Identification, Organic Synthesis, and Sensitive Analysis of a cis-Homosalate-Specific Exposure Biomarker.

Homosalate (HMS) is an organic UV filter used in sunscreens and personal care products. Despite its widespread use and detection in environmental matrices, little is known regarding its exposure in humans. HMS is used as a mixture of cis- and trans-isomers, and we recently revealed major differences in human toxicokinetics, indicating the need to consider these isomers separately in exposure and risk assessments. In the course of these previous investigations of human HMS toxicokinetics, we identified two trans-HMS-specific and one cis-HMS-specific biomarker candidates. However, the latter lacks sensitivity due to only low amounts excreted in urine, prompting the search for another cis-HMS-specific biomarker. Our toxicokinetic investigations revealed a total of five isomers of HMS carboxylic acid metabolites (HMS-CA). Of these, only one was specifically formed from cis-HMS (HMS-CA 5), but its full identity in terms of constitution and configuration had, so far, not been elucidated. Here, we describe the synthesis of three HMS-CA isomers, of which the isomer (1R,3S,5S)/(1S,3R,5R)-3-((2-hydroxybenzoyl)oxy)-1,5-dimethylcyclohexane-1-carboxylic acid turned out to be HMS-CA 5. Taken together with two previously synthesized HMS-CA isomers, we were able to identify the constitution and configuration of all five HMS-CA isomers observed in human metabolism. We integrated the newly identified cis-HMS-specific metabolite HMS-CA 5 into our previously published human biomonitoring LC-MS/MS method. Intra- and interday precisions had coefficients of variation below 2% and 5%, respectively, and the mean relative recovery was 96%. The limit of quantification in urine was 0.02 µg L-1, enabling the quantification of HMS-CA 5 in urine samples for at least 96 h after sunscreen application. The extended method thus enables the sensitive and separate monitoring of cis- and trans-HMS in future human biomonitoring studies for exposure and risk assessment.

Turn-on mode diarylethenes for bioconjugation and fluorescence microscopy of cellular structures.

The use of photoswitchable fluorescent diarylethenes (fDAEs) as protein labels in fluorescence microscopy and nanoscopy has been limited by labeling inhomogeneity and the need for ultraviolet light for fluorescence activation (on-switching). To overcome these drawbacks, we prepared "turn-on mode" fDAEs featuring thienyl substituents, multiple polar residues, and a reactive maleimide group in the core structure. Conjugates with antibodies and nanobodies displayed complete on-switching and excitation with violet (405 nm) and yellow-green (<565 nm) light, respectively. Besides, they afforded high signal-to-noise ratios and low unspecific labeling in fluorescence imaging. Irradiation with visible light at 532 nm or 561 nm led to transient on-off switching ("blinking") of the fDAEs of double-labeled nanobodies so that nanoscale superresolution images were readily attained through switching and localization of individual fluorophores.

Supramolecular Complex of Cucurbit[7]uril with Diketopyrrolopyrole Dye: Fluorescence Boost, Biolabeling and Optical Microscopy.

New photostable and bright supramolecular complexes based on cucurbit[7]uril (CB7) host and diketopyrrolopyrole (DPP) guest dyes having two positively charged 4-(trimethylammonio)phenyl groups were prepared; with spectra (H2O, abs. / emission max. 480 / 550 nm; e ~ 19 000, tfl > 4 ns), strong binding with hosts (~560 nM Kd) and a linker affording fluorescence detection of bioconjugates with antibody and nanobody. Combination of protein-functionalized DPP dye with CB7 improves photostability and affords up to 12-fold emission gain. Two-color confocal and stimulated emission depletion (STED) microscopy with 595 nm or 655 nm STED depletion lasers shows that the presence of CB7 not only leads to improved brightness and image quality, but also results in DPP becoming cell-permeable.

Quantitation of 6-chloronicotinic acid and 2-chloro-1,3-thiazole-5-carboxylic acid and their glycine conjugates in human urine to assess neonicotinoid exposure.

