Cell-based fluorescence screen for K+ channels and transporters using an extracellular triazacryptand-based K+ sensor

New nanostructured extracellular potassium ion probe for assay of cellular K+ transport

The concentration of potassium ion is an important indicator for human health, and its abnormality is often accompanied by various diseases. However, most tools currently used to study potassium ion transport are low throughput. Herein, we reported a new K⁺ fluorescent nanoprobe CP1-KS with high selectivity and sensitivity to K⁺ (fluorescence enhanced factor was up to 9.91 at 20 mM K⁺). The polymeric fluorescent probe CP1-KS was composed of the small-molecular K⁺ indicator KS and amphiphilic copolymer CP1. This sensor can be easily and uniformly dispersed in cell culture medium and is suitable for high throughput analysis. To assess the utility of the probe CP1-KS in biological field, this probe was employed as an extracellular fluorescent probe to monitor the efflux of K⁺ from cells (E coli, B. Subtilis 168, Hela and MCF-7 cells) under various stimulation including lysozyme, nigericin, digitonin, and ATP. Results demonstrated that CP1-KS is an effective analysis tool for extracellular K⁺ concentration. We believe that the nanoprobe has great potential in antibacterial drug screening, K⁺ ionophore function, K⁺ channel activity, cell membrane permeability analysis or other K⁺ related field in the future.

Fluorescent probes for the detection of disease-associated biomarkers

Fluorescent probes have emerged as indispensable chemical tools to the field of chemical biology and medicine. The ability to detect intracellular species and monitor physiological processes has not only advanced our knowledge in biology but has provided new approaches towards disease diagnosis. In this review, we detail the design criteria and strategies for some recently reported fluorescent probes that can detect a wide range of biologically important species in cells and in vivo. In doing so, we highlight the importance of each biological species and their role in biological systems and for disease progression. We then discuss the current problems and challenges of existing technologies and provide our perspective on the future directions of the research area. Overall, we hope this review will provide inspiration for researchers and prove as useful guide for the development of the next generation of fluorescent probes.

Molecular Probes, Chemosensors, and Nanosensors for Optical Detection of Biorelevant Molecules and Ions in Aqueous Media and Biofluids

Synthetic molecular probes, chemosensors, and nanosensors used in combination with innovative assay protocols hold great potential for the development of robust, low-cost, and fast-responding sensors that are applicable in biofluids (urine, blood, and saliva). Particularly, the development of sensors for metabolites, neurotransmitters, drugs, and inorganic ions is highly desirable due to a lack of suitable biosensors. In addition, the monitoring and analysis of metabolic and signaling networks in cells and organisms by optical probes and chemosensors is becoming increasingly important in molecular biology and medicine. Thus, new perspectives for personalized diagnostics, theranostics, and biochemical/medical research will be unlocked when standing limitations of artificial binders and receptors are overcome. In this review, we survey synthetic sensing systems that have promising (future) application potential for the detection of small molecules, cations, and anions in aqueous media and biofluids. Special attention was given to sensing systems that provide a readily measurable optical signal through dynamic covalent chemistry, supramolecular host-guest interactions, or nanoparticles featuring plasmonic effects. This review shall also enable the reader to evaluate the current performance of molecular probes, chemosensors, and nanosensors in terms of sensitivity and selectivity with respect to practical requirement, and thereby inspiring new ideas for the development of further advanced systems.

Molecular cages for biological applications

Artificial receptors able to recognise biologically relevant molecules or ions have gained interest in the chemical community because they offer a plethora of posibilities. Molecular cage compounds are polycyclic compounds with a cavity designed for the encapsulation of guest species. Once inside the host cavity, the substrate can be transported through membranes and protected from the action of enzymes or other reactive species, thus offering the possibility of interfering with biological systems. Commonly, enzymes have been an inspiration for chemists in the search and design of defined cavities for different purposes. However, the chemical preparation of molecular cages has struggled with many synthetic challenges but this effort is worthwhile as they are a very promising tool for many applications ranging from sensing, delivery, purification or even promotion of/prevention from chemical modifications. Since the early reports at the end of the 60s, this field has experienced a growing interest; this review summarises the progress in the preparation and study of cage-like compounds highlighting their importance in biological applications.

