Development of Two Eco-Friendly and High-Throughput Microwell Spectrophotometric Methods for Analysis of an Antibacterial Drug Tulathromycin in Pharmaceutical Bulk Form

BACKGROUND

Tulathromycin (TUL) is a triamilide antibacterial drug which has been approved for use in the European Union and the United States for the treatment and prevention of bovine respiratory diseases. The existing methods for determination of TUL in its pharmaceutical bulk form are very limited and suffer from major drawbacks.

OBJECTIVES

The aim of this study was the development of two innovative microwell spectrophotometric methods (MW-SPMs) for determination of TUL in its pharmaceutical bulk form.

METHODS

The formation of charge transfer complexes (CTCs) of TUL, as an electron donor, was investigated with 2,5-dihydroxy-3,6-dichlorocyclohexa-2,5-diene-1,4-dione (HCD) and 2,3-dichloro-5,6-dicyano-p-benzoquinone (CBQ), as π-electron acceptors. The CTCs were characterized by using UV-visible spectrophotometry and computational calculations. The reactions were employed for the development of two MW-SPMs with a one-step for the quantitative analysis of TUL.

RESULTS

The formation of CTCs was confirmed via the formation of characteristic absorption bands with maximum absorption at 520 and 460 nm for CTCs with HCD and CBQ, respectively. The stoichiometry of both CTCs was found to be 1:1, and the values of different spectroscopic and electronic constants confirmed the stability of the CTCs. The mechanisms of the reactions were postulated. The linear range of both MW-SPMs was 10-500 µg/mL. The limits of quantitation were 13.5 and 26.4 µg/mL for methods involving reactions with HCD and CBQ, respectively. Both methods were successfully applied to the quantitation of TUL in pharmaceutical bulk form with acceptable accuracy and precision. The results of eco-friendliness/greenness assessment proved that both MW-SPMs fulfill the requirements of green analytical approaches. In addition, the one-step reactions and simultaneous handling of a large number of samples with micro-volumes in the proposed methods gave them the advantage of high throughput analysis.

CONCLUSIONS

This study described two new MW-SPMs as valuable analytical tools for the determination of TUL.

Efficient capture of iodine in steam and water media by hydrogen bond-driven charge transfer complexes

Untreated radioactive iodine (129I and 131I) released from nuclear power plants poses a significant threat to humans and the environment, so the development of materials to capture iodine from water media and steam is critical. Here, we report a charge transfer complex (TCNQ-MA CTC) with abundant nitrogen atoms and π-conjugated system for adsorption of I2 vapor and I3- from aqueous solutions. Due to the synergistic binding mechanism of benzene/triazine rings and N-containing groups with iodine, special I-π and charge transfer interaction can be formed between the guest and the host, and thus efficient removal of I2 and I3- can be realized by TCNQ-MA CTC with the adsorption capacity up to 2.42 g/g and 800 mg/g, respectively. TCNQ-MA CTC can capture 92% of I3- within 2.5 min, showing extremely fast kinetics, excellent selectivity and high affinity (Kd = 5.68 × 106 mL/g). Finally, the TCNQ-MA CTC was successfully applied in the removal of iodine from seawater with the efficiency of 93.71%. This work provides new insights in the construction of charge transfer complexes and lays the foundation for its environmental applications.

Charge transfer complex-based spectrophotometric analysis of famotidine in pure and pharmaceutical dosage forms

A straightforward and efficient spectrum technique was created using Ortho-chloranil as the electron acceptor (-acceptor) in a charge transfer (CT) complex formation reaction to determine the concentration of famotidine (FMD) in solutions. Compared to the double-distilled blank solution, the reaction result detected a definite violet colour at a maximum absorption wavelength of 546 nm, For concentrations range 2-28 µg/ml, the technique demonstrated excellent compliance with Beer-Law and Lambert's, as evidenced by its molar absorptivity of 2159.648 L mol-1 cm-1. Lower detection limits of 0.3024 µg/ml and 1.471 µg/ml, respectively, were discovered. The complexes of famotidine and Ortho-chloranil were found to have a 2:1 stoichiometry. Additionally, the suggested approach effectively estimated famotidine concentrations in pharmaceutical formulations, particularly in tablet form.