Neonicotinoids and neonicotinoid-like compounds (NNIs) are widely used insecticides and their ubiquitous occurrence in the environment requires methods for exposure assessment in humans. The majority of the NNIs can be divided into 6-chloropyridinyl- and 2-chlorothiazolyl-containing compounds, suggesting the formation of the group-specific metabolites 6-chloronicotinic acid (6-CNA), 2-chloro-1,3-thiazole-5-carboxylic acid (2-CTA), and their respective glycine derivatives (6-CNA-gly, 2-CTA-gly). Here, we developed and validated an analytical method based on gas chromatography coupled to mass spectrometry (GC-MS/MS) to simultaneously analyze these four metabolites in human urine. As analytical standards for the glycine conjugates were not commercially available, we synthesized 6-CNA-gly, 2-CTA-gly, and their 13C2,15N-labeled analogs for internal standardization and quantitation by stable isotope dilution. We also ensured chromatographic separation of 6-CNA and its isomer 2-CNA. Enzymatic cleavage during sample preparation was proven unnecessary. The limits of quantitation were between 0.1 (6-CNA) and 0.4 µg/L (2-CTA-gly) and the repeatability was satisfactory (coefficient of variation was <19% over the calibration range). We analyzed 38 spot urine samples from the general population and were able to quantify 6-CNA-gly in 58% of the samples (median 0.2 µg/L). In contrast, no 6-CNA could be detected. The results are in line with well-known metabolic pathways specific in humans, that, compared to rodents, favor the formation and excretion of phase-II-metabolites (glycine derivatives) rather than phase-I metabolites (free carboxylic acids). Nevertheless, the exact source of exposure (i.e., the specific NNI) remains elusive in the general population, may even vary quantitatively between different NNIs, and also might be regional specific based on the respective use of individual NNIs. In sum, we developed a robust and sensitive analytical method for the determination of four group-specific NNI metabolites.

Photoactivatable Carbo- and Silicon-Rhodamines and Their Application in MINFLUX Nanoscopy.

New photoactivatable fluorescent dyes (rhodamine, carbo- and silicon-rhodamines [SiR]) with emission ranging from green to far red have been prepared, and their photophysical properties studied. The photocleavable 2-nitrobenzyloxycarbonyl unit with an alpha-carboxyl group as a branching point and additional functionality was attached to a polycyclic and lipophilic fluorescent dye. The photoactivatable probes having the HaloTagTM amine (O2) ligand bound with a dye core were obtained and applied for live-cell staining in stable cell lines incorporating Vimentin (VIM) or Nuclear Pore Complex Protein NUP96 fused with the HaloTag. The probes were applied in 2D (VIM, NUP96) and 3D (VIM) MINFLUX nanoscopy, as well as in superresolution fluorescence microscopy with single fluorophore activation (VIM, live-cell labeling). Images of VIM and NUPs labeled with different dyes were acquired and their apparent dimensions and shapes assessed on a lower single-digit nanometer scale. Applicability and performance of the photoactivatable dye derivatives were evaluated in terms of photoactivation rate, labeling and detection efficiency, number of detected photons per molecule and other parameters related to MINFLUX nanoscopy.

Supramolecular Complex of Photochromic Diarylethene and Cucurbit[7]uril: Fluorescent Photoswitching System for Biolabeling and Imaging.

Photoswitchable fluorophores─proteins and synthetic dyes─whose emission is reversibly switched on and off upon illumination, are powerful probes for bioimaging, protein tracking, and super-resolution microscopy. Compared to proteins, synthetic dyes are smaller and brighter, but their photostability and the number of achievable switching cycles in aqueous solutions are lower. Inspired by the robust photoswitching system of natural proteins, we designed a supramolecular system based on a fluorescent diarylethene (DAE) and cucurbit[7]uril (CB7) (denoted as DAE@CB7). In this assembly, the photoswitchable DAE molecule is encapsulated by CB7 according to the host-guest principle, so that DAE is protected from the environment and its fluorescence brightness and fatigue resistance in pure water improved. The fluorescence quantum yield (Φfl) increased from 0.40 to 0.63 upon CB7 complexation. The photoswitching of the DAE@CB7 complex, upon alternating UV and visible light irradiations, can be repeated 2560 times in aqueous solution before half-bleaching occurs (comparable to fatigue resistance of the reversibly photoswitchable proteins), while free DAE can be switched on and off only 80 times. By incorporation of reactive groups [maleimide and N-hydroxysuccinimidyl (NHS) ester], we prepared bioconjugates of DAE@CB7 with antibodies and demonstrated both specific labeling of intracellular proteins in cells and the reversible on/off switching of the probes in cellular environments under irradiations with 355 nm/485 nm light. The bright emission and robust photoswitching of DAE-Male3@CB7 and DAE-NHS@CB7 complexes (without exclusion of air oxygen and addition of any stabilizing/antifading reagents) enabled confocal and super-resolution RESOLFT (reversible saturable optical fluorescence transitions) imaging with apparent 70-90 nm optical resolution.