Rational Design of a Polymer-Based Ratiometric K+ Indicator for High-Throughput Monitoring Intracellular K+ Fluctuations

Highly selective fluorescent K⁺ sensors are of great importance for monitoring K⁺ fluctuations in various biological processes. In particular, highly efficient ratiometric K⁺ sensors that can emit in dual wavelengths and facilitate the quantitative determination of K⁺ are highly anticipated. Herein, we present the first polymer-based ratiometric fluorescent K⁺ indicator (PK1) for quantitatively detecting K⁺ in aqueous solutions and high-throughput monitoring K⁺ fluctuations in living cells. PK1 was synthesized by conjugating a small molecular K⁺ probe and a red emission reference dye to a hydrophilic polymer skeleton. The newly synthesized PK1 can form highly stable nanoparticles in aqueous solutions and work in 100% water without the aid of any organic solvents or surfactants. PK1 is sensitive to K⁺ with a fluorescence enhancement of sevenfold after interactions with K⁺ at 1000 mM and inert to other metal ions, physiological pH, or dye concentration vibrations. More importantly, the fluorescence intensity ratio at 572 and 638 nm is linearly correlated with log [K⁺] in the range of 2-500 mM (R² = 0.998), which will facilitate the quantitative detection of K⁺. Practical application of PK1 in detecting different K⁺-rich samples demonstrates its great potential in quantitative detection of K⁺. PK1 can be quickly internalized by live cells and shows no obvious cytotoxicity. We also demonstrate that PK1 could be used for monitoring K⁺ fluctuations under different stimulations by using a confocal microscope and especially a microplate reader, which is high throughput and time saving. The rational design of PK1 will broaden the design concept of ratiometric fluorescent K⁺ sensors and facilitate the quantitative detection of K⁺.

A dual-fluorophore sensor approach for ratiometric fluorescence imaging of potassium in living cells

Potassium is the most abundant intracellular metal in the body, playing vital roles in regulating intracellular fluid volume, nutrient transport, and cell-to-cell communication through nerve and muscle contraction. On the other hand, aberrant alterations in K+ homeostasis contribute to a diverse array of diseases spanning cardiovascular and neurological disorders to diabetes to kidney disease to cancer. There is an unmet need for studies of K+ physiology and pathology owing to the large differences in intracellular versus extracellular K+ concentrations ([K+]intra = 150 mM, [K+]extra = 3-5 mM). With a relative dearth of methods to reliably measure dynamic changes in intracellular K+ in biological specimens that meet the dual challenges of low affinity and high selectivity for K+, particularly over Na+, currently available fluorescent K+ sensors are largely optimized with high-affinity receptors that are more amenable for extracellular K+ detection. We report the design, synthesis, and biological evaluation of Ratiometric Potassium Sensor 1 (RPS-1), a dual-fluorophore sensor that enables ratiometric fluorescence imaging of intracellular potassium in living systems. RPS-1 links a potassium-responsive fluorescent sensor fragment (PS525) with a low-affinity, high-selectivity crown ether receptor for K+ to a potassium-insensitive reference fluorophore (Coumarin 343) as an internal calibration standard through ester bonds. Upon intracellular delivery, esterase-directed cleavage splits these two dyes into separate fragments to enable ratiometric detection of K+. RPS-1 responds to K+ in aqueous buffer with high selectivity over competing metal ions and is sensitive to potassium ions at steady-state intracellular levels and can respond to decreases or increases from that basal set point. Moreover, RPS-1 was applied for comparative screening of K+ pools across a panel of different cancer cell lines, revealing elevations in basal intracellular K+ in metastatic breast cancer cell lines vs. normal breast cells. This work provides a unique chemical tool for the study of intracellular potassium dynamics and a starting point for the design of other ratiometric fluorescent sensors based on two-fluorophore approaches that do not rely on FRET or related energy transfer designs.

