Development of Methods for the Determination of pKa Values

Biomimetic Charge-Neutral Anion Receptors for Reversible Binding and Release of Highly Hydrated Phosphate in Water

Control of phosphate capture and release is vital in environmental, biological, and pharmaceutical contexts. However, the binding of trivalent phosphate (PO4 3-) in water is exceptionally difficult due to its high hydration energy. Based on the anion coordination chemistry of phosphate, in this study, four charge-neutral tripodal hexaurea receptors (L1-L4), which were equipped with morpholine and polyethylene glycol terminal groups to enhance their solubility in water, were synthesized to enable the pH-triggered phosphate binding and release in aqueous solutions. Encouragingly, the receptors were found to bind PO4 3- anion in a 1 : 1 ratio via hydrogen bonds in 100 % water solutions, with L1 exhibiting the highest binding constant (1.2×103 M-1). These represent the first neutral anion ligands to bind phosphate in 100 % water and demonstrate the potential for phosphate capture and release in water through pH-triggered mechanisms, mimicking native phosphate binding proteins. Furthermore, L1 can also bind multiple bioavailable phosphate species, which may serve as model systems for probing and modulating phosphate homeostasis in biological and biomedical researches.

A Fusion-Growth Protocell Model Based on Vesicle Interactions with Pyrite Particles

Protocell models play a pivotal role in the exploration of the origin of life. Vesicles are one type of protocell model that have attracted much attention. Simple single-chain amphiphiles (SACs) and organic small molecules (OSMs) possess primitive relevance and were most likely the building blocks of protocells on the early Earth. OSM@SAC vesicles have been considered to be plausible protocell models. Pyrite (FeS2), a mineral with primitive relevance, is ubiquitous in nature and plays a crucial role in the exploration of the origin of life in the mineral-water interface scenario. "How do protocell models based on OSM@SAC vesicles interact with a mineral-water interface scenario that simulates a primitive Earth environment" remains an unresolved question. Hence, we select primitive relevant sodium monododecyl phosphate (SDP), isopentenol (IPN) and pyrite (FeS2) mineral particles to build a protocell model. The model investigates the basic physical and chemical properties of FeS2 particles and reveals the effects of the size, content and duration of interaction of FeS2 particles on IPN@SDP vesicles. This deepens the understanding of protocell growth mechanisms in scenarios of mineral-water interfaces in primitive Earth environments and provides new information for the exploration of the origin of life.

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.

How do microplastics alter molluscicidal activity? Effects of weathered microplastics and niclosamide in developing freshwater snails

Despite the wide distribution and persistence of microplastics (MPs), their interactive effects with molluscicides are unknown. Schistosomiasis, a neglected tropical disease, affects 236.6 million people worldwide. Niclosamide (NCL) is the only molluscicide recommended by the World Health Organization (WHO) and it is used to control the population of Schistosoma spp.'s intermediate host. Thus, this study aimed to evaluate of the interaction between polyethylene (PE) MPs and NCL, and their associated toxicity in the freshwater snail Biomphalaria glabrata (Say 1818). Weathered PE MPs were characterized and theoretical analysis of NCL-MP adsorption nature was made using quantum mechanical calculations. The toxicity of NCL isolated (0.0265 to 0.0809 mg L-1) and under interaction with PE MPs (3400 μg L-1) in B. glabrata embryos and newly hatched snails was analyzed. In silico analysis confirmed the adsorption mechanisms of NCL into PE MPs. PE MPs decreased the NCL toxicity to both B. glabrata developmental stages, increasing their survival and NCL lethal concentrations, indicating concerns regarding NCL use as molluscicide in aquatic environments polluted by MPs. In conclusion, MPs may change the efficiency of chemicals used in snail control programs.

