Determination of pKa values of active pharmaceutical ingredients

Low-frequency Raman spectroscopy as a new tool for understanding the behaviour of ionisable compounds in dispersed mesophases

HYPOTHESIS

Low-frequency Raman (LFR) spectroscopy is proposed as a novel non-destructive methodology to probe pH-related phase transitions in self-assembled lipid particles. In this case, dispersed lipid mesophases were composed of ionisable oleic acid (OA) or nicergoline (NG) in monoolein (MO). The sensitivity of LFR spectroscopy to low-energy intermolecular vibrations was hypothesised to be due to structural transformation in ionisable dispersed mesophases upon changes in pH.

METHOD/EXPERIMENT

Phase transitions of dispersed mesophases of MO mixed with OA or NG were induced by varying the pH of the aqueous buffer. The structural transformations were studied using LFR spectroscopy, recording the corresponding changes in the vibrational density of states (VDOS) upon changes in pH and analysed using principal component analysis (PCA). The results were correlated with structural transitions observed in simultaneous small-angle X-ray scattering (SAXS) measurements.

FINDINGS

The intensity of the VDOS signal of MO + OA mesophases scaled with phase-specific transformations, such as from the bi-continuous cubic Im3¯m phase (V2) or lamellar-based vesicles to the reversed hexagonal p6m phase (H2). For NG subtle changes in the lattice parameter of the V2 phase of NG + MO mesophases coincided with the apparent dissociation constant (pKaapp) of NG, however, slight variations between the pKaapp of NG determined by equilibrated samples analysed using SAXS and non-equilibrated samples analysed using LFR suggest structural hysteresis upon changes in the protonation state of NG. This approach offers an efficient method for studying the phase behaviour of lipid systems under varying pH and potentially other conditions such as temperature.

Biofilm formation and microbial interactions in moving bed-biofilm reactors treating wastewater containing pharmaceuticals and personal care products: A review

The risk of pharmaceuticals and personal care products (PPCPs) has been paid more attention after the outbreak of COVID-19, threatening the ecology and human health resulted from the massive use of drugs and disinfectants. Wastewater treatment plants are considered the final stop to restrict PPCPs from wide spreading into the environment, but the performance of conventional treatment is limited due to their concentrations and characteristics. Previous studies have shown the unreplaceable capability of moving bed-biofilm reactor (MBBR) as a cost-effective method with layered microbial structure for treating wastewater even with toxic compounds. The biofilm community and microbial interactions are essential for the MBBR process in completely degrading or converting types of PPCPs to secondary metabolites, which still need further investigation. This review starts with discussing the initiation of MBBR formation and its influencing parameters according to the research on MBBRs in the recent years. Then the efficiency of MBBRs and the response of biofilm after exposure to PPCPs are further addressed, followed by the bottlenecks proposed in this field. Some critical approaches are also recommended for mitigating the deficiencies of MBBRs based on the recently published publications to reduce the environmental risk of PPCPs. Finally, this review provides fundamental information on PPCPs removal by MBBRs with the main focus on microbial interactions, promoting the MBBRs to practical application in the real world of wastewater treatment.

Ecotoxicological Effects of Microplastics Combined With Antibiotics in the Aquatic Environment: Recent Developments and Prospects

Both microplastics and antibiotics are commonly found contaminants in aquatic ecosystems. Microplastics have the ability to absorb antibiotic pollutants in water, but the specific adsorption behavior and mechanism are not fully understood, particularly in relation to the impact of microplastics on toxicity in aquatic environments. We review the interaction, mechanism, and transport of microplastics and antibiotics in water environments, with a focus on the main physical characteristics and environmental factors affecting adsorption behavior in water. We also analyze the effects of microplastic carriers on antibiotic transport and long-distance transport in the water environment. The toxic effects of microplastics combined with antibiotics on aquatic organisms are systematically explained, as well as the effect of the adsorption behavior of microplastics on the spread of antibiotic resistance genes. Finally, the scientific knowledge gap and future research directions related to the interactions between microplastics and antibiotics in the water environment are summarized to provide basic information for preventing and treating environmental risks. Environ Toxicol Chem 2024;43:1950-1961. © 2024 SETAC.

A novel ratiometric electrochemical sensing platform combined with molecularly imprinted polymer and Fe-MOF-NH2/CNTs-NH2/MXene composite for efficient detection of ofloxacin

BACKGROUND

Ofloxacin (OFL) is often abused in medicine and animal husbandry, which poses a great threat to human health and ecological environment. Therefore, it is necessary to establish efficient method to detect OFL. Electrochemical sensor has attracted widespread attention due to the advantages of low cost and fast response. However, most electrochemical sensors usually use one response signal to detect the target, which makes it sensitive to the variable background noise in the complex environment, resulting in low robustness and selectivity. The ratio detection mode and employing molecularly imprinted polymer (MIP) are two strategies to solve these problems.

RESULTS

A novel molecular imprinting polymer-ratiometric electrochemical sensor (MIP-RECS) based on Fe-MOF-NH2/CNTs-NH2/MXene composite was prepared for the rapid and sensitive detection of OFL. The positively charged Fe-MOF-NH2 and CNTs-NH2 as interlayer spacers were introduced into the negatively charged MXene through a simple electrostatic self-assembly technique, which effectively prevented the agglomeration of MXene and increased the electrocatalytic activity. A glass carbon electrode was modified by the composite and a MIP film was electropolymerized on it using o-phenylenediamine and β-cyclodextrin as bifunctional monomers and OFL as template. Then a MIP-RECS was designed by adding dopamine (DA) into the electrolyte solution as internal reference, and OFL was quantified by the response current ratio of OFL to DA. The current ratio and the concentration of OFL displayed a satisfying linear relationship in the range of 0.1 μM-100 μM, with a limit of detection (LOD) of 13.2 nM.

SIGNIFICANCE

Combining molecular imprinting strategy and ratio strategy, the MIP-RECS has impressive selectivity compared with the non-imprinted polymer-RECS, and has better repeatability and reproducibility than non-ratiometric sensor. The MIP-RECS has high sensitivity and accuracy, which was applied for the detection of OFL in four different brands of milk and was verified by HPLC method with satisfactory results.