Close Temporal Relationship between Oscillating Cytosolic K+ and Growth in Root Hairs of Arabidopsis

Root hair elongation relies on polarized cell expansion at the growing tip. As a major osmotically active ion, potassium is expected to be continuously assimilated to maintain cell turgor during hair tip growth. However, due to the lack of practicable detection methods, the dynamics and physiological role of K⁺ in hair growth are still unclear. In this report, we apply the small-molecule fluorescent K⁺ sensor NK3 in Arabidopsis root hairs for the first time. By employing NK3, oscillating cytoplasmic K⁺ dynamics can be resolved at the tip of growing root hairs, similar to the growth oscillation pattern. Cross-correlation analysis indicates that K⁺ oscillation leads the growth oscillations by approximately 1.5 s. Artificially increasing cytoplasmic K⁺ level showed no significant influence on hair growth rate, but led to the formation of swelling structures at the tip, an increase of cytosolic Ca²⁺ level and microfilament depolymerization, implying the involvement of antagonistic regulatory factors (e.g., Ca²⁺ signaling) in the causality between cytoplasmic K⁺ and hair growth. These results suggest that, in each round of oscillating root hair elongation, the oscillatory cell expansion accelerates on the heels of cytosolic K⁺ increment, and decelerates with the activation of antagonistic regulators, thus forming a negative feedback loop which ensures the normal growth of root hairs.

A mitochondria-targeting NIR fluorescent potassium ion sensor: real-time investigation of the mitochondrial K+ regulation of apoptosis in situ

TAC-Rh, as the first mitochondria-targeting NIR K⁺ sensor, was applied to explore mutual regulation between mitochondrial K⁺ and apoptosis.

Synthesis, structure and metal ion coordination of novel benzodiazamacrocyclic ligands bearing pyridyl and picolinate pendant side-arms

Complex formation of benzodiazacrown ethers with heavy and transition metal ions was studied using NMR spectroscopy, potentiometry and X-ray crystallography.

A highly selective, colorimetric, and environment-sensitive optical potassium ion sensor

Potassium ions (K⁺) play vital roles in many biological processes and thus highly selective sensors for K⁺ are critical for disease diagnosis and health monitoring. Herein, we report a colorimetric K⁺ sensor (KS7) in which a hemicyanine dye was used as a fluorophore and phenylaza-[18]crown-6 lariat ether (ACLE) was utilized as a K⁺ ligand. The maximum absorption peak of KS7 shifted hypsochromically by 77 nm (from 515 to 438 nm) with an isosbestic point at 452 nm upon the addition of K⁺ to its aqueous solution accompanied by a color change from red to yellow. This sensor exhibited a linear response range to K⁺ from 1 to 200 mM, indicating its wide detection range for cellular, urinary, and environmental potassium ions. Further, this sensor is solvent-sensitive, implying its environmental sensitivity. For the demonstration of its applications, we prepared filter paper-based K⁺ test strips, which were used to detect K⁺ in urine conveniently.

Potassium-Ion-Selective Fluorescent Sensors To Detect Cereulide, the Emetic Toxin of B. cereus, in Food Samples and HeLa Cells

We report the development of new chemical probes for cereulide, a toxic metabolite produced by specific strains of Bacillus cereus, through displacement of potassium cations from a preformed specific complex and a subsequent change in the fluorescence emission. For this purpose, we designed fluorescent probes for potassium cations that were suitable for displacement assays with cereulide from organic extracts. The fluorescence detection of natural cereulide in rice samples was achieved by using synthetic cereulide as a reference and a potassium fluorescent reporter, and this was found to be useful as a portable and fast method for the in situ detection of cereulide in food extracts. To study the fate of cereulide in live cells, we designed a procedure that was suitable for live-cell microscopy imaging of HeLa cells by comparing the cellular location of the potassium fluorogenic probe, which stained intracellular endolysosomes, in the absence and presence of cereulide; we concluded that in the presence of cereulide, the fluorescence of the probe was decreased because of complexation of the potassium ions by cereulide.

Recent progress in the design and applications of fluorescence probes containing crown ethers

Crown ethers, discovered by the winner of the Nobel Prize Charles Pedersen, are cyclic chemical compounds that consist of a ring or multiple rings containing several ether groups that are capable of binding various ions.