Recent Progress in the Endosomal Escape Mechanism and Chemical Structures of Polycations for Nucleic Acid Delivery

Nucleic acid-based therapies are seeing a spiralling surge. Stimuli-responsive polymers, especially pH-responsive ones, are gaining widespread attention because of their ability to efficiently deliver nucleic acids. These polymers can be synthesized and modified according to target requirements, such as delivery sites and the nature of nucleic acids. In this regard, the endosomal escape mechanism of polymer-nucleic acid complexes (polyplexes) remains a topic of considerable interest owing to various plausible escape mechanisms. This review describes current progress in the endosomal escape mechanism of polyplexes and state-of-the-art chemical designs for pH-responsive polymers. The importance is also discussed of the acid dissociation constant (i.e., pKa) in designing the new generation of pH-responsive polymers, along with assays to monitor and quantify the endosomal escape behavior. Further, the use of machine learning is addressed in pKa prediction and polymer design to find novel chemical structures for pH responsiveness. This review will facilitate the design of new pH-responsive polymers for advanced and efficient nucleic acid delivery.

Acidity constants and protonation sites of cyclic dinucleotides determined by capillary electrophoresis, quantum chemical calculations, and NMR spectroscopy

Cyclic dinucleotides (CDNs) are important second messengers in bacteria and eukaryotes. Detailed characterization of their physicochemical properties is a prerequisite for understanding their biological functions. Herein, we examine acid-base and electromigration properties of selected CDNs employing capillary electrophoresis (CE), density functional theory (DFT), and nuclear magnetic resonance (NMR) spectroscopy to provide benchmark pKa values, as well as to unambiguously determine the protonation sites. Acidity constants (pKa) of the NH+ moieties of adenine and guanine bases and actual and limiting ionic mobilities of CDNs were determined by nonlinear regression analysis of the pH dependence of their effective electrophoretic mobilities measured by CE in aqueous background electrolytes in a wide pH range (0.98-11.48), at constant temperature (25°C), and constant ionic strength (25 mM). The thermodynamic pKa values were found to be in the range 3.31-4.56 for adenine and 2.28-3.61 for guanine bases, whereas the pKa of enol group of guanine base was in the range 10.21-10.40. Except for systematic shifts of ∼2 pKa, the pKa values calculated by the DFT-D3//COSMO-RS composite protocol that included large-scale conformational sampling and "cross-morphing" were in a relatively good agreement with the pKas determined by CE and predict N1 atom of adenine and N7 atom of guanine as the protonation sites. The protonation of the N1 atom of adenine and N7 atom of guanine in acidic background electrolytes (BGEs) and the dissociation of the enol group of guanine in alkaline BGEs was confirmed also by NMR spectroscopy.

A pH-enhanced resolution in benchtop NMR spectroscopy

NMR spectroscopy is one of the most potent methods in analytical chemistry. NMR titration experiments are particularly useful since they measure molecular binding affinities and other concentration-dependent effects. These experiments, however, require a long series of measurements. An alternative to these serial measurements has recently been presented, exploiting a pH (or generally - a concentration) gradient along the NMR tube. The proposed experiment, although efficient, was based on the sensitivity- and hardware-demanding chemical shift imaging (CSI) method. Thus, it is practically limited to high-resolution NMR spectrometers. This paper proposes modifying and adapting the approach to the popular and cost-efficient benchtop NMR machines. Instead of CSI, we use a device that shifts the NMR tube vertically to measure the spectra of different sample volumes, which have different pH values due to the established gradient along the tube. We demonstrate the potential of the method on the test samples of L-tyrosine and 2,6-lutidine, and two real samples from the food industry - an infant formula and an energy drink. The proposed method boosts spectral resolution and allows for the sampling of a broader range of pH values when compared to the original approach.

Which Lipid Nanoparticle (LNP) Designs Work? A Simple Kinetic Model Linking LNP Chemical Structure to In Vivo Delivery Performance

Although lipid nanoparticles (LNPs) are the predominant nanocarriers for short-interfering RNA (siRNA) delivery, most therapies use nearly identical formulations that have taken 30 years to design but lack the diverse property ranges necessary for versatile application. This dearth in variety and the extended timeline for implementation are attributed to a limited understanding of how LNP properties facilitate overcoming biological barriers. Herein, a simple kinetic model was developed by using major rate-limiting steps for siRNA delivery, and this model enabled the identification of a critical parameter to predict LNP efficacy without extensive experimental testing. A volume-averaged log D, the "solubility" of charged molecules as a function of pH weighted by component volume fractions, resulted in a good correlation between LNP composition and siRNA delivery. Both the effects of modifying the structures of ionizable lipids and LNP composition on gene silencing were easily captured in the model predictions. Thus, this approach provides a robust LNP structure-activity relationship to dramatically accelerate the realization of effective LNP formulations.