Oxidation and mineralization rates of harmful organic chemicals in hydroxyl radical induced reactions

In most of advanced oxidation processes (AOPs) used to destroy harmful organic chemicals in water/wastewater hydroxyl radical (•OH) reactions oxidize (increasing the oxygen/carbon ratio in the molecules) and mineralize (transforming them to inorganic molecules, H2O, CO2, etc.) these contaminants. In this paper, we used the radiolysis of water to produce •OH and characterised the rate of oxidation and mineralization by the dose dependences of the Chemical Oxygen Demand (COD) and Total Organic Carbon (TOC) content values. Analysis of the dose dependences for 34 harmful organic compounds showed large differences in the oxidation and mineralization rates and these parameters are characteristic to the given group of chemicals. E.g., the rate of oxidation is relatively low for fluoroquinolone antibiotics; it is high for β-blocker medicines. Mineralization rates are low for both fluoroquinolones and β-blockers. The one-electron-oxidant •OH in most cases induces two - four-electron-oxidations. Most of the degradation takes place gradually, through several stable molecule intermediates. However, based on the results it is likely, that some part of the oxidation and mineralization takes place parallel. The organic radicals formed in •OH reactions react with several O2 molecules and release several inorganic fragments during the radical life cycle.

Comparative electrochemical study of veterinary drug danofloxacin at glassy carbon electrode and electrified liquid-liquid interface

This work compares the electroanalytical performance of two electroanalytical systems based on (1) the glassy carbon electrode (GCE), and (2) the electrified liquid-liquid interface (eLLI), for the detection of fluoroquinolone antibiotic-danofloxacin (DANO). Our aim was to define the optimal conditions to detect the chosen analyte with two employed systems, extract a number of electroanalytical parameters, study the mechanism of the charge transfer reactions (oxidation at GCE and ion transfer across the eLLI), and to provide physicochemical constants for DANO. Detection of the chosen analyte was also performed in the spiked milk samples. To the best of our knowledge, this is the first work that directly compares the electroanalytical parameters obtained with solid electrode (in this case GCE) and eLLI. We have found that for DANO the latter provides better electroanalytical parameters (lower LOD and LOQ) as well as good selectivity when the milk was analyzed.

Determination of physicochemical parameters of (bio)molecules and (bio)particles by capillary electromigration methods

The review provides an overview of recent developments and applications of capillary electromigration (CE) methods for the determination of important physicochemical parameters of various (bio)molecules and (bio)particles. These parameters include actual and limiting (absolute) ionic mobilities, effective electrophoretic mobilities, effective charges, isoelectric points, electrokinetic potentials, hydrodynamic radii, diffusion coefficients, relative molecular masses, acidity (ionization) constants, binding constants and stoichiometry of (bio)molecular complexes, changes of Gibbs free energy, enthalpy and entropy and rate constants of chemical reactions and interactions, retention factors and partition and distribution coefficients. For the determination of these parameters, the following CE methods are employed: zone electrophoresis in a free solution or in sieving media, isotachophoresis, isoelectric focusing, affinity electrophoresis, electrokinetic chromatography, and electrochromatography. In the individual sections, the procedures for the determination of the above parameters by the particular CE methods are described.

Removal of sarafloxacin from aqueous solution through Ni/Al-layered double hydroxide@ZIF-8

In recent years, excessive amounts of drugs such as antibiotics have been used to combat COVID-19 and newly discovered viruses. This has led to the production and release of significant amounts of drugs and their metabolites as toxic pollutants in aquatic systems. Therefore, pharmaceutical wastes must be removed efficiently before entering the environment and entering water sources. In this research, Ni/Al-LDH@ZIF-8 nanocomposite was synthesized from layered double hydroxides and metal-organic frameworks and used to remove the antibiotic sarafloxacin (SRF) in the aqueous medium. The work aimed to develop the performance and combine the features of the adsorbent compounds such as high surface area, adjustable porosity, and low-density structure. Different methods implemented to analyze the nanocomposite, such as Fourier transform infrared spectroscopy, X-ray diffraction, field emission scanning electron microscopy, and energy dispersive X-ray spectroscopy. The experiment utilized the central composite design to evaluate statistics and the response level method to optimize the factors affecting the absorption process. The initial concentration of SRF, adsorbent dose, pH, and contact time were considered in this experiment. The results showed an increase in the removal efficiency of SRF to 97%. Statistical studies showed that the optimal adsorption conditions are as follows: initial concentration of SRF 40 mg·L-1, pH 6.3, adsorbent dose of Ni/Al-LDH@ZIF-8 49 mg, and contact time of 44 min. According to the model of isotherms parameters, the adsorption process is more consistence with the Freundlich model with the absorption capacity of 79.7 mg·g-1. The pseudo-second-order model described the adsorption kinetics data.

Graphical abstract

Experimental data and modeling of sulfadiazine adsorption onto raw and modified clays from Tunisia

In recent years, the increasing detection of emerging pollutants (particularly antibiotics, such as sulfonamides) in agricultural soils and water bodies has raised growing concern about related environmental and health problems. In the current research, sulfadiazine (SDZ) adsorption was studied for three raw and chemically modified clays. The experiments were carried out for increasing doses of the antibiotic (0, 1, 5, 10, 20, and 40 μmol L-1) at ambient temperature and natural pH with a contact time of 24 h. The eventual fitting to Freundlich, Langmuir and Linear adsorption models, as well as residual concentrations of antibiotics after adsorption, was assessed. The results obtained showed that one of the clays (HJ1) adsorbed more SDZ (reaching 99.9 % when 40 μmol L-1 of SDZ were added) than the other clay materials, followed by the acid-activated AM clay (which reached 99.4 % for the same SDZ concentration added). The adsorption of SDZ followed a linear adsorption isotherm, suggesting that hydrophobic interactions, rather than cation exchange, played a significant role in SDZ retention. Concerning the adsorption data, the best adjustment corresponded to the Freundlich model. The highest Freundlich KF scores were obtained for the AM acid-treated and raw HJ1 clays (606.051 and 312.969 Ln μmol1-n kg-1, respectively). The Freundlich n parameter ranged between 0.047 and 1.506. Regarding desorption, the highest value corresponded to the AM clay, being generally <10 % for raw clays, <8 % for base-activated clays, and <6 % for acid-activated clays. Chemical modifications contributed to improve the adsorption capacity of the AM clay, especially when the highest concentrations of the antibiotic were added. The results of this research can be considered relevant as regard environmental and public health assessment since they estimate the feasibility of three Tunisian clays in SDZ removal from aqueous solutions.