Protein-Coupled Fluorescent Probe To Visualize Potassium Ion Transition on Cellular Membranes

K(+) is the most abundant metal ion in cells, and changes of [K(+)] around cell membranes play important roles in physiological events. However, there is no practical method to selectively visualize [K(+)] at the surface of cells. To address this issue, we have developed a protein-coupled fluorescent probe for K(+), TLSHalo. TLSHalo is responsive to [K(+)] in the physiological range, with good selectivity over Na(+) and retains its K(+)-sensing properties after covalent conjugation with HaloTag protein. By using cells expressing HaloTag on the plasma membrane, we successfully directed TLSHalo specifically to the outer surface of target cells. This enabled us to visualize localized extracellular [K(+)] change with TLSHalo under a fluorescence microscope in real time. To confirm the experimental value of this system, we used TLSHalo to monitor extracellular [K(+)] change induced by K(+) ionophores or by activation of a native Ca(2+)-dependent K(+) channel (BK channel). Further, we show that K(+) efflux via BK channel induced by electrical stimulation at the bottom surface of the cells can be visualized with TLSHalo by means of total internal reflection fluorescence microscope (TIRFM) imaging. Our methodology should be useful to analyze physiological K(+) dynamics with high spatiotemporal resolution.

Fluorescent probes and bioimaging: alkali metals, alkaline earth metals and pH

All living species and life forms have an absolute requirement for bio-functional metals and acid-base equilibrium chemistry owing to the critical roles they play in biological processes. Hence, a great need exists for efficient methods to detect and monitor biometals and acids. In the last few years, great attention has been paid to the development of organic molecule based fluorescent chemosensors. The availability of new synthetic fluorescent probes has made fluorescence microscopy an indispensable tool for tracing biologically important molecules and in the area of clinical diagnostics. This review highlights the recent advances that have been made in the design and bioimaging applications of fluorescent probes for alkali metals and alkaline earth metal cations, including lithium, sodium and potassium, magnesium and calcium, and for pH determination within biological systems.

Xanthurenic acid: a natural ionophore with high selectivity and sensitivity for potassium ions in an aqueous solution

Synopsis: A well-known tryptophan metabolite, xanthurenic acid, a natural non-fluorescent intermediate siderophore, showed a very selective turn-on response to K⁺ over other competing metal ions and the detection limit of this natural ionophore was found to be 53 nM at physiological pH.

A highly selective mitochondria-targeting fluorescent K(+) sensor

Regulation of intracellular potassium (K(+) ) concentration plays a key role in metabolic processes. So far, only a few intracellular K(+) sensors have been developed. The highly selective fluorescent K(+) sensor KS6 for monitoring K(+) ion dynamics in mitochondria was produced by coupling triphenylphosphonium, borondipyrromethene (BODIPY), and triazacryptand (TAC). KS6 shows a good response to K(+) in the range 30-500 mM, a large dynamic range (Fmax /F0 ≈130), high brightness (ϕf =14.4 % at 150 mM of K(+) ), and insensitivity to both pH in the range 5.5-9.0 and other metal ions under physiological conditions. Colocalization tests of KS6 with MitoTracker Green confirmed its predominant localization in the mitochondria of HeLa and U87MG cells. K(+) efflux/influx in the mitochondria was observed upon stimulation with ionophores, nigericin, or ionomycin. KS6 is thus a highly selective semiquantitative K(+) sensor suitable for the study of mitochondrial potassium flux in live cells.

Optical probes for the detection of protons, and alkali and alkaline earth metal cations

Luminescent sensors and switches continue to play a key role in shaping our understanding of key biochemical processes, assist in the diagnosis of disease and contribute to the design of new drugs and therapies. Similarly, their contribution to the environment cannot be understated as they offer a portable means to undertake field testing for hazardous chemicals and pollutants such as heavy metals. From a physiological perspective, the Group I and II metal ions are among the most important in the periodic table with blood plasma levels of H(+), Na(+) and Ca(2+) being indicators of several possible disease states. In this review, we examine the progress that has been made in the development of luminescent probes for Group I and Group II ions as well as protons. The potential applications of these probes and the mechanism involved in controlling their luminescent response upon analyte binding will also be discussed.

A label-free fluorescence assay for potassium ions using riboflavin as a G-quadruplex ligand

A label-free fluorescence K⁺assay was developed using riboflavin, a new G-quadruplex ligand, and a G-quadruplex sequence (PW17).