Accurately Predicting Protein pKa Values Using Nonequilibrium Alchemy

The stability, solubility, and function of a protein depend on both its net charge and the protonation states of its individual residues. pKa is a measure of the tendency for a given residue to (de)protonate at a specific pH. Although pKa values can be resolved experimentally, theory and computation provide a compelling alternative. To this end, we assess the applicability of a nonequilibrium (NEQ) alchemical free energy method to the problem of pKa prediction. On a data set of 144 residues that span 13 proteins, we report an average unsigned error of 0.77 ± 0.09, 0.69 ± 0.09, and 0.52 ± 0.04 pK for aspartate, glutamate, and lysine, respectively. This is comparable to current state-of-the-art predictors and the accuracy recently reached using free energy perturbation methods (e.g., FEP+). Moreover, we demonstrate that our open-source, pmx-based approach can accurately resolve the pKa values of coupled residues and observe a substantial performance disparity associated with the lysine partial charges in Amber14SB/Amber99SB*-ILDN, for which an underused fix already exists.

Automatic Rhodopsin Modeling with Multiple Protonation Microstates

Automatic Rhodopsin Modeling (ARM) is a simulation protocol providing QM/MM models of rhodopsins capable of reproducing experimental electronic absorption and emission trends. Currently, ARM is restricted to a single protonation microstate for each rhodopsin model. Herein, we incorporate an extension of the minimal electrostatic model (MEM) into the ARM protocol to account for all relevant protonation microstates at a given pH. The new ARM+MEM protocol determines the most important microstates contributing to the description of the absorption spectrum. As a test case, we have applied this methodology to simulate the pH-dependent absorption spectrum of a toy model, showing that the single-microstate picture breaks down at certain pH values. Subsequently, we applied ARM+MEM toAnabaenasensory rhodopsin, confirming an improved description of its absorption spectrum when the titration of several key residues is considered.

Accurate Computation of Aqueous pKas of Biologically Relevant Organic Acids: Overcoming the Challenges Posed by Multiple Conformers, Tautomeric Equilibria, and Disparate Functional Groups with the Fully Black-Box pK-Yay Method

The use of static electronic structure calculations to compute solution-phase pKas offers a great advantage in that a macroscopic bulk property could be computed via microscopic computations involving very few molecules. There are various sources of errors in the quantum chemical calculations though. Overcoming these errors to accurately compute pKas of a plethora of acids is an active area of research in physical chemistry pursued by both computational as well as experimental chemists. We recently developed the pK-Yay method in our attempt to accurately compute aqueous pKas of strong and weak acids. The method is fully black-box, computationally inexpensive, and is very easy for even a nonexpert to use. However, the method was thus far tested on very few molecules (only 16 in all). Herein, in order to assess the future applicability of pK-Yay, we study the effect of multiple conformers, the presence of tautomers under equilibrium, and the impact of a wide variety of functional groups (derivatives of acetic acid with substituents at various positions, dicarboxylic acids, aromatic carboxylic acids, amines and amides, phenols and thiols, and fluorine bearing organic acids). Starting with more than 1000 conformers and tautomers, this study establishes that overall errors of ∼ 1.0 pKa units are routinely obtained for a majority of the molecules. Larger errors are noted in cases where multiple charges, intramolecular hydrogen bonding, and several ionizable functional groups are simultaneously present. An important conclusion to emerge from this work is that, the computed pKas are insensitive (difference <0.5) to whether we consider multiple conformers/tautomers or only choose the most stable conformer/tautomer. Further, pK-Yay captures the stereoelectronic effects arising due to differing axial vs equatorial pattern, and is useful to predict the dominant acid-base equilibrium in a system featuring several equilibria. Overall, pK-Yay may be employed in several chemical applications featuring organic molecules and biomonomers.

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.