Synthesis, spectroscopic and computational characterization of charge transfer complex of remdesivir with chloranilic acid: Application to development of novel 96-microwell spectrophotometric assay

Remdesivir (REM) is an adenosine triphosphate analog antiviral drug that has received authorization from European Commission and approval from the U.S. Food and Drug Administration for treatment of coronavirus disease 2019 (Covid-19). This study, describes, for the first time, the synthesis of a novel charge transfer complex (CTC) between REM, as electron donor, with chloranilic acid (CLA), as π electron acceptor. The CTC was characterized using different spectroscopic and thermogravimetric techniques. UV-visible spectroscopy ascertained the formation of the CTC in methanol via formation of a new broad absorption band with maximum absorption peak (λ_max) at 530 nm. The molar absorptivity (ε) of the complex was 3.33 × 10³ L mol^-1 cm^-1 and its band gap energy was 1.91 eV. The stoichiometric ratio of REM:CLA was found to be 1:1. The association constant of the complex was 1.11 × 10⁹ L mol^-1, and its standard free energy was 5.16 × 10⁴ J mole^-1. Computational calculation for atomic charges of energy minimized REM was conducted, the site of interaction on REM molecule was assigned and the mechanism of the reaction was postulated. The solid-state CTC was further characterized by FT-IR and ¹H NMR spectroscopic techniques. Both FT-IR and ¹H NMR confirmed the formation of the CTC and its structure. The reaction was adopted as a basis for developing a novel 96-microwell spectrophotometric method (MW-SPA) for REM. The assay limits of detection and quantitation were 3.57 and 10.83 µg/well, respectively. The assay was validated, and all validation parameters were acceptable. The assay was implemented successfully with great precision and accuracy to the determination of REM in its bulk form and pharmaceutical formulation (injection). This assay is simple, economic, and more importantly, has high throughput property. Therefore, the assay can be valuable for routine in quality control laboratories for analysis of REM's bulk form and pharmaceutical injection.

Synthesis, spectroscopic and computational studies on hydrogen bonded charge transfer complex of duvelisib with chloranilic acid: Application to development of novel 96-microwell spectrophotometric assay

Duvelisib (DUV) is a is a small-molecule with inhibitory action for phosphoinositide 3-kinase (PI3K). It has been recently approved for the effective treatment of chronic lymphocytic leukemia (CLL) and small lymphocytic lymphoma (SLL). Novel charge transfer complex (CTC) between DUV, as electron donor, with chloranilic acid (CLA), as π electron acceptor has been synthesized and characterized using different spectroscopic and thermogravimetric techniques. UV-visible spectroscopy ascertained the formation of the CTC in different solvents of varying polarity indexes and dielectric constants via formation of new broad absorption band with maximum absorption peak (λ_max) in the range of 488-532 nm. The molar absorptivity of the CTC was dependent on the polarity index and dielectric constant of the solvent; the correlation coefficients were 0.9955 and 0.9749, respectively. The stoichiometric ratio of DUV:CLA was 1:1. Electronic spectral analysis was conducted for characterization of the complex in terms of its electronic constants. Computational calculation for atomic charges of energy minimized DUV was conducted and the site of interaction on DUV molecule was assigned. The solid-state CTC of DUV:CLA (1:1) was synthesized, and its structure was characterized by UV-visible, mass, FT-IR, and ¹H NMR spectroscopic techniques. Both FT-IR and ¹H NMR confirmed that both CT and hydrogen bonding contributed to the molecular composition of the complex. The reaction was adopted as a basis for developing a novel 96-microwell spectrophotometric assay (MW-SPA) for DUV. The assay limits of detection and quantitation were 0.57 and 1.72 µg/well, respectively. The assay was validated and all validation parameters were acceptable. The method was implemented successfully with great precision and accuracy to the analysis of the DUV in its bulk and capsules.

