Why Do Ionic Surfactants Significantly Alter the Chemiluminogenic Properties of Acridinium Salt?

Acridinium esters, due to their capability for chemiluminescence (CL), are employed as indicators and labels in biomedical diagnostics and other fields. In this work, the influence of ionic surfactants, hexadecyltrimethylammonium chloride and bromide (CTAC and CTAB, cationic) and sodium dodecyl sulphate (SDS, anionic) on the CL parameters and mechanism of representative emitter, 10-methyl-9-[(2-methylphenoxy)carbonyl]acridinium trifluoromethanesulphonate (2MeX) in a H2O2/NaOH environment, is studied. Our investigations revealed that the type of surfactant and its form in solution have an impact on the CL kinetic constants and integral efficiencies, while changes in those emission properties resulting from the type of ion (Cl- vs. Br-) are negligible. The major changes were recorded for systems containing surfactants at concentrations higher than the critical micelle concentration. The cationic surfactants (CTAC, CTAB) cause a substantial increase in CL emission kinetics and a moderate increase in its integral efficiency. At the same time, the opposite effect is observed in the case of SDS. Molecular dynamics simulations suggest that changes in emission parameters are likely due to differences in the binding strength of 2MeX substrate with surfactant molecules, which is higher for SDS than for CTAC. The results can help in rational designing of optimal acridinium CL systems and demonstrate their usefulness in distinguishing the pre- and post-micellar environment and the charge of surfactants.

What Other Than Acridinium Esters? Computational Search for New Acridinium-Based Chemiluminogens.

The rapid increase in disease prevalence in the world makes it extremely important to search for new or develop existing diagnostic methods, for example, chemiluminescent labeling used in immunodiagnostics. At present, acridinium esters are willingly used as chemiluminogenic fragments of labels. However, the search for new chemiluminogens that are particularly efficient is the main task of our studies. The density functional theory (DFT) and time-dependent (TD) DFT methods were used to obtain thermodynamic and kinetic results concerning the chemiluminescence and competitive dark reactions, which indicated whether some of the scrutinized derivatives have better characteristics than the chemiluminogens used so far. Synthesis of these candidates for efficient chemiluminogens, followed by studies of their chemiluminescent properties, and ultimately in chemiluminescent labeling, are further steps to confirm their potential applicability in immunodiagnostics.

Solvent Effect on the Chemiluminescence of Acridinium Thioester: A Computational Study.

Chemiluminescent labelling, which is one of the promising procedures of modern immunodiagnostics, is increasingly carried out using acridinium derivatives, an oxidant, and an alkaline aqueous environment. However, the efficiency of the chemiluminescence of luminol or acridinium esters is higher in non-aqueous solvents such as dimethyl sulfoxide or acetonitrile. Therefore, the search for a new environment for the chemiluminescence reaction, especially the one characterized by a higher quantum yield of chemiluminescence, is one of the aims of current research. Using computational methods (DFT and TD DFT with PCM model of solvent), we examined thermodynamic and kinetic data concerning the chemiluminescence and competitive dark pathways. Our results suggest that better characteristics of the chemiluminescence reaction of acridinium thioester are observed in nonpolar solvents, such as methylcyclohexane, n-hexane and n-pentane, than in aqueous media used so far. Further experimental verification is necessary to confirm the possible application of proposed nonpolar solvents in chemiluminescent labelling and hence in immunodiagnostics.

New luminometric method for quantification of biological sulfur nucleophiles with the participation of 9-cyano-10-methylacridinium salt.