Cleavable Linker Incorporation into a Synthetic Dye-Nanobody-Fluorescent Protein Assembly: FRET, FLIM and STED Microscopy.

A bright and photostable fluorescent dye with a disulfide (S-S) linker and maleimide group (Rho594-S2-mal), as cleavable and reactive sites, was synthesized and conjugated with anti-GFP nanobodies (NB). The binding of EGFP (FRET donor) with anti-GFP NB labeled with one or two Rho594-S2-mal residues was studied in vitro and in cellulo. The linker was cleaved with dithiothreitol recovering the donor (FP) signal. The bioconjugates (FP-NB-dye) were applied in FRET-FLIM assays, confocal imaging, and superresolution STED microscopy.

Bis-Rhodamines Bridged with a Diazoketone Linker: Synthesis, Structure, and Photolysis.

Two fluorophores bound with a short photoreactive bridge are fascinating structures and remained unexplored. To investigate the synthesis and photolysis of such dyes, we linked two rhodamine dyes via a diazoketone bridge (-COCN2-) attached to position 5' or 6' of the pendant phenyl rings. For that, the mixture of 5'- or 6'-bromo derivatives of the parent dye was prepared, transformed into 1,2-diarylacetylenes, hydrated to 1,2-diarylethanones, and converted to diazoketones Ar1COCN2Ar2. The high performance liquid chromatography (HPLC) separation gave four individual regioisomers of Ar1COCN2Ar2. Photolysis of the model compound─C6H5COCN2C6H5─in aqueous acetonitrile at pH 7.3 and under irradiation with 365 nm light provided diphenylacetic acid amide (Wolff rearrangement). However, under the same conditions, Ar1COCN2Ar2 gave mainly α-diketones Ar1COCOAr2. The migration ability of the very bulky dye residues was low, and the Wolff rearrangement did not occur. We observed only moderate fluorescence increase, which may be explained by the insufficient quenching ability of diazoketone bridge (-COCN2-) and its transformation into another (weaker) quencher, 1,2-diarylethane-1,2-dione.

Rhodamines with a Chloronicotinic Acid Fragment for Live Cell Superresolution STED Microscopy*.

Formylation of 2,6-dichloro-5-R-nicotinic acids at C-4 followed by condensation with 3-hydroxy-N,N-dimethylaniline gave analogs of the popular TAMRA fluorescent dye with a 2,6-dichloro-5-R-nicotinic acid residues (R=H, F). The following reaction with thioglycolic acid is selective, involves only one chlorine atom at the carbon between pyridine nitrogen and the carboxylic acid group and affords new rhodamine dyes absorbing at 564/ 573 nm and emitting at 584/ 597 nm (R=H/ F, in aq. PBS). Conjugates of the dyes with "small molecules" provided specific labeling (covalent and non-covalent) of organelles as well as of components of the cytoskeleton in living cells and were combined with fluorescent probes prepared from 610CP and SiR dyes and applied in two-color STED microscopy with a 775 nm STED laser.

Photoactivatable Fluorophore for Stimulated Emission Depletion (STED) Microscopy and Bioconjugation Technique for Hydrophobic Labels.

The use of photoactivatable dyes in STED microscopy has so far been limited by two-photon activation through the STED beam and by the fact that photoactivatable dyes are poorly solvable in water. Herein, we report ONB-2SiR, a fluorophore that can be both photoactivated in the UV and specifically de-excited by STED at 775 nm. Likewise, we introduce a conjugation and purification protocol to effectively label primary and secondary antibodies with moderately water-soluble dyes. Greatly reducing dye aggregation, our technique provides a defined and tunable degree of labeling, and improves the imaging performance of dye conjugates in general.