Reliable and accurate prediction of basic pK[Formula: see text] values in nitrogen compounds: the pK[Formula: see text] shift in supramolecular systems as a case study

This article presents a quantitative structure-activity relationship (QSAR) approach for predicting the acid dissociation constant (pK[Formula: see text]) of nitrogenous compounds, including those within supramolecular complexes based on cucurbiturils. The model combines low-cost quantum mechanical calculations with QSAR methodology and linear regressions to achieve accurate predictions for a broad range of nitrogen-containing compounds. The model was developed using a diverse dataset of 130 nitrogenous compounds and exhibits excellent predictive performance, with a high coefficient of determination (R[Formula: see text]) of 0.9905, low standard error (s) of 0.3066, and high Fisher statistic (F) of 2142. The model outperforms existing methods, such as Chemaxon software and previous studies, in terms of accuracy and its ability to handle heterogeneous datasets. External validation on pharmaceutical ingredients, dyes, and supramolecular complexes based on cucurbiturils confirms the reliability of the model. To enhance usability, a script-like tool has been developed, providing a streamlined process for users to access the model. This study represents a significant advancement in pK[Formula: see text] prediction, offering valuable insights for drug design and supramolecular system optimization.

Sample pH Can Drift during Native Mass Spectrometry Experiments: Results from Ratiometric Fluorescence Imaging

The ability of nanoelectrospray ionization (nanoESI) to generate a continuous flow of charged droplets relies on the electrolytic nature of the process. This electrochemistry can lead to the accumulation of redox products in the sample solution. This consequence can have significant implications for native mass spectrometry (MS), which aims to probe the structures and interactions of biomolecules in solution. Here, ratiometric fluorescence imaging and a pH-sensitive, fluorescent probe are used to quantify changes in solution pH during nanoESI under conditions relevant to native MS. Results show that the extent and rate of change in sample pH depends on several experimental parameters. There is a strong correlation between the extent and rate of change in solution pH and the magnitude of both the nanoESI current and electrolyte concentration. Smaller changes in solution pH are observed during experiments when a negative potential is applied than for those when a positive potential is applied. Finally, we make specific recommendations for designing native MS experiments that control for these effects.

Colloidal Stability Window for Carboxylated Cellulose Nanocrystals: Considerations for Handling, Characterization, and Formulation

The scale of production of cellulose nanocrystals (CNCs) has increased dramatically to meet the growing demand for sustainably sourced materials. This work defines the colloidal stability window for commercially produced carboxylated CNCs (DextraCel) compared to the more traditional sulfated CNCs. Phase diagrams showing the stable, reversibly agglomerated, irreversibly aggregated/sedimented, and colloidal glass "zones" as a function of suspension pH, ionic strength, CNC surface charge content, counterion, and concentration are presented. The pKa of carboxylated CNCs was measured to be 5.1, and suspensions of carboxylated CNCs (0.5-1.5 wt %) were visually stable from pH 3 to 11 (without salt). Carboxylated CNCs were highly sensitive to ionic strength, demonstrating some agglomeration with as little as 5 mM NaCl, supporting that weak acid surface groups and lower charge contents make CNCs more sensitive to solution conditions. Surface charge content had the greatest influence on colloidal stability followed by the counterion; carboxylated CNCs were more stable in the "as-received" sodium form, whereas sulfated CNCs had improved stability in acid form after ion exchange. The stability of carboxylated CNCs with industrially relevant additives (ionic and nonionic surfactants and initiators) was also investigated. Less concentrated suspensions were more colloidally stable, emphasizing that characterization and processing of CNCs favor dilute conditions. If carboxylated CNCs are subjected to conditions outside of their colloidal stability window, simple dilution or pH adjustment does not return them to colloidally stable discrete nanoparticles; however, ultrasonication can redisperse agglomerates. This study offers guidelines for handling carboxylated CNCs to broaden the range of products that can be improved by their incorporation.

