.024 µg L(-1), far lower than the official maximum residue limits and close to the sensitivity of regular instrumental assays. This ELISA was shown to be robust to buffer changes and tolerant to the presence of little amounts of methanol, ethanol and acetonitrile. Finally, the developed assay was applied to the analysis of pyrimethanil in carrot juice samples, and a limit of quantification of 0.040 mg L(-1) was determined.

Development and validation of a direct competitive monoclonal antibody-based immunoassay for the sensitive and selective analysis of the phytoregulator forchlorfenuron.

Forchlorfenuron is a synthetic phytohormone with cytokinin-like activity used worldwide as a plant growth regulator to increase fruit size in a number of crops, mostly in kiwifruit and grape vines. A monoclonal antibody-based enzyme-linked immunosorbent assay (ELISA) for the determination of forchlorfenuron has been characterized and optimized. The selected immunoreagents afforded a highly selective assay with a limit of detection of 10 ng L(-1) in buffer. This direct competitive ELISA was validated in terms of trueness, precision, and robustness using both commercial juice and whole fruit samples. Recoveries from fortified kiwifruit juices and white and red musts were between 97 % and 131 %, with relative standard deviations below 16 %. When homogenized whole fruits were analysed after acetonitrile extraction, recoveries between 96 % and 113 % were found, with a limit of quantification of 5 µg kg(-1). The proposed immunoassay was validated by comparison with a reference chromatographic method using fruits from in-field treated grape and kiwifruit vines. Linear regression analysis of ELISA and HPLC-UV determinations showed an excellent correlation (r(2)=0.998), whereas analysis of the slope (0.99±0.01) and of the intercept (-1±3) clearly proved that the developed competitive immunoassay provided results that were statistically comparable to those obtained by the instrumental method for the analysis of forchlorfenuron in fruits at trace levels.

Design of a novel hapten and development of a sensitive monoclonal immunoassay for dicamba analysis in environmental water samples.

Weed resistance to glyphosate has been a driving force behind the increased use of alternative herbicides in agriculture. Recently, dicamba-tolerant recombinant plants were introduced to the market, which may result in residues of this agrochemical contaminating environmental waters. Given that restrictions on the use of dicamba have consequently been established by regulatory agencies, it is therefore also desirable to conduct extensive controls on dicamba residues. Immunoassays are currently the most powerful bioanalytical technology for the rapid monitoring of chemical residues and contaminants. In the present study, a novel hapten was designed maintaining unaltered all the antigenic moieties of the target molecule, and this was used to generate high-affinity monoclonal antibodies against dicamba for the first time. Additionally, a collection of haptens with different linker composition or linker tethering site was synthesized and conjugated to proteins. Using these novel immunoreagents, a direct competitive enzyme-linked immunosorbent assay with a limit of detection for dicamba of 0.24 ng/mL was developed and validated. Analysis of water samples from different origins afforded recovery values between 90 % and 120 %, and coefficients of variation below 20 % were obtained. These results indicate that the developed immunochemical assay is suitable for the rapid determination of dicamba residues in environmental water samples.

Alternative Hapten Design for Zearalenone Immunoreagent Generation.

Appropriate hapten design and synthesis have been identified as critical steps to generate high-performance immunoreagents and to develop sensitive and selective immunoanalytical methods. Antibodies and immunoassays for the major mycotoxin zearalenone have been reported and marketed. However, zearalenone haptens have mostly been prepared by the oxime active ester technique, and hapten characterization has generally been poor or non-existent. In the present study, novel haptens of zearalenone with longer linkers and with alternative tethering sites have been designed for immunizing and assay conjugate preparation. All of these molecules were purified and spectroscopically verified, and a structure-activity relationship evaluation was carried out. This approach revealed that the hapten with the linker at the carbonyl group generated antibodies with a higher affinity than the hapten functionalized at the phenyl moiety. Antibodies produced with the latter hapten, on the other hand, showed lower cross-reactivity values to the major zearalenone metabolites. Finally, similar immunoassay sensitivity was achieved with all of the antibodies when heterologous haptens were employed. Furthermore, by altering the structure of the competing antigen, the immunoassay selectivity was modified. These results demonstrate that immunochemical methods for zearalenone rapid analysis can still be improved in terms of sensitivity and selectivity.

Exploring alternative hapten tethering sites for high-affinity anti-picoxystrobin antibody generation.

The relevance of the linker tethering site in haptens was investigated for antibody generation and immunoassay development. Three derivatives of the strobilurin fungicide picoxystrobin were synthesized with the same functionalized spacer arm located at three different positions. Protein conjugates of those haptens were employed as immunogens, and novel polyclonal antibodies were produced and characterized. All haptens afforded highly specific antibodies, but different affinities to the free analyte were observed among the obtained antisera. Next, competitive enzyme-linked immunosorbent assays were studied in several formats, and site heterology was confirmed as an effective strategy for detectability improvement. An indirect heterologous immunoassay was eventually selected and optimized, showing a limit of detection for picoxystrobin of 0.02 µg/L and a working range between 0.03 and 1.30 µg/L. Finally, the developed extraction and analytical procedures revealed a practical limit of quantification of 5 µg/kg for this fungicide in soybean sprouts, well below the maximum residue limits in the European Union.

Synthesis of site-heterologous haptens for high-affinity anti-pyraclostrobin antibody generation.

The design and synthesis of functional chemical derivatives of small organic molecules is usually a key step for the intricate production of a variety of bioconjugates. In this respect, the derivatization site at which the spacer arm is introduced in immunizing conjugates constitutes a highly critical parameter for the generation of high-affinity and selective antibodies. However, due to the usual complexity of the required synthetic procedures, the appropriate comparison of alternative tethering positions has often been neglected. In the present study, meticulous strategies were followed to prepare synthetic derivatives of pyraclostrobin with the same linkers located at diverse rationally-chosen sites. Activity appraisal of antibodies and bioconjugates was carried out by bidimensional competitive direct and indirect immunoassays, and a superior performance of two of the three synthesized haptens was found. Finally, a detailed analysis of the conformations of the target molecule and the synthesized haptens in aqueous solution was done using computer assisted molecular modeling techniques. This study suggested that the lower titers and affinities of one set of antibodies are most probably due to conformational effects of the spacer arm in the immunizing bioconjugate.

Forchlorfenuron-mimicking haptens: from immunogen design to antibody characterization by hierarchical clustering analysis.

To obtain highly-specific and selective forchlorfenuron binders, a collection of functionalized derivatives with different spacer arm locations and lengths was prepared. By immunization with target-mimicking haptens, a large battery of monoclonal and polyclonal antibodies against this synthetic cell regulator was produced and exhaustively characterized in two immunoassay formats using homologous and heterologous conjugates. Antibodies with IC(50) values lower than 0.3 nM were successfully raised from the prepared immunogens, thus evidencing the efficacy of the explored strategies. In order to identify significant epitopes in the antibody-antigen interaction, a series of new chemical forchlorfenuron analogues, with slight modifications at both rings of the target molecule, were synthesized and evaluated in competitive assays. As a novel approach in hapten recognition studies, data processing was performed by computational classification methods based on hierarchical clustering. This strategy was shown to be highly valuable for a straightforward profiling of antibodies according to analogue recognition patterns. A relationship could be established between the antigen binding properties of antibodies and the structure of the immunogen. Whereas antibodies with equivalent affinities had been obtained from all of the derivatives, their specificity was found to be largely influenced by the differential exposition of the molecule to the immune system.