Human Metabolism and Urinary Excretion Kinetics of Nonylphenol in Three Volunteers after a Single Oral Dose.

Nonylphenol (NP) is an endocrine-disrupting anthropogenic chemical that is ubiquitous in the environment. Human biomonitoring data and knowledge on internal NP exposure are still sparse, and its human metabolism is largely unknown. Therefore, in this study, we investigated human metabolism and urinary excretion of NP. Three male volunteers received a single oral dose of 1 mg 13C6-labeled NP (10.6-11.7 µg/kg body weight). Consecutive full urine voids were collected for 48 h. A metabolite screening identified nine ring- and/or side chain-oxidized metabolites. We chose the most promising hits, the alkyl chain-oxidized metabolites hydroxy-NP (OH-NP) and oxo-NP, for quantitative investigation next to the parent NP. For this purpose, we newly synthesized specific n - 1-oxidized monoisomeric analytical standards. Quantification of the polyisomeric metabolites was performed via online-solid phase extraction-LC-MS/MS with stable isotope dilution using a previously published consensus method. Alkyl chain hydroxylation (OH-NP) constituted the major metabolism pathway representing 43.7 or 62.2% (depending on the mass transition used for quantification) of the NP dose excreted in urine. The urinary excretion fraction (FUE) for oxo-NP was 6.0 or 9.3%. The parent NP, quantified via an analogous isomeric 13C6-NP standard, represented 6.6%. All target analytes were excreted predominately as glucuronic acid conjugates. Excretion was rather quick, with concentration maxima in urine 2.3-3.4 h after dosing and biphasic elimination kinetics (elimination half-times first phase: 1.0-1.5 h and second phase: 5.2-6.8 h). Due to its high FUE and insusceptibility to external contamination (contrary to parent NP), OH-NP represents a robust and sensitive novel exposure biomarker for NP. The novel FUEs enable to robustly back-calculate the overall NP intakes from urinary metabolite levels in population samples for a well-informed cumulative exposure and risk assessment.

Direct Visualization of Amlodipine Intervention into Living Cells by Means of Fluorescence Microscopy.

Amlodipine, a unique long-lasting calcium channel antagonist and antihypertensive drug, has weak fluorescence in aqueous solutions. In the current paper, we show that direct visualization of amlodipine in live cells is possible due to the enhanced emission in cellular environment. We examined the impact of pH, polarity and viscosity of the environment as well as protein binding on the spectral properties of amlodipine in vitro, and used quantum chemical calculations for assessing the mechanism of fluorescence quenching in aqueous solutions. The confocal fluorescence microscopy shows that the drug readily penetrates the plasma membrane and accumulates in the intracellular vesicles. Visible emission and photostability of amlodipine allow confocal time-lapse imaging and the drug uptake monitoring.

Fluorescence Assisted Capillary Electrophoresis of Glycans Enabled by the Negatively Charged Auxochromes in 1-Aminopyrenes.

A compact and negatively charged acceptor group, N-(cyanamino)sulfonyl, is introduced for dye design and its influence on the absorption and emission spectra of the "push-pull" chromophores is demonstrated with 1,3,6-tris[(cyanamino)sulfonyl]-8-aminopyrene. The new sulfonamides, including O-phosphorylated (3-hydroxyazetidine)-N-sulfonyl, are negatively charged electron acceptors and auxochromes. 1-Aminopyrenes decorated with the new sulfonamides have three or six negative charges (pH ≥8), low m/z ratios, high mobilities in an electric field, and yellow to orange emission. We labeled maltodextrin oligomers by reductive amination, separated the products by electrophoresis, and demonstrated their high brightness in a commercial DNA analyzer and the distribution of the emission signal among the detection channels.

Negatively Charged Red-Emitting Acridine Dyes for Facile Reductive Amination, Separation, and Fluorescent Detection of Glycans.