Charge-transfer complex-based spectrophotometric method for the determination of mesotrione in environmental samples

A simple, accurate, sensitive, and selective spectrophotometric method has been developed for the determination of mesotrione. This method is based on the reaction of mesotrione with Fe(III) to form a charge transfer metal complex having λ_max at 348 nm. Beer's law was obeyed in the concentration range of 0.2-10.0 µg mL^-1 with limit of detection (LOD) and limit of quantification (LOQ) equal to 0.053 and 0.162 µg mL^-1, respectively. The percent recovery of mesotrione from different environmental and agricultural samples was found to be 95.00-106.50% at various levels. Notably, the developed method was successfully employed for the determination of mesotrione in environmental (pond water, canal water, tap water, and soil) and agricultural (maize grains) samples.

Validated spectrophotometric determination of maduramicin ammonium using three charge transfer complexation reactions

Direct spectrophotometric determination of Maduramicin ammonium (MAD) represents an analytical challenge since it is a weak UV-absorbing and lacking a strong chromophore. This work represents the first spectrophotometric determination of MAD as no direct spectrophotometric or colorimetric determination methods for MAD are available in the literature. The present study illustrates the development of three simple, rapid and inexpensive colorimetric methods for the routine quality control analysis of MAD based on the formation of colored charge transfer complexes with three electron acceptors namely p-chloranilic acid (p-CA), 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and picric acid (PA). The color products of MAD with p-CA, DDQ and PA were measured at 519, 588 and 405nm respectively. The proposed methods were validated in terms of linearity, ranges, precision, accuracy, robustness and limits of detection and quantification. MAD was effectively determined over concentration ranges of 100-1000, 25-250 and 30-150μg/mL using p-CA, DDQ and PA, respectively with good linearity as shown by the values of correlation coefficients not less than 0.9991. The developed methods were successfully implemented in the assay of MAD powder pharmaceutical formulation for veterinary use.

Theoretical study on cyclophane amide molecular receptors and its complexation behavior with TCNQ

Complexation behavior of cyclophane amide molecular receptors towards 7,7,8,8-tetracyanoquinodimethane (TCNQ) studied. TD-B3LYP/6-31 + G(d,p) based density functional theory was employed to investigate the photophysical characteristics of the complexes obtained. Syn isomers of cyclophane amide molecular hosts show preferred conformation over other conformations. Molecular Orbital analysis indicates the electronic structure change, which reflects in the absorption spectra of the cyclophane amide-1@TCNQ, and cyclophane amide-2@TCNQ charge-transfer (CT) complexes. Binding energy studies with B3LYP-D3/6-31 + G (d,p) theory demonstrated that the more effective binding of the pyridine-2,6-dicarboxamide macrocycles than for their isophthalamide analogs. Both the CT complexes show intermolecular bifurcated hydrogen bonding (N-H_(host)···N_(guest)···H-N_(host)) interactions (2.06 to 2.08 Å), and π_(host)···π_(guest) interactions (3.2 to 3.4 Å). Calculated BSSE corrected complexation energy (ΔE) be associated with the formation of the inclusion complexes in the range - 28 to -37 kJ mol^-1, indicating spontaneity of host-guest complex formation in both the cases. From the calculated vibrational spectra of these complexes, the formation of inclusion complexes via N - H_(host)···N_(guest) and π_(host)···π_(guest) intermolecular interactions established by the frequency shift in the N - H vibrations. Mulliken population analysis performed to recognize the CT process and the variation in charges between the free and complex TCNQ molecules suggests the intermolecular charge transfer. This study indicates that these cyclophane amides can be a decent CT complexation host for the guests like TCNQ.

Kinetic and structural insight into a role of the re-face Tyr328 residue of the homodimer type ferredoxin-NADP+ oxidoreductase from Rhodopseudomonas palustris in the reaction with NADP+/NADPH