Calculating the Aqueous pKa of Phenols: Predictions for Antioxidants and Cannabinoids

We aim to develop a theoretical methodology for the accurate aqueous pK_a prediction of structurally complex phenolic antioxidants and cannabinoids. In this study, five functionals (M06-2X, B3LYP, BHandHLYP, PBE0, and TPSS) and two solvent models (SMD and PCM) were combined with the 6-311++G(d,p) basis set to predict pK_a values for twenty structurally simple phenols. None of the direct calculations produced good results. However, the correlations between the calculated Gibbs energy difference of each acid and its conjugate base, ΔGaq(BA)°=ΔGaqA-°-ΔGaq(HA)°, and the experimental aqueous pK_a values had superior predictive accuracy, which was also tested relative to an independent set of ten molecules of which six were structurally complex phenols. New correlations were built with twenty-seven phenols (including the phenols with experimental pK_a values from the test set), which were used to make predictions. The best correlation equations used the PCM method and produced mean absolute errors of 0.26-0.27 pK_a units and R² values of 0.957-0.960. The average range of predictions for the potential antioxidants (cannabinoids) was 0.15 (0.25) pK_a units, which indicates good agreement between our methodologies. The new correlation equations could be used to make pK_a predictions for other phenols in water and potentially in other solvents where they might be more soluble.

Experimental and numerical elucidation of the fate and transport of antibiotics in aquatic environment: A review

This review highlights various experimental and mathematical modeling strategies to investigate the fate and transport of antibiotics that elucidate antimicrobial selective pressure in aquatic environments. Globally, the residual antibiotic concentrations in effluents from bulk drug manufacturing industries were 30- and 1500-fold greater than values reported in municipal and hospital effluents, respectively. The antibiotic concentration from different effluents enters the waterbodies that usually get diluted as they go downstream and undergo various abiotic and biotic reactive processes. In aquatic systems, photolysis is the predominant process for antibiotic reduction in the water matrix, while hydrolysis and sorption are frequently reported in the sediment compartment. The rate of antibiotic reduction varies widely with influencing factors such as the chemical properties of the antibiotics and hydrodynamic conditions of river streams. Among all, tetracycline was found to more unstable (log Kow =  - 0.62 to - 1.12) that can readily undergo photolysis and hydrolysis; whereas macrolides were more stable (log Kow = 3.06 to 4.02) that are prone to biodegradation. The processes like photolysis, hydrolysis, and biodegradation followed first-order reaction kinetics while the sorption followed a second-order kinetics for most antibiotic classes with reaction rates occurring in the decreasing order of Fluoroquinolones and Sulphonamides. The reports from various experiments on abiotic and biotic processes serve as input parameters for an integrated mathematical modeling to predict the fate of the antibiotics in the aquatic environment. Various mathematical models viz. Fugacity level IV, RSEMM, OTIS, GREAT-ER, SWAT, QWASI, and STREAM-EU are discussed for their potential capabilities. However, these models do not account for microscale interactions of the antibiotics and microbial community under real-field conditions. Also, the seasonal variations for contaminant concentrations that exert selective pressure for antimicrobial resistance has not been accounted. Addressing these aspects collectively is the key to exploring the emergence of antimicrobial resistance. Therefore, a comprehensive model involving antimicrobial resistance parameters like fitness cost, bacterial population dynamics, conjugation transfer efficiency, etc. is required to predict the fate of antibiotics.

Proton nuclear magnetic resonance (1H NMR) of flammable organic chemicals in radioactive high-level supernatant waste at the Savannah River Site (SRS)

The Savannah River Site stores approximately 36 million gallons of radioactive and hazardous waste that contains approximately 245 million curies. The waste is sent through various chemical processes to reduce its volume and to separate various components. The facility plans to replace formic acid (a chemical used to reduce soluble mercury) with glycolic acid. Recycle solution with glycolate may flow back to the tank farm, where the glycolate can generate hydrogen gas by thermal and radiolytic mechanisms. The current analytical method for detecting glycolate (ion chromatography) in supernatant requires a large dilution to reduce interference from the nitrate anions. Hydrogen nuclear magnetic resonance is an analytical method that requires less sample dilution. It takes advantage of the CH₂ group in glycolate. Liquid samples were spiked with four different levels of glycolate to build a calibration line, as it is recommended in the standard addition method. The detection and quantitation limits determined were 1 and 5 ppm, respectively, for 32 scans, which is well below the process limit of 10 ppm. In one test, 800 scans of a supernatant spiked with 1 ppm glycolate resulted in a -CH₂ peak with a signal-to-noise ratio of 36.