Biologically active compounds containing sulfhydryl groups (RSHs: N-acetyl-l-cysteine, d-penicillamine, glutathione and acetylthiocholine chloride) were used to develop a luminometric method for their quantification. The title substrate capable of chemiluminescence (CL) was isolated in a highly pure state as a chloride salt (99.9% using RP-HPLC) and identified using mass spectrometry (ESI Q-TOF) and 1 H NMR spectroscopy. The cation included in the salt, 9-CMA+ , underwent oxidation in an alkaline environment containing RSHs by molecular oxygen, generating CL of various intensities, with no need for the use of hydrogen peroxide. The amount of produced light was linearly proportional to the content of investigated analytes in the system over the concentration range ~0.2-2 µM, with the detection limits in the range 0.19-1.73 µM. The mechanism of chemiluminogenic oxidation of 9-CMA+ in the presence of RSHs and molecular oxygen is proposed, using computational methods at the density-functional theory level. The presence of RSHs in an alkaline medium seems to be crucial to produce hydroperoxide anions (- OOH), which initiate the 'light path' of 9-CMA+ transformations, ending with the excretion of electronically excited molecules of 10 methyl-acridan-9-one.

Structural characterization and theoretical calculations of the monohydrate of the 1:2 cocrystal salt formed from acriflavine and 3,5-dinitrobenzoic acid.

The synthesis and structural characterization of the monohydrated 1:2 cocrystal salt of acriflavine with 3,5-dinitrobenzoic acid [systematic name: 3,6-diamino-10-methylacridin-10-ium 3,5-dinitrobenzoate-3,5-dinitrobenzoic acid-water (1/1/1), C14H14N3+·C7H3N2O6-·C7H4N2O6·H2O] are reported. Single-crystal X-ray diffraction measurements show that the title solvated monohydrate salt crystalizes in the monoclinic space group P21 with one acriflavine cation, a 3,5-dinitrobenzoate anion, a 3,5-dinitrobenzoic acid molecule and a water molecule in the asymmetric unit. The neutral and anionic forms of 3,5-dinitrobenzoic acid are linked via O-H...O hydrogen bonds to form a monoanionic dimer. Neighbouring monoanionic dimers of 3,5-dinitrobenzoic acid are linked by nitro-nitro N-O...N and nitro-acid N-O...π intermolecular interactions to produce a porous organic framework. The acriflavine cations are linked with carboxylic acid molecules directly via amine-carboxy N-H...O, amine-nitro N-H...O and acriflavine-carboxy C-H...O hydrogen bonds, and carboxy-acriflavine C-O...π, nitro-acriflavine N-O...π and acriflavine-nitro π-π interactions, or through the water molecule by amino-water N-H...O and water-carboxy O-H...O hydrogen bonds, and are located in the voids of the porous organic framework. The intermolecular interactions were studied using the CrystalExplorer program to provide information about the interaction energies and the dispersion, electrostatic, polarization and repulsion contributions to the lattice energy.

Effect of Aromatic System Expansion on Crystal Structures of 1,2,5-Thia- and 1,2,5-Selenadiazoles and Their Quaternary Salts: Synthesis, Structure, and Spectroscopic Properties.

Rational manipulation of secondary bonding interactions is a crucial factor in the construction of new chalcogenadiazole-based materials. This article reports detailed experimental studies on phenanthro[9,10-c][1,2,5]chalcogenadiazolium and 2,1,3-benzochalcogenadiazolium salts and their precursors. The compounds were synthesized, characterized employing NMR and UV-Vis spectroscopy. TD-DFT calculations were also performed. The influence of the size of the aromatic system on the molecular motifs formed by the compounds in the solid state has been studied by means of single-crystal X-ray diffraction. In case of the salts, the nature of an anion was also taken into consideration. The results show that cyclic [E···N]2 supramolecular synthon connects neighboring molecules of phenanthro[9,10-c][1,2,5]chalcogenadiazoles, with a relatively large aromatic system, in dimers regardless of the chalcogen atom in the molecule. Both N-methyl-2,1,3-benzothiadiazolium and N-methylphenanthro[9,10-c][1,2,5]chalcogenadiazolium cations have a strong affinity for triflate and iodide anions, therefore the formation of S···N or Se···N secondary bonding interactions is observed only in two out of the eight quaternary salts. Less coordinating anions must be used to enable the building blocks studied to form cyclic [E···N]2 synthons. Moreover, for two of the triflate salts, which are isostructural, a new supramolecular motif has been observed.