Among the thioredoxin reductase-type ferredoxin-NAD(P)⁺ oxidoreductase (FNR) family, FNR from photosynthetic purple non‑sulfur bacterium Rhodopseudomonas palustris (RpFNR) is distinctive because the predicted residue on the re-face of the isoalloxazine ring portion of the FAD prosthetic group is a tyrosine. Here, we report the crystal structure of wild type RpFNR and kinetic analyses of the reaction of wild type, and Y328F, Y328H and Y328S mutants with NADP⁺/NADPH using steady state and pre-steady state kinetic approaches. The obtained crystal structure of wild type RpFNR confirmed the presence of Tyr328 on the re-face of the isoalloxazine ring of the FAD prosthetic group through the unique hydrogen bonding of its hydroxyl group. In the steady state assays, the substitution results in the decrease of K_d for NADP⁺ and K_M for NADPH in the diaphorase assay; however, the k_cat values also decreased significantly. In the stopped-flow spectrophotometry, mixing oxidized RpFNRs with NADPH and reduced RpFNRs with NADP⁺ resulted in rapid charge transfer complex formation followed by hydride transfer. The observed rate constants for the hydride transfer in both directions were comparable (>400 s^-1). The substitution did not drastically affect the rate of hydride transfer, but substantially slowed down the subsequent release and re-association of NADP⁺/NADPH in both directions. The obtained results suggest that Tyr328 stabilizes the stacking of C-terminal residues on the isoalloxazine ring portion of the FAD prosthetic group, which impedes the access of NADP⁺/NADPH on the isoalloxazine ring portions, in turn, enhancing the release of the NADP⁺/NADPH and/or reaction with electron transfer proteins.

Ground state charge transfer complex formation of some metalloporphyrins with aromatic solvents: Further theoretical and experimental investigations

In our earlier work (Chem. Phys. Letts. 592 (2014) 149-154), a new broad band was observed in the near infrared region (700-900nm) of the steady state absorption spectra of some metalloporphyrins (zinc tetraphenylporphyrin, zinc octaethylporphyrin and magnesium octaethylporphyrin) in aromatic solvents (chlorobenzene, 1,2-dichlorobenzene, benzonitrile, benzene and toluene) at high concentrations (~10^-4molL^-1). The band was ascribed to be due to ground state charge transfer complexation between solute and solvent molecules. In the present work, density functional theory calculations are carried out to study the possibility of such ground state charge transfer complex formation between zinc tetraphenylporphyrin and four aromatic solvents viz., benzene, toluene, chlorobenzene and benzonitrile with 1:1 and 2:1 solvent-solute stoichiometries. Also, we determined the association constants for the ground state charge transfer complex formation of zinc tetraphenylporphyrin and zinc octaethylporphyrin with two aromatic solvents (benzene and benzonitrile) by Benesi-Hildebrand method.

The key role of glutamate 172 in the mechanism of type II NADH:quinone oxidoreductase of Staphylococcus aureus

Type II NADH:quinone oxidoreductases (NDH-2s) are membrane bound enzymes that deliver electrons to the respiratory chain by oxidation of NADH and reduction of quinones. In this way, these enzymes also contribute to the regeneration of NAD⁺, allowing several metabolic pathways to proceed. As for the other members of the two-Dinucleotide Binding Domains Flavoprotein (tDBDF) superfamily, the enzymatic mechanism of NDH-2s is still little explored and elusive. In this work we addressed the role of the conserved glutamate 172 (E172) residue in the enzymatic mechanism of NDH-2 from Staphylococcus aureus. We aimed to test our earlier hypothesis that E172 plays a key role in proton transfer to allow the protonation of the quinone. For this we performed a complete biochemical characterization of the enzyme's variants E172A, E172Q and E172S. Our steady state kinetic measurements show a clear decrease in the overall reaction rate, and our substrate interaction studies indicate the binding of the two substrates is also affected by these mutations. Interestingly our fast kinetic results show quinone reduction is more affected than NADH oxidation. We have also determined the X-ray crystal structure of the E172S mutant (2.55Ǻ) and compared it with the structure of the wild type (2.32Ǻ). Together these results support our hypothesis for E172 being of central importance in the catalytic mechanism of NDH-2, which may be extended to other members of the tDBDF superfamily.

Synthesis of new chromeno-carbamodithioate derivatives and preliminary evaluation of their antioxidant activity and molecular docking studies

New chromeno carbamodithioates (7a-i), have been synthesized from 2, 3-dimethyl-7-(oxiran-2-ylmethoxy)-4H-chromen-4-one (5), carbondisulphide and commercially available acyclic and cyclic secondary amines in acetonitrile with good to excellent yields. The free radical scavenging activity of novel chromone-carbamodithioate analogues was quantitatively estimated by spectrophotometric method. Whereas, molecular docking studies were performed with the active site of cyclooxygenase-2 to identify hydrogen bonding, hydrophobic and ionic interactions between protein and ligands. The compounds 7g and 7h demonstrated potent antioxidant activity with IC₅₀ of 1.405±0.019mM and 1.382±0.35mM respectively compared to Ascorbic acid.