Capillary gel electrophoresis with laser-induced fluorescence detection (CGE-LIF) has become a key method in high-throughput glycan analysis. At present, CGE-LIF relies on the green fluorophore 8-aminopyrene-1,3,6-trisulfonic acid (APTS). However, APTS has moderate reactivity in labeling of glycans and a fixed selectivity profile. Here, we report synthesis of red-emitting and highly reactive fluorescent tags for glycan derivatization. The design is based on a 9-aminoacridine scaffold with various acceptor groups at C-2 (CN, SO2R) and a primary amino group at C-7 for conjugation via reductive amination. These reactive dyes exhibit absorption maxima close to 450 nm and emission above 600 nm. They readily undergo conjugation with reducing sugars at the desired 1:1 stoichiometry. The red emission of conjugates with a maximum at 610-630 nm can be observed under excitation with 488 nm light and detected separately from the APTS-labeled oligosaccharides. Phosphorylated 7,9-diaminoacridine-2-SO2R derivatives with variable amounts of negative charges provide high mobilities of glycoconjugates on polyacrylamide gel electrophoresis (PAGE), as compared with those of APTS. We further demonstrate their utility by labeling and separating a maltodextrin ladder and sialyllactose isomers. The new dyes are expected to cross-validate and increase the glycan identification precision in CGE-LIF and help to reveal "heavy" glycans, yet undetectable with the APTS label.

Synthesis of Fluorescent Jasplakinolide Analogues for Live-Cell STED Microscopy of Actin.

The nanometer thickness of filaments and the dynamic behavior of actin-a protein playing a crucial role in cellular function and motility-make it attractive for observation with super-resolution optical microscopy. We developed the solution-phase synthesis of des-bromo-des-methyl-jasplakinolide-lysine, used as the "recognition unit" (ligand) for F-actin in living cells. The first amino acid-Fmoc-O-TIPS-ß-tyrosine-was prepared in 78% yield (two steps in one pot). The new solution-phase synthesis involves 2-phenylisopropyl protection of the carboxyl group and does not require excesses of commercially unavailable amino acids. The overall yield of the target intermediate obtained in nine steps is about 8%. The 2-phenylisopropyl group can be cleaved from carboxyl with 2-3% (v/v) of TFA in acetonitrile (0-10 °C), without affecting TIPS protection of the phenolic hydroxyl in ß-tyrosine and N-Boc protection in lysine. Des-bromo-des-methyl-jasplakinolide-lysine was coupled with red-emitting fluorescent dyes 580CP and 610CP (via 6-aminohexanoate linker). Actin in living cells was labeled with 580CP and 610CP probes, and the optical resolution measured as full width at half-maximum of line profiles across actin fibers was found to be 300-400 nm and 100 nm under confocal and STED conditions, respectively. The solution-phase synthesis of des-bromo-des-methyl-jasplakinolide-lysine opens a way to better fluorescent probe perspective for actin imaging.

Negatively Charged Yellow-Emitting 1-Aminopyrene Dyes for Reductive Amination and Fluorescence Detection of Glycans.

1-Aminopyrenes with three ω-hydroxylated N-alkylsulfonamido or alkylsulfonyl residues in positions 3, 6, and 8 were prepared, O-phosphorylated, and applied for reductive amination of oligosaccharides. The dyes (ϵ≈20 000 m-1 cm-1 ) with six negative charges (pH≥8) and low m/z ratios enable labeling and fluorescence detection of reducing sugars (glycans) related to the most structurally and functionally diverse class of natural products. Under excitation with a 488 nm laser, the new glycoconjugates emit yellow light of about 560 nm, outperforming (with respect to brightness and faster electrophoretic mobilities) the corresponding APTS derivatives (benchmark dye with green emission in conjugates).

Reversibly Photoswitchable Fluorescent Diarylethenes Resistant against Photobleaching in Aqueous Solutions.