Protecting the Last Line of Defense: Analytical Approaches for Sample Preparation and Determination of the Reserve Group of Antibiotics in the Environment

Drug resistance in microorganisms is a serious threat to life and health due to the limited number of antibiotics that show efficacy in treating infections and the difficulty in discovering new compounds with antibacterial activity. To address this issue, the World Health Organization created the AWaRe classification, a tool to support global and national antimicrobial stewardship programs. The AWaRe list categorizes antimicrobials into three groups - Access, Watch, and Reserve - according to their intended use. The Reserve group comprises "last resort" medicines used solely for treating infections caused by bacterial strains that are resistant to other treatments. It is therefore necessary to protect them, not only by using them as prudently as possible in humans and animals, but also by monitoring their subsequent fate. Unmetabolized antibiotics enter the environment through hospital and municipal wastewater or from manure, subsequently contaminating bodies of water and soils, thus contributing to the emergence and spread of antibiotic resistance. This article presents a review of determination methods for the Reserve group of antimicrobials in water, wastewater, and manure. Procedures for extracting and determining these substances in environmental samples are described, showing the limited research available, which is typically on a local level.

Tetracycline adsorption/desorption by raw and activated Tunisian clays

Clay-based adsorbents have applications in environmental remediation, particularly in the removal of emerging pollutants such as antibiotics. Taking that into account, we studied the adsorption/desorption process of tetracycline (TC) using three raw and acid- or base-activated clays (AM, HJ1 and HJ2) collected, respectively, from Aleg (Mazzouna), El Haria (Jebess, Maknessy), and Chouabine (Jebess, Maknessy) formations, located in the Maknessy-Mazzouna basin, center-western of Tunisia. The main physicochemical properties of the clays were determined using standard procedures, where the studied clays presented a basic pH (8.39-9.08) and a high electrical conductivity (446-495 dS m-1). Their organic matter contents were also high (14-20%), as well as the values of the effective cation exchange capacity (80.65-97.45 cmolckg-1). In the exchange complex, the predominant cations were Na and Ca, in the case of clays HJ1 and AM, while Mg and Ca were dominant in the HJ2 clay. The sorption experimental setup consisted in performing batch tests, using 0.5 g of each clay sample, adding the selected TC concentrations, then carrying out quantification of the antibiotic by means of HPL-UV equipment. Raw clays showed high adsorption potential for TC (close to 100%) and very low desorption (generally less than 5%). This high adsorption capacity was also present in the clays after being activated with acid or base, allowing them to adsorb TC in a rather irreversible way for a wide range of pH (3.3-10) and electrical conductivity values (3.03-495 dS m-1). Adsorption experimental data were studied as regards their fitting to the Freundlich, Langmuir, Linear and Sips isotherms, being the Sips model the most appropriate to explain the adsorption of TC in these clays (natural or activated). These results could help to improve the overall knowledge on the application of new low-cost methods, using clay based adsorbents, to reduce risks due to emerging pollutants (and specifically TC) affecting the environment.

Ecofriendly single-step HPLC and TLC methods for concurrent analysis of ternary antifungal mixture in their pharmaceutical products

Two accurate, sensitive, and selective methods for simultaneous determination of miconazole nitrate (MIC), nystatin (NYS), and metronidazole (MET) in pure state or drug product were established and verified. First, RP-HPLC-DAD was designed. Separation was accomplished using a ZOBRAX Eclipse Plus RP-C8 column that was running under an isocratic elution of methanol: 0.05% aqueous solution of sodium dodecyl sulphate (40: 60 v/v), with a flow rate that was regulated at 0.8 mL/min. The column temperature was adjusted at 25 °C and diode array detector was monitored at 220 nm. The linearity range of the proposed method was achieved at the concentration of 5-50, 4-50, and 4-40 µg/mL and the attained retention time for the studied drugs was 2.52, 3.52 and 4.99 min for MIC, NYS, and MET, correspondingly. Second, a TLC-densitometric approach was used to resolve the three compounds. Resolution of the three cited drugs was carried out using TLC aluminum plates pre-coated with 0.25 mm silica gel 60 F254. A developing solvent comprised ethyl acetate: toluene: methanol: triethyl amine: formic acid (3: 1: 7: 0.3: 0.1 by volume) (pH = 5.5) was utilized and scanning of the resolved bands at 215 nm. Linearity of the developed TLC method was evaluated and evident to be 0.4-2, 0.4-2.2, and 0.4-2 μg/band for MIC, NYS, and MET, in that order. The suggested chromatographic methods were verified according to ICH directives. The findings of the developed chromatographic procedures were statistically compared with the results of the reported ones using student's t-test and F-test. Furthermore, two green assessment tools evaluated the indicated methods' level of greenness (GAPI and AGREE).

Interaction between antibiotics and microplastics: Recent advances and perspective

Both microplastics and antibiotics are emerging pollutants, which are ubiquitous in aquatic environments. With small size, high specific surface area, and attached biofilm, microplastics are capable of adsorbing or biodegrading antibiotic pollutants across aquatic environments. However, the interactions between them are poorly understood, especially factors that affect microplastics' chemical vector effects and the mechanisms driving these interactions. In this review, the properties of microplastics and their interaction behavior and mechanisms towards antibiotics were comprehensively summarized. Particularly, the impact of weathering properties of microplastics and the growth of attached biofilm was highlighted. We concluded that compared with virgin microplastics, aged microplastics usually adsorb more types and quantities of antibiotics from aquatic environments, whilst the attached biofilm could further enhance the adsorption capacities and biodegrade some antibiotics. This review can answer the knowledge gaps of the interaction between microplastics and antibiotics (or other pollutants), offer basic information for evaluating their combined toxicity, provide insights into the distribution of both emerging pollutants in the global water chemical cycle, and inform measures to remove microplastic-antibiotic pollution.

Advantages of Multidimensional Biasing in Accelerated Dynamics: Application to the Calculation of the Acid pKa for Acetic Acid

The use of accelerated sampling methods such as metadynamics has shown a significant advantage in calculations that involve infrequent events, which would otherwise require sampling a prohibitive number of configurations to determine the difference in free energies between two or more chemically distinct states such as in the calculation of acid dissociation constants Ka. In this case, the most common method is to bias the system via a single collective variable (CV) representing the coordination number of the proton donor group, which yields results in reasonable agreement with experiments. Here we study the deprotonation of acetic acid using the reactive force field ReaxFF and observe a significant dependence of Ka on the simulation box size when biasing only the coordination number CV, which is due to incomplete sampling of the deprotonated state for small simulation systems and inefficient sampling for larger ones. Incorporating a second CV representing the distance between the H3O+ cation and the acetate anion results in substantially more efficient sampling, both accelerating the dynamics and virtually eliminating the computational box size dependence.