Computational Insights on the Mechanism of the Chemiluminescence Reaction of New Group of Chemiluminogens-10-Methyl-9-thiophenoxycarbonylacridinium Cations.

Immunodiagnostics, in which one of the promising procedures is the chemiluminescent labelling, is essential to facilitate the detection of infections in a human organism. One of the standards commonly used in luminometric assays is luminol, which characterized by low quantum yield in aqueous environments. Acridinium esters have better characteristics in this topic. Therefore, the search for new derivatives, especially those characterized by the higher quantum yield of chemiluminescence, is one of the aims of the research undertaken. Using the proposed mechanism of chemiluminescence, we examined the effect of replacing a single atom within a center of reaction on the efficient transformation of substrates into electronically excited products. The density functional theory (DFT) and time dependent (TD) DFT calculated thermodynamic and kinetic data concerning the chemiluminescence and competitive dark pathways suggests that some of the scrutinized derivatives have better characteristics than the chemiluminogens used so far. Synthesis of these candidates for efficient chemiluminogens, followed by studies of their chemiluminescent properties, and ultimately in chemiluminescent labelling, are further steps to confirm their potential applicability in immunodiagnostics.

Effective chemiluminogenic systems based on acridinium esters bearing substituents of various electronic and steric properties.

A series of 10-methyl-9-(phenoxycarbonyl)acridinium trifluoromethanesulfonates (XAEs), bearing substituents of various characteristics in the lateral benzene ring (2-halogen, 2,6-dihalogen, 2-trifluoromethyl, 2-nitro, 2-methoxy, 3-halogen and 4-halogen) were synthesized with high yields, identified and subjected to a physicochemical and theoretical investigation. The main task of the work was to assess the mechanism and optimal conditions of light emission in various liquid systems based on the above salts in order to evaluate their potential usefulness as chemiluminescence (CL) labels and indicators in ultra-sensitive analyses. Density functional theory (DFT) calculations were performed to investigate the detailed mechanism of the oxidation of 9-substituted 10-methylacridinium cations involved in XAEs by hydrogen peroxide in alkaline media. Three general pathways were drawn, which are termed the "light path" (chemiluminogenic) and there were two "dark paths" (non-chemiluminogenic): hydrolytic and "pseudobase". The CL time profiles, triggered in alkaline solutions containing hydrogen peroxide, enabled us to establish crucial physicochemical parameters, including pseudo-first order kinetic constants of CL decay and relative efficiencies of emission. In order to optimize the systems' luminogenic performance, different bases, such as sodium hydroxide, tetrabutylammonium hydroxide (TBAOH) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), as well as enhancers, such as cationic, zwitterionic and neutral surfactants (cetyltrimethylammonium chloride (CTAC), N,N-dimethyldodecylammonio-1,3-propane sulfonate (DDAPS) and Triton X-100, respectively) were tested. The results revealed the optimal CL systems, which enabled us to obtain substantially higher emissions than typical ones, based on acridinium esters or luminol. The derived parameters, characterizing the potential utility of the acridinium esters, such as stability in aqueous environments and usefulness (the product of emission efficiency and stability at a given pH), enabled us to reveal the best candidates and their practical applications. The post-reaction mixtures, analyzed by means of chromatography (RP-HPLC) and mass spectrometry (ESI-MS), allowed us to verify the occurrence and population of the products that were theoretically predicted, i.e. 10-methyl-9-acridinone (NMAON), 10-methylacridinium-9-carboxylic acid (NMACA) and substituted phenols (RPhOHs).

Tautomerism and behavior of 3-hydroxy-2-phenyl-4H-chromen-4-ones (flavonols) and 3,7-dihydroxy-2,8-diphenyl-4H,6H-pyrano[3,2-g]chromene-4,6-diones (diflavonols) in basic media: spectroscopic and computational investigations.