A highly K(+)-selective phenylaza-[18]crown-6-lariat-ether-based fluoroionophore and its application in the sensing of K+ ions with an optical sensor film and in cells

Herein, we report the synthesis of two phenylaza-[18]crown-6 lariat ethers with a coumarin fluorophore (1 and 2) and we reveal that compound 1 is an excellent probe for K(+) ions under simulated physiological conditions. The presence of a 2-methoxyethoxy lariat group at the ortho position of the anilino moiety is crucial to the substantially increased stability of compounds 1 and 2 over their lariat-free phenylaza-[18]crown-6 ether analogues. Probe 1 shows a high K(+)/Na(+) selectivity and a 2.5-fold fluorescence enhancement was observed in the presence of 100 mM K(+) ions. A fluorescent membrane sensor, which was prepared by incorporating probe 1 into a hydrogel, showed a fully reversible response, a response time of 150 s, and a signal change of 7.8% per 1 mM K(+) within the range 1-10 mM K(+). The membrane was easily fabricated (only a single sensing layer on a solid polyester support), yet no leaching was observed. Moreover, compound 1 rapidly permeated into cells, was cytocompatible, and was suitable for the fluorescent imaging of K(+) ions on both the extracellular and intracellular levels.

K, Ca complexes with a sulfonic ligand: structure and DNA-binding properties

A novel 4-(2,4-dihydroxybenzylideneamino)benzenesulfonic acid (HL), and its kalium(I), calcium(II) complexes [M(L)n]·2nH(2)O·Cl(n) (M=K(1) n=1, M=Ca(2) n=2), have been prepared and characterized. The crystal and molecular structures of 1 and 2 were determined by single-crystal X-ray diffraction. The interaction of 1, 2 and ligand (L) with calf thymus DNA was investigated by UV-visible (UV-vis), fluorescence and viscosity measurements. Experimental results indicate that 1, 2 and L could bind to DNA via the intercalation mode, and the binding affinity of 1 is stronger than that of 2 and L. The intrinsic binding constants of 1, 2 and L were 5.60×10(5), 6.53×10(5) and 1.44×10(5)M(-1), respectively. The cleavage reaction on plasmid DNA has been monitored by agarose gel electrophoresis. The results indicated that 1 and 2 could cleave pBR322 DNA.

A new highly selective fluorescent K+ sensor

We describe the synthesis, properties, and application of a new fluorescent potassium chemosensor, KS2, for K(+) sensing and imaging in live cells. By virtue of a strong electron-withdrawing group, 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF), with a triazacryptand ligand, the new sensor can respond to K(+) up to 1.6 M. This is the first highly selective intracellular sensor suitable for sensing K(+) over a broad and high concentration range. Confocal fluorescence microscopy has established the utility of KS2 for live-cell K(+) detection. The application of KS2 combined with other sensors will be of great benefit for investigating cellular metabolism, detecting and diagnosing diseases including cancer, and monitoring responses to therapy.

Triazacryptand-based fluorescent sensors for extracellular and intracellular K+ sensing

A 4-amino-naphthalimide derived fluorophore with a triazacryptand moiety ligand was synthesized as a potassium ion (K(+)) sensor (KS1). This sensor is a monomer possessing a polymerizable vinyl group. By taking advantage of the polymerizable characteristics of the vinyl group, KS1 was polymerized with 2-hydroxyethyl methacrylate (HEMA) and acrylamide (AM) to form K(+) sensing films for extracellular sensing. The sensitivity of the films to potassium ions can be further tuned through the adjustment of the HEMA and AM weight ratios as well as introduction of positive or negative charge-containing segments. KS1 and its poly(2-hydroxyethyl methacrylate)-co-poly(acrylamide) (PHEMA-co-PAM) thin films show high selectivity for K(+) over competing sodium ions (Na(+)) at physiological concentrations. Extracellular sensing was demonstrated using a KS1-conjugated PHEMA-co-PAM thin film to measure the K(+) efflux of Escherichia coli (E. coli) and Bacillus subtilis (B. subtilis) stimulated by lysozyme. Meanwhile, KS1 itself permeates human glioblastoma U87MG and human esophagus premalignant CP-A cell lines. KS1 was used to monitor K(+) efflux stimulated by adenosine-5'-triphosphate (ATP), amphotericin, and a mixture of nigericin, bumetanide and ouabain, demonstrating application of this material as an intracellular potassium ion sensor.