Flavonoids in the Spotlight: Bridging the Gap between Physicochemical Properties and Formulation Strategies

Flavonoids are hydroxylated polyphenols that are widely distributed in plants with diverse health benefits. Despite their popularity, the bioavailability of flavonoids is often overlooked, impacting their efficacy and the comparison of products. The study discusses the bioavailability-related physicochemical properties of flavonoids, with a focus on the poorly soluble compounds commonly found in dietary supplements and herbal products. This review sums up the values of pKa, log P, solubility, permeability, and melting temperature of flavonoids. Experimental and calculated data were compiled for various flavonoid subclasses, revealing variations in their physicochemical properties. The investigation highlights the challenges posed by poorly soluble flavonoids and underscores the need for enabling formulation approaches to enhance their bioavailability and therapeutic potential. Compared to aglycones, flavonoid glycosides (with sugar moieties) tend to be more hydrophilic. Most of the reviewed aglycones and glycosides exhibit relatively low log P and high melting points, making them "brick dust" candidates. To improve solubility and absorption, strategies like size reduction, the potential use of solid dispersions and carriers, as well as lipid-based formulations have been discussed.

Kinetic Barriers to Disproportionation of Salts of Weakly Basic Drugs

Disproportionation is a major issue in formulations containing salts of weakly basic drugs. Despite considerable interest in risk assessment approaches for disproportionation, the prediction of salt-to-base conversion remains challenging. Recent studies have highlighted several confounding factors other than pHmax that appear to play an important role in salt disproportionation and have suggested that kinetic barriers need to be considered in addition to the thermodynamic driving force when assessing the risk of a salt to undergo conversion to parent free base. Herein, we describe the concurrent application of in situ Raman spectroscopy and pH monitoring to investigate the disproportionation kinetics of three model salts, pioglitazone hydrochloride, sorafenib tosylate, and atazanavir sulfate, in aqueous slurries. We found that even for favorable thermodynamic conditions (i.e., pH ≫ pHmax), disproportionation kinetics of the salts were very different despite each system having a similar pHmax. The importance of free base nucleation kinetics was highlighted by the observation that the disproportionation conversion time in the slurries showed the same trend as the free base nucleation induction time. Pioglitazone hydrochloride, with a free base induction time of <1 min, rapidly converted to the free base in slurry experiments. In contrast, atazanavir sulfate, where the free base induction time was much longer, took several hours to undergo disproportionation in the slurry for pH ≫ pHmax. Additionally, we altered an established thermodynamically based modeling framework to account for kinetic effects (representing the nucleation kinetic barrier) to estimate the solid-state stability of salt formulations. In conclusion, a solution-based thermodynamic model is mechanistically appropriate to predict salt disproportionation in a solid-state formulation, when kinetic barriers are also taken into consideration.

Determination of the microscopic acid dissociation constant of piperacillin and identification of dissociated molecular forms

For amphoteric ß-lactam antibiotics, the acid dissociation constant (pK a) is a fundamental parameter to characterize physicochemical and biochemical properties of antibiotics and to predict persistence and removal of drugs. pK a of piperacillin (PIP) is determined by potentiometric titration with a glass electrode. Electrospray ionization mass spectrometry (ESI-MS) is creatively applied to verify the reasonable pK a value at every dissociation step. Two microscopic pK a values (3.37 ± 0.06 and 8.96 ± 0.10) are identified and attributed to the direct dissociation of the carboxylic acid functional group and one secondary amide group, respectively. Different from other ß-lactam antibiotics, PIP presents a dissociation pattern where direct dissociation is involved instead of protonation dissociation. Moreover, the degradation tendency of PIP in an alkaline solution may alter the dissociation pattern or dismiss the corresponding pK a of the amphoteric ß-lactam antibiotics. This work offers a reliable determination of the acid dissociation constant of PIP and a clear interpretation of the effect of stability of antibiotics on the dissociation process.

Rate constants for the reactions of chloride monoxide radical (ClO•) and organic molecules of environmental interest

ClO^• plays a key role in the UV/chlorine process besides Cl^•, Cl2^• - , and ^•OH. In many experiments, ClO^• proved to be the main reactant that destroyed the organic pollutants in advanced oxidation process. About 200 rate constants of ClO^• reactions were collected from the literature, grouped together according to the chemical structure, and the molecular structure dependencies were evaluated. In most experiments, ClO^• was produced by the photolytic reaction of HClO/ClO-. For a few compounds, the rate constants were determined by the absolute method, pulse radiolysis. Most values were obtained in steady-state experiments by competitive technique or by complex kinetic calculations after measuring the pollutant degradation in the UV/chlorine process. About 30% of the listed rate constant values were derived in quantum chemical or in structure-reactivity (QSAR) calculations. The values show at least six orders of magnitude variations with the molecular structure. Molecules having electron-rich parts, e.g., phenol/phenolate, amine, or sulfite group have high rate constants in the range of 10⁸-10⁹ mol^-1 dm³ s^-1. ClO^• is inactive in reactions with saturated molecules, alcohols, or simple aromatic molecules.

A systematic review of various pKa determination techniques

The pK_a values of functional groups is crucial in determining the pharmacokinetic properties of a drug, affecting its absorption and thus bioavailability. This physicochemical property is also vital for the designing of drug excipients and vehicles. There are currently 13 known methods of determining a pK_a value, namely: potentiometric titration, spectrometry, fluorometry, NMR, HPLC, conductometry, electrophoresis, voltammetry, solubility, partition coefficient, calorimetry, computational, and surface tension. Some of these techniques are more widely utilized and well-established compared to others, with each having their inherent advantages and disadvantages. This review discusses each of the aforementioned techniques with emphasis on their pros and cons.