Low photostability in aqueous solutions is the main drawback of synthetic photochromic dyes, which limits their switching performance and utility in biology, medicine, and life sciences. Even the most promising photochromes-reversibly photoswitchable diarylethenes (DAEs) with fluorescent "closed" forms-are known to undergo only several tens (typically 20-30) of switching cycles in aqueous solutions. In this work, we introduce water-soluble and highly photostable 1,2-[bis(2-ethyl/2-isobutyl-1-benzothiophene-1,1-dioxide-6-phenyl-3-yl)]perfluorocyclopentenes decorated with four -CONHC(CH2R)3 residues capped with 12 carboxylic acid groups (R = CH2CO2H, O(CH2)2CO2H). Under irradiation with UV (365 nm) and visible light (470 nm), they provide several hundred (for the "rapid" DAEs with isobutyl groups, up to 1000) full switching cycles in aqueous solutions without exclusion of air oxygen (outperforming the photoswitchable and fluorescent protein Dreiklang). The new branching approach based on secondary carboxamides with N-tert-alkyl residues decorated with polar groups may serve as a blueprint for the design of highly fatigue resistant and reversibly photoswitchable fluorescent probes applicable in life sciences as aqueous solutions.

Green-Emitting Rhodamine Dyes for Vital Labeling of Cell Organelles Using STED Super-Resolution Microscopy.

Fluorescence microscopy reveals the localization, spatial distribution, and temporal dynamics of the specifically labeled organelles in living cells. Labeling with exogenous conjugates prepared from fluorescent dyes and small molecules (ligands) is an attractive alternative to the use of fluorescent proteins, but proved to be challenging due to insufficient cell-permeability of the probes, unspecific staining, or low dye brightness. We evaluated four green-emitting rhodamine dyes and their conjugates intended for the specific labeling of lysosomes, mitochondria, tubulin, and actin in living cells. The imaging performance of the probes in living human fibroblasts has been studied by using confocal and stimulated emission depletion (STED) super-resolution microscopy with a commercial 595 nm STED laser. Two bright and photostable dyes (LIVE 510 and LIVE 515) provide specific and versatile staining.

Mono- and bithiophene-substituted diarylethene photoswitches with emissive open or closed forms.

We present a new series of photochromic 1,2-bis(2-ethylbenzo[b]thiophen-3-yl)perfluorocyclopentenes with an oxidized benzothiophene core (O) or a nonoxidized one, decorated with mono- (Th1) and bithiophene (Th2) units attached to positions 6 and 6' (Sy = symmetric) or only to position 6 (As = asymmetric). "Oxidized" compounds have highly fluorescent closed forms emitting in the visible region (yellow to red). The dyes with nonoxidized benzothiophenes possess fluorescent open forms with rather low emission efficiency. The photoswitching kinetics was studied at several wavelengths with UV and visible light. New diarylethenes underwent ring-closure reactions by irradiation with UV light (365 nm, 405 nm), and the reversible ring-opening by irradiation with visible light (470 nm, 530 nm). The on-switching of fluorescence due to the ring-closure reaction was observed also with visible light of 470 nm (to an extent of 10% for compound SyOTh 1 ) and attributed to the Urbach tail effect. Due to a high degree of fluorescence modulation (>270), good fatigue resistance and large fluorescence quantum yield, compound SyOTh 1 emerged as a candidate for single-molecule based super-resolution fluorescence microscopy.

Determination of Urinary Metabolites of the Emerging UV Filter Octocrylene by Online-SPE-LC-MS/MS.

Octocrylene (OC) is an emerging UV filter, which is used in the majority of sunscreens as well as other personal care products (PCP) and consumer products. Its presence in various environmental matrices has been reported. However, information on the internal OC exposure in humans is not available, due to the lack of appropriate biomarkers of exposure and analytical methods. Here, we describe a rugged, precise, and accurate analytical method for the determination of three OC metabolites (ester hydrolysis and alkyl chain oxidation products) in human urine by stable isotope dilution analysis. Urine samples are incubated with ß-glucuronidase (E. coli K12) and then analyzed by liquid chromatography-electrospray ionization-triple quadrupole-tandem mass spectrometry with online turbulent flow chromatography for sample cleanup and analyte enrichment (online-SPE-LC-MS/MS). Syntheses of analytical standards, including deuterium-labeled internal standards, are also described. In a pilot study, we investigated the applicability of the metabolites as biomarkers of exposure in urine samples from the general population (n = 35). OC metabolites were detected in 91% of the samples, with the highest concentrations for three individuals having used sunscreen within 5 days prior to sample collection. We will apply the method in future human biomonitoring studies for OC exposure and risk assessment.