The K+ channel opener 1-EBIO potentiates residual function of mutant CFTR in rectal biopsies from cystic fibrosis patients

Background

The identification of strategies to improve mutant CFTR function remains a key priority in the development of new treatments for cystic fibrosis (CF). Previous studies demonstrated that the K⁺ channel opener 1-ethyl-2-benzimidazolone (1-EBIO) potentiates CFTR-mediated Cl⁻ secretion in cultured cells and mouse colon. However, the effects of 1-EBIO on wild-type and mutant CFTR function in native human colonic tissues remain unknown.

Methods

We studied the effects of 1-EBIO on CFTR-mediated Cl⁻ secretion in rectal biopsies from 47 CF patients carrying a wide spectrum of CFTR mutations and 57 age-matched controls. Rectal tissues were mounted in perfused micro-Ussing chambers and the effects of 1-EBIO were compared in control tissues, CF tissues expressing residual CFTR function and CF tissues with no detectable Cl⁻ secretion.

Results

Studies in control tissues demonstrate that 1-EBIO activated CFTR-mediated Cl⁻ secretion in the absence of cAMP-mediated stimulation and potentiated cAMP-induced Cl⁻ secretion by 39.2±6.7% (P<0.001) via activation of basolateral Ca²⁺-activated and clotrimazole-sensitive KCNN4 K⁺ channels. In CF specimens, 1-EBIO potentiated cAMP-induced Cl⁻ secretion in tissues with residual CFTR function by 44.4±11.5% (P<0.001), but had no effect on tissues lacking CFTR-mediated Cl⁻conductance.

Conclusions

We conclude that 1-EBIO potentiates Cl⁻secretion in native CF tissues expressing CFTR mutants with residual Cl⁻ channel function by activation of basolateral KCNN4 K⁺ channels that increase the driving force for luminal Cl⁻ exit. This mechanism may augment effects of CFTR correctors and potentiators that increase the number and/or activity of mutant CFTR channels at the cell surface and suggests KCNN4 as a therapeutic target for CF.

Fluorescent indicators based on BODIPY

This critical review covers the advances made using the 4-bora-3a,4a-diaza-s-indacene (BODIPY) scaffold as a fluorophore in the design, synthesis and application of fluorescent indicators for pH, metal ions, anions, biomolecules, reactive oxygen species, reactive nitrogen species, redox potential, chemical reactions and various physical phenomena. The sections of the review describing the criteria for rational design of fluorescent indicators and the mathematical expressions for analyzing spectrophotometric and fluorometric titrations are applicable to all fluorescent probes (206 references).

Function-Oriented Synthesis of a Didesmethyl Triazacryptand Analogue for Fluorescent Potassium Ion Sensing

Triazacryptand (TAC)-based fluorescent K+ sensors have broad biomedical utility, yet their advancement has been hindered because of their challenging synthesis. Herein, an efficient synthesis is reported that delivers a didesmethyl tri-azacryptand (ddTAC) K+ sensor in twofold fewer steps and ninefold higher overall yield than the original TAC synthesis. Our synthesis utilizes a C-O dianionic oxidative macrocyclization and reports new examples of aminoarylations and a microwave route to xanthythilium chromophores. The K+ sensitivity and selectivity of the ddTAC-based sensor are comparable to the TAC-based sensor.

Selective fluorescence sensing of deoxycytidine 5'-monophosphate (dCMP) employing a bis(diphenylphosphate)diimine ligand

A new bis(diphenylphosphate)diimine ligand (BP1) was prepared and evaluated for its ability for selective detection of deoxycytidine 5'-monophosphate (dCMP). BP1 exhibited off-type fluorescence in the presence of dCMP. The fluorescence of BP1 was significantly quenched upon the addition of 2.5 × 10(-4) M dCMP and the detection limit was 1.25 × 10(-5) M in MeCN-H(2)O (1:1, v/v). The binding ratio between BP1 and dCMP was determined to be 1:1 with the binding constant of 3.98 ± 0.60 × 10(-3) M(-1).

Synthesis of a sensitive and selective potassium-sensing fluoroionophore

An efficient synthesis is reported that delivers in 5 steps and 52% overall yield a new structurally simplified fluorescent K(+) sensor with improved K(+) sensitivity and selectivity over existing K(+) sensors. The synthesis procedure utilizes a new template-directed oxidative C-N bond-forming macrocyclization reaction and reports new approaches to Pd(0), Sandmeyer-like and metal-free aminoarylations, as well as organotitanium additions to vinylogous sulfonates.