Study of the Influence of the Wastewater Matrix in the Adsorption of Three Pharmaceuticals by Powdered Activated Carbon

The use of powdered activated carbon (PAC) as an absorbent has become a promising option to upgrade wastewater treatment plants (WWTPs) that were not designed to remove pharmaceuticals. However, PAC adsorption mechanisms are not yet fully understood, especially with regard to the nature of the wastewater. In this study, we tested the adsorption of three pharmaceuticals, namely diclofenac, sulfamethoxazole and trimethoprim, onto PAC under four different water matrices: ultra-pure water, humic acid solution, effluent and mixed liquor from a real WWTP. The adsorption affinity was defined primarily by the pharmaceutical physicochemical properties (charge and hydrophobicity), with better results obtained for trimethoprim, followed by diclofenac and sulfamethoxazole. In ultra-pure water, the results show that all pharmaceuticals followed pseudo-second order kinetics, and they were limited by a boundary layer effect on the surface of the adsorbent. Depending on the water matrix and compound, the PAC capacity and the adsorption process varied accordingly. The higher adsorption capacity was observed for diclofenac and sulfamethoxazole in humic acid solution (Langmuir isotherm, R² > 0.98), whereas better results were obtained for trimethoprim in the WWTP effluent. Adsorption in mixed liquor (Freundlich isotherm, R² > 0.94) was limited, presumably due to its complex nature and the presence of suspended solids.

Abiotic transformations of sulfamethoxazole by hydroxylamine, nitrite and nitric oxide during wastewater treatment: Kinetics, mechanisms and pH effects

Hydroxylamine (NH₂OH), nitrite (NO₂^-) and nitric oxide (NO), intermediates enzymatically formed during biological nitrogen removal processes, can engage in chemical (abiotic) transformations of antibiotics. This study determined the kinetics, mechanisms and pathways of abiotic transformations of the antibiotic sulfamethoxazole (SMX) by NH₂OH, NO₂^- and NO in a series of batch tests under different pH and oxygen conditions. While NH₂OH was not able to directly transform SMX, NO₂^- (with HNO₂ as the actual reactant) and NO can chemically transform SMX primarily through hydroxylation, nitration, deamination, nitrosation, cleavage of S-N, N-C and C-S bonds, and coupling reactions. There were substantial overlaps in transformation product formations during abiotic transformations by HNO₂^- and NO. The second order rate constants of SMX with NO₂^- and NO were determined in the range of 1.5 × 10^-1 - 4.8 × 10³ M^-1 s^-1 and 1.0 × 10² - 3.1 × 10⁴ M^-1 s^-1, respectively, under varying pH (4 - 9) and anoxic or oxic conditions. Acidic pH significantly enhanced abiotic transformation kinetics, and facilitated nitration, nitrosation, and cleavage of S-N and N-C bonds. The findings advance our understanding of the fate of antibiotics during biological nitrogen removal, and highlight the role of enzymatically formed reactive nitrogen species in the antibiotic degradation.

Azithromycin Adsorption onto Different Soils

The antibiotic azithromycin (AZM) is one of the most persistent in the environment, with potential to cause serious health and environmental problems. As some polluting discharges containing this antibiotic can reach the soil, it is clearly relevant determining the ability of soils with different characteristics to retain it. In this research, AZM adsorption and desorption were studied for a variety of soils, using batch-type experiments. The results show that, at low doses of antibiotic added (less than or equal to 50 µmol L−1), the adsorption always reached 100%, while when higher concentrations were added (between 200 and 600 µmol L−1) the highest adsorption corresponded to soils with higher pH values. Adsorption data were fitted to the Linear, Langmuir and Freundlich models, with the latter showing the best fit, in view of the determination coefficient. No desorption was detected, indicating that AZM is strongly adsorbed to the soils evaluated, suggesting that the risks of environmental problems due to this contaminant are minimized for these edaphic media. These results can be considered relevant with respect to risk assessment and possible programming of measures aimed at controlling environmental contamination by emerging contaminants, especially from the group of antibiotics, and in particular from AZM.

Tetracycline degradation by nonthermal plasma: removal efficiency, degradation pathway, and toxicity evaluation

Tetracyclines (TCs) are often discussed as one of the emerging contaminants detected in water matrices and studied for their persistence towards conventional water treatment technologies. In this work, the treatment of TC in aqueous solutions with nonthermal plasma gliding arc process was investigated. The degradation efficiency of TC was studied along with the effect of initial concentration, working gas, pH, and the presence of a radical scavenger. The generation of reactive oxidative species was characterized by the quantification of radical hydroxyl, hydrogen peroxide, ozone, nitrite, and nitrate. Mineralization efficiency was examined by assessment of Total organic carbon evolution. Experimental results have shown that the gliding arc plasma is effective for the treatment of TC. At an initial concentration of 5 mg/L: degradation rates of 94.95% and 60.45% were achieved, while mineralization rates were 81.3% and 57.34% under O₂ and air plasma, respectively. O₂ plasma exhibited an immense potential for the generation of reactive oxygen species. Meanwhile, air plasma showed better degradation performance in the presence of a radical scavenger. Moreover, degradation products were identified by mass spectroscopy analysis and degradation pathway was proposed. The gliding arc process proposed in this work is promising for the removal of TC antibiotics.

Influence of pH on the adsorption-desorption of doxycycline, enrofloxacin, and sulfamethoxypyridazine in soils with variable surface charge

In this research, the adsorption/desorption of the antibiotics doxycycline (DC), enrofloxacin (ENR), and sulfamethoxypyradazine (SMP) was studied in 6 agricultural soils with predominance of variable charge, both before and after removing organic matter by calcination. DC adsorption was high at acidic pH, and decreased at pH values above 8. Removal of organic matter with calcination caused just a slight decrease in adsorption, and even in some soils adsorption was similar to that in non-calcined samples. The adsorption coefficients (K_d) were higher for the DC^- species compared to DC⁺, DC⁰ and DC^2-. Regarding DC desorption, the values were very low throughout the pH range covered in the study (2-12), both in the calcined samples and in those not subjected to calcination. ENR showed a similar behavior to DC regarding the effect of pH, since ENR adsorption also decreased at basic pH, but the effect of removing organic matter was different, as it caused a clear decrease in ENR adsorption. The species with the highest K_d was in this case ENR⁰, although ENR⁺ is also quantitatively important as regards K_d value in calcined samples. For this antibiotic, no differences in desorption were observed between calcined and non-calcined samples. Finally, SMP adsorption also decreased as pH increased, and, in addition, similarly to what happened with ENR, in general, there was a strong decrease in SMP adsorption when organic matter was removed. The species with the highest K_d in this case was SMP⁺ in non-calcined samples, but SMP⁰ and SMP^- become more relevant in calcined samples. The percentages of SMP desorption were higher than those for the other two antibiotics, and an increase occurs at intermediate pH values, being higher for calcined samples. These results can be considered relevant in terms of increasing the knowledge as regards the possible evolution and fate of the three antibiotics studied. Specifically, for different pH conditions and with different organic matter contents, when they reach soils and other environmental compartments after being discharged as contaminants. This could have important repercussions on public health and the overall environment.