Kinetics of NADP+/NADPH reduction-oxidation catalyzed by the ferredoxin-NAD(P)+ reductase from the green sulfur bacterium Chlorobaculum tepidum

Ferredoxin-NAD(P)⁺ oxidoreductase (FNR, [EC 1.18.1.2], [EC 1.18.1.3]) from the green sulfur bacterium Chlorobaculum tepidum (CtFNR) is a homodimeric flavoprotein with significant structural homology to bacterial NADPH-thioredoxin reductases. CtFNR homologs have been found in many bacteria, but only in green sulfur bacteria among photoautotrophs. In this work, we examined the reactions of CtFNR with NADP⁺, NADPH, and (4S-²H)-NADPD by stopped-flow spectrophotometry. Mixing CtFNR_ox with NADPH yielded a rapid decrease of the absorbance in flavin band I centered at 460 nm within 1 ms, and then the absorbance further decreased gradually. The magnitude of the decrease increased with increasing NADPH concentration, but even with ~50-fold molar excess NADPH, the absorbance change was only ~45 % of that expected for fully reduced protein. The absorbance in the charge transfer (CT) band centered around 600 nm increased rapidly within 1 ms, then slowly decreased to about 70 % of the maximum. When CtFNR_red was mixed with excess NADP⁺, the absorbance in the flavin band I increased to about 70 % of that of CtFNR_ox with an apparent rate of ~4 s^-1, whereas almost no absorption changes were observed in the CT band. Obtained data suggest that the reaction between CtFNR and NADP⁺/NADPH is reversible, in accordance with its physiological function.

Validated spectrophotometric methods for determination of sodium valproate based on charge transfer complexation reactions

This work presents the development, validation and application of four simple and direct spectrophotometric methods for determination of sodium valproate (VP) through charge transfer complexation reactions. The first method is based on the reaction of the drug with p-chloranilic acid (p-CA) in acetone to give a purple colored product with maximum absorbance at 524nm. The second method depends on the reaction of VP with dichlone (DC) in dimethylformamide forming a reddish orange product measured at 490nm. The third method is based upon the interaction of VP and picric acid (PA) in chloroform resulting in the formation of a yellow complex measured at 415nm. The fourth method involves the formation of a yellow complex peaking at 361nm upon the reaction of the drug with iodine in chloroform. Experimental conditions affecting the color development were studied and optimized. Stoichiometry of the reactions was determined. The proposed spectrophotometric procedures were effectively validated with respect to linearity, ranges, precision, accuracy, specificity, robustness, detection and quantification limits. Calibration curves of the formed color products with p-CA, DC, PA and iodine showed good linear relationships over the concentration ranges 24-144, 40-200, 2-20 and 1-8μg/mL respectively. The proposed methods were successfully applied to the assay of sodium valproate in tablets and oral solution dosage forms with good accuracy and precision. Assay results were statistically compared to a reference pharmacopoeial HPLC method where no significant differences were observed between the proposed methods and reference method.

Studies on the weak interactions and CT complex formations between chloranilic acid, 2,3-dichloro-5,6-dicyano-p-benzoquinone, tetracyanoethylene and papaverine in acetonitrile and their thermodynamic properties, theoretically, spectrophotometrically aided by FTIR