Absorption and emission spectroscopic investigations and computational predictions have shown that neutral molecules of flavonols and diflavonols can exist in the ground and excited states in one or two tautomeric forms stabilized by intramolecular (in aprotic media) or intermolecular (with solvent molecule(s), in protic media) hydrogen bonds. Electronic excitation creates conditions for the transformation of tautomeric forms, accompanied by proton transfer, reflected in fluorescence spectra. Proton transfer is also probable in monoanions of diflavonols in protic media. The OH groups involved in hydrogen bonds exhibit a proton-donating ability characterized by the respective acidity constants. The electronically excited diflavonols are relatively strong acids if they lose one proton. With increasing basicity of the medium, anionic forms occur, which exhibit spectral characteristics and emission abilities different from those of neutral molecules. These features open up possibilities for the analytical use of these compounds as spectral probes sensitive to the properties of liquid phases--from neutral to strongly basic. The less intensively studied diflavonols seem to be more promising than flavonols for these purposes, since they are more lipophilic, polarizable, polar, and sensitive to basic features of the environment.

Chemiluminogenic features of 10-methyl-9-(phenoxycarbonyl)acridinium trifluoromethanesulfonates alkyl substituted at the benzene ring in aqueous media.

10-Methyl-9-(phenoxycarbonyl)acridinium trifluoromethanesulfonates bearing alkyl substituents at the benzene ring were synthesized, purified, and identified. In the reaction with OOH(-) in basic aqueous media, the cations of the compounds investigated were converted to electronically excited 10-methyl-9-acridinone, whose relaxation was accompanied by chemiluminescence (CL). The kinetic constants of CL decay, relative efficiencies of light emission, chemiluminescence quantum yields, and resistance toward alkaline hydrolysis were determined experimentally under various conditions. The mechanism of CL generation is considered on the basis of thermodynamic and kinetic parameters of the reaction steps predicted at the DFT level of theory. The chemiluminescence efficiency is the result of competition of the electrophilic center at C(9) between nucleophilic substitution by OOH(-) or OH(-) and the ability of the intermediates thus formed to decompose to electronically excited 10-methyl-9-acridinone. Identification of stable and intermediate reaction products corroborated the suggested reaction scheme. The results obtained, particularly the dependency of the "usefulness" parameter, which takes into account the CL quantum yield and the susceptibility to hydrolysis, on the cavity volume of the entity removed during oxidation, form a convenient framework within which to rationally design chemiluminescent 10-methyl-9-(phenoxycarbonyl)acridinium cations.

Chemiluminogenic properties of 10-methyl-9-(phenoxycarbonyl)acridinium cations in organic environments.

The chemiluminogenic (CL) properties of aryl esters of 9-carboxy-10-methylacridinium acid and 9-carboxy-2-methoxy-10-methylacridinium acid (AE), variously substituted in the benzene ring (2-H, 2-CH(3), 2-Cl) were investigated in aliphatic alcohols, acetonitrile, and dimethyl sulfoxide in the presence of hydrogen peroxide and different bases-potassium hydroxide, tetra-n-butylammonium hydroxide, and 1,8-diazabicyclo[5.4.0]undec-7-ene. The dependence of their CL properties (decay rate constants (k(CL)) and relative efficiencies (RE)) on solvent parameters, the nature and concentration of base, as well as H(2)O(2) concentration were investigated. Comparison of the various AE revealed that substituents at the benzene ring strongly influence the reaction kinetics, while 2-OCH(3) substitution of the acridine nucleus is manifested, in general, by a red shift in the emission spectrum and slight increase in CL efficiency. The values of k(CL) depend linearly on polarity and acid-base properties of solvents as well as on concentration of bases (over certain concentration ranges) and demonstrate a nonlinear dependence on H(2)O(2) concentration. RE values depend on solvent polarity and nucleophilicity but are rather weakly dependent on base and oxidant concentrations. The CL properties of the above systems are discussed in the context of their physicochemical features gained from fluorescence spectroscopy, spectrophotometric titration, MS, and HPLC. Electronically excited 10-methyl-9-acridinones are the light-emitting entities in both organic and aqueous environments. It was also found that the tendency for an unwanted side-process, the production of a pseudobase form of AE, to take place was similar in alcoholic and aqueous media, although 2-methoxy ring-substituted derivatives seemed to be less susceptible to this dark-type conversion. On the basis of these results new CL systems are postulated that are more efficient than their aqueous counterparts.