Molar absorption coefficients and acid dissociation constants for fluoroquinolone, sulfonamide, and tetracycline antibiotics of environmental concern

Antibiotics are priority contaminants of emerging concern due to their pseudo-persistence in the environment and contribution to the development of antimicrobial resistance. In solution, antibiotics undergo (de)protonation reactions that affect their UV absorbance and, therefore, photolytic fate in natural and engineered systems. This study employed enhanced spectrophotometric methods to determine the acid dissociation constants (as pK_a values) and molar absorption coefficients for 12 fluoroquinolone, 9 sulfonamide, and 7 tetracycline antibiotics of environmental relevance. Molar absorption coefficient heatmaps were generated for all 28 antibiotics at 200-500 nm and pH 1.8-12.2. The data in the heatmaps were deconvoluted to calculate pK_a values and specific molar absorption coefficients at each wavelength. All antibiotics had at least one pK_a value in the environmentally relevant range of 5.5-8.5, and pK_a values were reported for methacycline, moxifloxacin, nadifloxacin, rolitetracycline, sulfadoxine, and sulfapyridine for the first time. Deprotonation of the carboxylic acid associated with pK_a,1 (5.5-6.7) exerted the strongest effects on the UV absorbance of fluoroquinolones. For tetracyclines, deprotonation of the tertiary amine at pK_a,3 (7.8-10.2) was responsible for major shifts in UV absorbance. Although sulfonamides have conserved pK_a sites, no general trends were observed for the molar absorption coefficients. The structural similarity of fluoroquinolones and tetracyclines supported the potential for a class-based approach to identifying molar absorbance as a function of pH. Overall, the reported pK_a values and specific molar absorption coefficients will serve as important resources for future studies on antibiotic fate in natural and engineered systems.

Rate constants of chlorine atom reactions with organic molecules in aqueous solutions, an overview

Rate constants of chlorine atom (Cl^•) reactions (k_Cl•) determined using a large variation of experimental methods, including transient measurements, steady-state and computation techniques, were collected from the literature and were discussed together with the reaction mechanisms. The k_Cl• values are generally in the 10⁸-10⁹ mol^-1 dm³ s^-1 range when the basic reaction between the Cl^• and the target molecule is H-atom abstraction. When Cl^• addition to double bonds dominates the interaction, the k_Cl• values are in the 1 × 10⁹-2 × 10¹⁰ mol^-1 dm³ s^-1 range. In the k_Cl• = 1 × 10¹⁰-4 × 10¹⁰ mol^-1 dm³ s^-1 range, single-electron-transfer reactions may also contribute to the mechanism. The Cl^• reactions with organic molecules in many respects are similar to those of ^•OH, albeit Cl^• seems to be less selective as ^•OH. However, there is an important difference, as opposed to Cl^• in the case of ^•OH single-electron-transfer reactions have minor importance. The uncertainty of Cl^• rate constant determinations is much higher than those of ^•OH. Since Cl^• reactions play very important role in the emerging UV/chlorine water purification technology, some standardization of the rate constant measuring techniques and more k_Cl• measurements are recommended.

Recognition of quinolone antibiotics by the multidrug efflux transporter MexB of Pseudomonas aeruginosa

The drug/proton antiporter MexB is the engine of the major efflux pump MexAB-OprM in Pseudomonas aeruginosa. This protein is known to transport a large variety of compounds, including antibiotics, thus conferring a multi-drug resistance phenotype. Due to the difficulty of producing co-crystals, only two X-ray structures of MexB in a complex with ligands are available to date, and mechanistic aspects are largely hypothesized based on the body of data collected for the homologous protein AcrB of Escherichia coli. In particular, a recent study (Ornik-Cha, Wilhelm, Kobylka et al., Nat. Commun., 2021, 12, 6919) reported a co-crystal structure of AcrB in a complex with levofloxacin, an antibiotic belonging to the important class of (fluoro)-quinolones. In this work, we performed a systematic ensemble docking campaign coupled to the cluster analysis and molecular-mechanics optimization of docking poses to study the interaction between 36 quinolone antibiotics and MexB. We additionally investigated surface complementarity between each molecule and the transporter and thoroughly assessed the computational protocol adopted against the known experimental data. Our study reveals different binding preferences of the investigated compounds towards the sub-sites of the large deep binding pocket of MexB, supporting the hypothesis that MexB substrates oscillate between different binding modes with similar affinity. Interestingly, small changes in the molecular structure translate into significant differences in MexB–quinolone interactions. All the predicted binding modes are available for download and visualization at the following link: https://www.dsf.unica.it/dock/mexb/quinolones.

Putative binding modes (BMs) of quinolones to the bacterial efflux transporter MexB were identified. Multiple interaction patterns are possible, supporting the hypothesis that substrates oscillate between different BMs with similar affinity.

Degradation of sulfamethoxazole using PMS activated by cobalt sulfides encapsulated in nitrogen and sulfur co-doped graphene

In this study cobalt sulfides (Co₉S₈) coated on the nitrogen and sulfur co-doped graphene (Co₉S₈@S-N-RG) was firstly prepared and used for degradation of antibiotic sulfamethoxazole (SMX). The results showed that SMX could be completely degraded by Co₉S₈@S-N-RG-activated peroxymonosulfate (PMS) within 20 min with its mineralization efficiency of 38.7%. The SMX degradation rate followed pseudo first-order kinetics with kinetic constant of 0.377 min^-1 that was higher than that induced by Co₉S₈, N-RG, S-N-RG and Co₉S₈@S-RG, indicating Co₉S₈@S-N-RG had superior catalytic activity. Co₉S₈@S-N-RG can activate PMS to produce sulfate radicals and hydroxyl radicals, while sulfate radicals played major role. Co₉S₈ participated in PMS activation in which Co²⁺ was involved in sulfate radicals formation, while sulfur species facilitated the conversion of Co³⁺ to Co²⁺. In addition, carbon defects, CO, pyridinic N and pyrrolic N also contributed to PMS activation.The superior catalytic activity was attributed to the synergistic effect of Co₉S₈ and S-N-RG. This study could provide an efficient and stable PMS activator, and insight into the PMS activation mechanism by Co₉S₈@S-N-RG.