Spectrophotometric, FTIR and theoretical studies of the charge-transfer complexes between mild narcotic drug papaverine and the acceptors chloranilic acid (Cl-A), 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and tetracyanoethylene (TCNE) in acetonitrile, their association constants, thermodynamic (ΔG(0), ΔH(0) and ΔS(0)) and other related properties had been described. Papaverine was found to form colored charge-transfer complexes with Cl-A, DDQ and TCNE in acetonitrile. The absorption maxima of the complexes were 518.5, 584.0 and 464.0 nm for Cl-A complex, DDQ complex, and TCNE complex respectively. The compositions of the papaverine complexes were determined to be 1:1 from Job's method of continuous variation. Solid complexes formed between papaverine and the acceptors were isolated. Comparison of the FTIR spectra of the solid complexes between papaverine and the acceptors and their constituents showed considerable shift in absorption peaks, changes in intensities of the peaks and formation of the new bands on complexation. However, no attempt has been made to purify the complexes and study the detailed spectra both theoretically and experimentally. The energies hν(CT) of the charge-transfer complexes were compared with the theoretical values of hν(CT) of the complexes obtained from HOMO and LUMO of the donor and the acceptors. The reasons for the differences in hνCT values were explained. Density function theory was used for calculation. hν(CT) (experimental) values of the transition energies of the complexes in acetonitrile differed from hν(CT) (theoretical) values. ID(V) value of papaverine was calculated. Charge-transfer complexes were assumed to be partial electrovalent compounds with organic dative ions D(+) and A(-) (in the excited state) and attempts had been made to correlate the energy changes for the formation of the complexes with the energy changes for the formation of electrovalent compounds between M(+) and X(-) ions.

Spectrophotometric investigation of interaction between iodine and pentadentate Schiff base ligands

The interaction between iodine as an electron acceptor (A), and three pentadentate Schiff bases, 1,3-bis(salicylideneamino)-2-propanol (SB1), 1,3-bis(2-hydroxy-1-naphthylideneamino)-2-propanol (SB2), and 1,3-bis[1-(pyridine-2-yl)methylideneamino]-2-propanol (SB3), as electron donor systems (D), was studied spectrophotometrically in methanol at 28 °C. Equilibrium constants K(AD) and molar extinction coefficients ε(AD) of the donor-acceptor complexes (AD) were determined using the modified Benesi-Hildebrand equation in conjunction with the non linear fit analysis. The method shows the formation of 1:1 type complexes as major species in solution. The free energy changes ΔG° and the energy of the charge transfer band E(CT) were also calculated for all complexes. The iodine complex derived from SB2 seems to be more stable than those derived from SB3 and SB1. On the other hand, the ionization potential I(D) of each Schiff base was estimated from the corresponding complex band energy, using an empirical equation. An inverse relationship between I(D) and K(AD) values was found. Blue and red shift observed for the 445 nm band of iodine were also discussed on the basis of theoretical considerations.

Synthesis, growth, spectral, and thermal studies of a new organic molecular charge transfer complex crystal: 3-nitroaniline 4-methyl benzene sulfonate

A new organic intermolecular charge transfer complex 3-nitroaniline 4-methyl benzene sulfonate (NATS) has been successfully synthesized and good optical quality single crystals grown by slow solvent evaporation solution growth technique at room temperature using methanol as the solvent. The (1)H and (13)C NMR spectra were recorded to establish the molecular structure of the title complex. The crystal structure of NATS has been determined by single crystal XRD analysis and it belongs to orthorhombic crystal system with space group Pbca. Fourier transform infrared (FT-IR) spectral study has been carried out to confirm the presence of various functional groups present in the complex. Electronic absorption spectrum was recorded to find the prevalent charge transfer activity in the complex. The UV-Vis-NIR transmission spectrum was recorded in the range 200-2500 nm, to find the optical transmittance window and lower cut off wavelength of the title crystal. The thermal stability of the title complex crystal was studied by using thermo-gravimetric and differential thermal analyses and found that the compound is stable up to 215 °C.

Synthesis, growth and spectroscopic investigation of an organic molecular charge transfer crystal: 8-hydroxy quinolinium 4-nitrobenzoate 4-nitrobenzoic acid

An organic intermolecular charge transfer complex salt of 8-Hydroxy quinolinium 4-nitrobenzoate 4-nitrobenzoic acid (OPNB) has been synthesized. Single crystals of OPNB were grown by slow solvent evaporation solution growth technique at room temperature. The (1)H and (13)C NMR spectra were recorded to confirm the molecular structure of the complex salt. The crystal structure of OPNB has been determined by single crystal XRD analysis and it belongs to triclinic crystal system with space group P-1. Fourier transform infrared (FT-IR) spectral study has been carried out to identify the various functional groups present. The UV-Vis-NIR transmittance spectrum was recorded in the range 200-2500nm, to find the suitability of the single crystal for various optical applications. The thermal stability of the crystal was investigated using thermogravimetric (TG) and differential thermal analyses (DTA).