2-Meth-oxy-9-phenoxy-acridine.

The mol-ecules in the crystal structure of the title compound, C(20)H(15)NO(2), form inversion dimers connected through the C-H⋯N and π-π inter-actions. These dimers are further linked by C-H⋯π inter-actions. The meth-oxy group is nearly coplanar with the acridine ring system [dihedral angle = 4.5 (1)°], whereas the phen-oxy fragment is nearly perpendicular to it [dihedral angle = 85.0 (1)°]. The mean planes of the acridine ring systems are either parallel or inclined at angles of 14.3 (1), 65.4 (1) and 67.3 (1)° in the crystal.

9-Benzyl-10-methyl-acridinium trifluoro-methane-sulfonate.

In the crystal structure of the title compound, C(21)H(18)N(+)·CF(3)OS(3) (-), the cations form inversion dimers through π-π inter-actions between the acridine ring systems. These dimers are further linked by C-H⋯π inter-actions. The cations and anions are connected by C-H⋯O, C-F⋯π and S-O⋯π inter-actions. The acridine and benzene ring systems are oriented at a dihedral angle of 76.8 (1)°with respect to each other. The acridine moieties are either parallel or inclined at an angle of 62.4 (1)° in the crystal structure.

9-Phenyl-10H-acridinium trifluoro-methane-sulfonate.

In the crystal structure of the title compound, C(19)H(14)N(+)·CF(3)SO(3) (-), the cations are linked to each other by very weak C-H⋯π inter-actions, while the cations and anions are connected by N-H⋯O, C-H⋯O and S-O⋯π inter-actions. The acridine ring system and the phenyl ring are oriented at an angle of 80.1 (1)° with respect to each other. The mean planes of adjacent acridine units are either parallel or inclined at an angle of 35.6 (1)°. The trifluoro-methane-sulfonate anions are disordered over two positions; the site occupancy factors are 0.591 (8) and 0.409 (8).

9-(Methyl-sulfan-yl)acridinium trifluoro-methane-sulfonate.

In the crystal structure of the title compound, C(14)H(12)NS(+)·CF(3)SO(3) (-), N-H⋯O hydrogen bonds link cations and anions into ion pairs. Inversely oriented ion pairs form stacks through multidirectional π-π inter-actions among the acridine units. The crystal structure features a network of C-H⋯O inter-actions among stacks and also long-range electrostatic inter-actions among ions. In the packing of the mol-ecules, the acridine units are nearly parallel in stacks or inclined at an angle of 33.07 (2)° in the four adjacent stacks with which they inter-act via weak C-H⋯O inter-actions. The methyl-sulfanyl group is twisted through an angle of 60.53 (2)° with respect to the acridine ring.

9-Chloro-2,4-dimethoxy-acridinium trifluoro-methane-sulfonate.

In the mol-ecular structure of the title compound, C(15)H(13)ClNO(2) (+)·CF(3)SO(3) (-), the meth-oxy groups are nearly coplanar with the acridine ring system, making dihedral angles of 0.4 (2) and 5.1 (2)°. Multidirectional π-π contacts between acridine units are observed in the crystal structure. N-H⋯O and C-H⋯O hydrogen bonds link cations and anions, forming a layer structure.

9-Ethyl-10-methyl-acridinium trifluoro-methane-sulfonate.

In the mol-ecule of the title compound, C(16)H(16)N(+)·CF(3)SO(3) (-), the central ring adopts a flattened-boat conformation, and the two aromatic rings are oriented at a dihedral angle of 3.94 (2)°. In the crystal structure, weak inter-molecular hydrogen bonds link the mol-ecules. There are π-π contacts between the aromatic rings and the central ring and one of the aromatic rings [centroid-centroid distances = 3.874 (2), 3.945 (2) and 3.814 (2) Å]. There is also an S-O⋯π contact between the central ring and one of the O atoms of the anion.