The Role of Grass Compost and Zea Mays in Alleviating Toxic Effects of Tetracycline on the Soil Bacteria Community

Given their common use for disease treatment in humans, and particularly in animals, antibiotics pose an exceptionally serious threat to the soil environment. This study aimed to determine the response of soil bacteria and oxidoreductases to a tetracycline (Tc) contamination, and to establish the usability of grass compost (G) and Zea mays (Zm) in mitigating adverse Tc effects on selected microbial properties of the soil. The scope of microbiological analyses included determinations of bacteria with the conventional culture method and new-generation sequencing method (NGS). Activities of soil dehydrogenases and catalase were determined as well. Tc was found to reduce counts of organotrophic bacteria and actinobacteria in the soils as well as the activity of soil oxidoreductases. Soil fertilization with grass compost (G) and Zea mays (Zm) cultivation was found to alleviate the adverse effects of tetracycline on the mentioned group of bacteria and activity of oxidoreductases. The metagenomic analysis demonstrated that the bacteria belonging to Acidiobacteria and Proteobacteria phyla were found to prevail in the soil samples. The study results recommend soil fertilization with G and Zm cultivation as successful measures in the bioremediation of tetracycline-contaminated soils and indicate the usability of the so-called core bacteria in the bioaugmentation of such soils.

Use of hydroxypropyl β-cyclodextrin hybrid monolithic material as adsorbent for dispersive solid-phase extraction of fluoroquinolones from environmental water samples

In this study, the hydroxypropyl β-cyclodextrin hybrid monolithic material was fabricated and firstly applied as adsorbent for dispersive solid-phase extraction coupled with high-performance liquid chromatography to detect trace-level seven fluoroquinolones in water samples. The prepared hydroxypropyl β-cyclodextrin hybrid monolithic material was characterized by fourier transform infrared spectroscopy, scanning electron microscopy and adsorption experiments, which showed excellent specific adsorption to the target fluoroquinolones. Under the optimized conditions, the extraction methodology showed satisfactory precision with relative standard deviations between 2.6 and 5.6%, good linearity (R² ≥0.9990) and satisfactory recoveries (82.5∼91.8%). The limits of detection and limits of quantification of the method were in the range of 0.4∼1.2 ng mL^-1 and 1.4∼4.0 ng mL^-1 respectively, which confirmed the possibility of quantifying trace levels. Furthermore, the material could be reused at least five times. These results demonstrated that the hydroxypropyl β-cyclodextrin hybrid monolithic material was a promising adsorbent for fluoroquinolones, and the established method combined dispersive solid-phase extraction with high-performance liquid chromatography was suitable for the determination of fluoroquinolones in aqueous samples. This article is protected by copyright. All rights reserved.

AB-DB: Force-Field parameters, MD trajectories, QM-based data, and Descriptors of Antimicrobials

Antibiotic resistance is a major threat to public health. The development of chemo-informatic tools to guide medicinal chemistry campaigns in the efficint design of antibacterial libraries is urgently needed. We present AB-DB, an open database of all-atom force-field parameters, molecular dynamics trajectories, quantum-mechanical properties, and curated physico-chemical descriptors of antimicrobial compounds. We considered more than 300 molecules belonging to 25 families that include the most relevant antibiotic classes in clinical use, such as β-lactams and (fluoro)quinolones, as well as inhibitors of key bacterial proteins. We provide traditional descriptors together with properties obtained with Density Functional Theory calculations. Noteworthy, AB-DB contains less conventional descriptors extracted from μs-long molecular dynamics simulations in explicit solvent. In addition, for each compound we make available force-field parameters for the major micro-species at physiological pH. With the rise of multi-drug-resistant pathogens and the consequent need for novel antibiotics, inhibitors, and drug re-purposing strategies, curated databases containing reliable and not straightforward properties facilitate the integration of data mining and statistics into the discovery of new antimicrobials.

Synthesis, Characterization and Anticancer Efficacy Evaluation of Benzoxanthone Compounds toward Gastric Cancer SGC-7901

Three benzoxanthone derivatives were synthesized through a new photochemical strategy. The in vitro cytotoxic activity of these compounds was evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, and their partition coefficients (logP) were measured by shake flask method. The pK_a values of the compounds were detected by potentionmetric titration. The interaction of the compounds with calf thymus DNA (CT-DNA) was investigated by electronic absorption, luminescence spectra and viscosity. A molecular docking analysis was performed. The antitumor efficacy of the compounds was evaluated by cell apoptosis, cell cycle arrest, intracellular Ca²⁺ concentrations and reactive oxygen species (ROS) levels. The mitochondrial membrane potential was assayed using JC-1 (5,5′,6,6′-tetrachloro-1,1,3′,3′-tetraethyl-imidacarbocyanine iodide) as the fluorescence probe. The expression of Bcl-2 family protein, caspase 3 and poly ADP-ribose polymerase (PARP) was explored by western blot. The results showed that the compounds induced apoptosis through a ROS-mediated mitochondrial dysfunction pathway. This work provides an efficient approach to synthesize benzoxanthone derivatives, and is helpful for understanding the apoptotic mechanism.

Upcycling fruit peel waste into a green reductant to reduce graphene oxide for fabricating an electrochemical sensing platform for sulfamethoxazole determination in aquatic environments

Fruit and vegetable wastes contribute to a substantial proportion of global food waste. While these wastes could potentially be repurposed for a wide range of applications, the majority of them are discarded without effective utilization. To address the current challenges of fruit waste accumulation and sustainable nanomaterial synthesis, natural reductants derived from discarded dragon fruit (Hylocereus polyrhizus) peels are proposed as an alternative to conventional hazardous reductants for graphene-based material synthesis. Given that the chemical reduction of graphene oxide (GO) is the major route for graphene production, the effectiveness of the proposed reductants derived from peels of dragon fruit on graphene oxide reduction was evaluated. The reducing constituents (i.e., betanin substances) were recovered from dragon fruit peel wastes using facile aqueous extraction processes, where suitable extraction treatments (e.g., pH conditions) were found to be critical for boosting the reducing power of the obtained reductants. The compiled results indicated that the proposed fruit waste-derived reducing agents demonstrated great promise for GO reduction through S_N2 nucleophilic reactions, mainly driven by the extracted betanin. The obtained reduced GO serves as a promising platform for electrochemical determination of sulfamethoxazole in aquatic environments, realizing both food waste valorization and environmentally benign material synthesis.

Engineered macroalgal and microalgal adsorbents: Synthesis routes and adsorptive performance on hazardous water contaminants

Colourants, micropollutants and heavy metals are regarded as the most notorious hazardous contaminants found in rivers, oceans and sewage treatment plants, with detrimental impacts on human health and environment. In recent development, algal biomass showed great potential for the synthesis of engineered algal adsorbents suitable for the adsorptive management of various pollutants. This review presents comprehensive investigations on the engineered synthesis routes focusing mainly on mechanical, thermochemical and activation processes to produce algal adsorbents. The adsorptive performances of engineered algal adsorbents are assessed in accordance with different categories of hazardous pollutants as well as in terms of their experimental and modelled adsorption capacities. Due to the unique physicochemical properties of macroalgae and microalgae in their adsorbent forms, the adsorption of hazardous pollutants was found to be highly effective, which involved different mechanisms such as physisorption, chemisorption, ion-exchange, complexation and others depending on the types of pollutants. Overall, both macroalgae and microalgae not only can be tailored into different forms of adsorbents based on the applications, their adsorption capacities are also far more superior compared to the conventional adsorbents.

The spectrophotometric determination of lipophilicity and dissociation constants of ciprofloxacin and levofloxacin

Lipophilicity plays a significant role in the permeability of the drugs through cell membranes and impacts the drug activity in the human body. In this paper, the spectrophotometric method was used to determine the apparent partition coefficients of two amphoteric drugs: ciprofloxacin and levofloxacin. The apparent partition coefficient was determined with the classic shake-flask method with n-octanol according to OECD guidelines. The lipophilicity profiles in a wide range of pH were determined and described quantitatively with the quadratic function. Basing on the macro- and microdissociation constants, the true partition coefficient for both drugs was calculated. Both levofloxacin and ciprofloxacin were lipophilic. The neutral forms, i.e., zwitterionic and uncharged, dominate in the pH relevant to the one in the intestines, the place from which they are absorbed.

New bis- and tetrakis-1,2,3-triazole derivatives: Synthesis, DNA cleavage, molecular docking, antimicrobial, antioxidant activity and acid dissociation constants

In this study, a series of bis- and tetrakis-1,2,3-triazole derivatives were synthesized using copper-catalyzed azide-alkyne cycloaddition (CuAAC) click chemistry in 73-95% yield. The bis- and tetrakis-1,2,3-triazoles exhibited significant DNA cleavage activity while the tetrakis-1,2,3-triazole analog 6g completely degraded the plasmid DNA. Molecular docking simulations suggest that compound 6g acts as minor groove binder of DNA by binding through several noncovalent interactions with base pairs. All bis- and tetrakis-1,2,3-triazole derivatives were screened for antibacterial activity against E. coli, B. cereus, S. aureus, P. aeruginosa, E. hirae, L. pneumophila subsp. pneumophila strains and antifungal activity against microfungus C. albicans and C. tropicalis strains. Compound 4d exhibited the best antibacterial activity among bis-1,2,3-triazoles against E. coli and E. hirae, while 6c exhibited the best antibacterial activity among tetrakis-1,2,3-triazoles against E. hirae. Furthermore, the best antifungal activity against C. albicans and C. tropicalis was reported for the compound 5, while 6d displayed the best antifungal activity against C. tropicalis and C. albicans. Reasonable iron chelating activities and DPPH radical scavenging abilities were found for some of the compounds. Finally, the acid dissociation constants (pK_a) of the bis-1,2,3-triazoles were also determined with the help of HYPERQUAD program using the data obtained from potentiometric titrations. The reported data here concludes that the bis- and tetrakis-1,2,3-triazoles are important cores that should be considered for further development of especially new anticancer agents acting through the DNA cleavage activity.

Synthesis of surface dual-template molecularly imprinted silica nanoparticles for extraction of ciprofloxacin and norfloxacin

Synthesis of selective dual-template molecularly imprinted silica nanoparticles (MI-SiNPs) on the surface of graphene quantum dots (GQDs) for the simultaneous extraction of ciprofloxacin (CIP) and norfloxacin (NOR) from biological samples.

MAPPING TRANSITION STATE STRUCTURES FOR THIOPHOSPHINOYL GROUP TRANSFER BETWEEN OXYANIONIC NUCLEOPHILES IN WATER AND AQUEOUS ETHANOL SOLVENTS

Second-order rate constants were measured for thiophosphinoyl group transfer from the substrates 3a-g to oxygen nucleophiles in 50% water-50% ethanol and 30% water-70% ethanol mixtures, to obtain solvent-independent Brønsted coefficients...

A review of antibiotic removal from domestic wastewater using the activated sludge process: removal routes, kinetics and operational parameters

Many advanced technologies have shown encouraging results in removing antibiotics from domestic wastewater. However, as activated sludge treatment is the most common sewage treatment system employed worldwide, improving its effect on antibiotic removal would be more desirable. Understanding the removal mechanisms, kinetics and factors that affect antibiotic removal in the activated sludge process is important as it would allow us to improve the treatment performance. Although these have been discussed in various literature covering different types of antibiotics and wastewater, a specific review on antibiotics and domestic wastewater is clearly missing. This review paper collates, discusses and analyses the removal of antibiotics from sewage in the activated sludge process along with the removal mechanisms and kinetics. The antibiotics are categorised into six classes: β-lactam, dihydrofolate reductase inhibitor, fluoroquinolone, macrolide, sulfonamides and tetracycline. Furthermore, the factors affecting the system performance with regard to antibiotic removal are examined.

Determination of the aqueous pKa of very insoluble drugs by capillary electrophoresis: Internal standards for methanol-water extrapolation

A fast determination of acidity constants (pK_a) of very insoluble drugs has become a necessity in drug discovery process because it often produces molecules that are highly lipophilic and sparingly soluble in water. In this work the high throughput internal standard capillary electrophoresis (IS-CE) method has been adapted to the determination of pK_a of water insoluble compounds by measurement in methanol/aqueous buffer mixtures. For this purpose, the reference pK_a values for a set of 46 acid-base compounds of varied structure (internal standards) have been established in methanol-water mixtures at several solvent composition levels (with a maximum of 40% methanol). The IS-CE method has been successfully applied to seven test drugs of different chemical nature with intrinsic solubilities lower than 10^-6 M. pK_a values have been determined at different methanol/aqueous buffer compositions and afterwards Yasuda-Shedlovsky extrapolation method has been applied to obtain the aqueous pK_a. The obtained results have successfully been compared to literature ones obtained by other methods. It is concluded that the IS-CE method allows the determination of aqueous pK_a values using low proportions of methanol, becoming then more accurate in the extrapolation procedure than other reference methods.