Combined NMR Spectroscopy and Quantum-Chemical Calculations in Fluorescent 1,2,3-Triazole-4-carboxylic Acids Fine Structures Analysis

The peculiarities of the optical properties of 2-aryl-1,2,3-triazole acids and their sodium salts were investigated in different solvents (1,4-dioxane, dimethyl sulfoxide DMSO, methanol MeOH) and in mixtures with water. The results were discussed in terms of the molecular structure formed by inter- and intramolecular noncovalent interactions (NCIs) and their ability to ionize in anions. Theoretical calculations using the Time-Dependent Density Functional Theory (TDDFT) were carried out in different solvents to support the results. In polar and nonpolar solvents (DMSO, 1,4-dioxane), fluorescence was provided by strong neutral associates. Protic MeOH can weaken the acid molecules' association, forming other fluorescent species. The fluorescent species in water exhibited similar optical characteristics to those of triazole salts; therefore, their anionic character can be assumed. Experimental ¹H and ¹³C-NMR spectra were compared to their corresponding calculated spectra using the Gauge-Independent Atomic Orbital (GIAO) method and several relationships were established. All these findings showed that the obtained photophysical properties of the 2-aryl-1,2,3-triazole acids noticeably depend on the environment and, therefore, are good candidates as sensors for the identification of analytes with labile protons.

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.

Benchmarking In Silico Tools for Cysteine pKa Prediction

Accurate estimation of the pK_a's of cysteine residues in proteins could inform targeted approaches in hit discovery. The pK_a of a targetable cysteine residue in a disease-related protein is an important physiochemical parameter in covalent drug discovery, as it influences the fraction of nucleophilic thiolate amenable to chemical protein modification. Traditional structure-based in silico tools are limited in their predictive accuracy of cysteine pK_a's relative to other titratable residues. Additionally, there are limited comprehensive benchmark assessments for cysteine pK_a predictive tools. This raises the need for extensive assessment and evaluation of methods for cysteine pK_a prediction. Here, we report the performance of several computational pK_a methods, including single-structure and ensemble-based approaches, on a diverse test set of experimental cysteine pK_a's retrieved from the PKAD database. The dataset consisted of 16 wildtype and 10 mutant proteins with experimentally measured cysteine pK_a values. Our results highlight that these methods are varied in their overall predictive accuracies. Among the test set of wildtype proteins evaluated, the best method (MOE) yielded a mean absolute error of 2.3 pK units, highlighting the need for improvement of existing pK_a methods for accurate cysteine pK_a estimation. Given the limited accuracy of these methods, further development is needed before these approaches can be routinely employed to drive design decisions in early drug discovery efforts.

Plasma Protein-Mediated Uptake and Contradictions to the Free Drug Hypothesis: A Critical Review

According to the free drug hypothesis (FDH), only free, unbound drug is available to interact with biological targets. This hypothesis is the fundamental principle that continues to explain the vast majority of all pharmacokinetic and pharmacodynamic processes. Under the FDH, the free drug concentration at the target site is considered the driver of pharmacodynamic activity and pharmacokinetic processes. However, deviations from the FDH are observed in hepatic uptake and clearance predictions, where observed unbound intrinsic hepatic clearance (CL_int,u) is larger than expected. Such deviations are commonly observed when plasma proteins are present and form the basis of the so-called plasma protein-mediated uptake effect (PMUE). This review will discuss the basis of plasma protein binding as it pertains to hepatic clearance based on the FDH, as well as several hypotheses that may explain the underlying mechanisms of PMUE. Notably, some, but not all, potential mechanisms remained aligned with the FDH. Finally, we will outline possible experimental strategies to elucidate PMUE mechanisms. Understanding the mechanisms of PMUE and its potential contribution to clearance underprediction is vital to improving the drug development process.

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.

Lactose Mother Liquor Stream Valorisation Using an Effective Electrodialytic Process

The integrated electrodialysis (ED) process supports valorisation of a lactose-rich side stream from the dairy industry, creating an important source of milk sugar used in various branches of the industry. This work focuses on the optimization of the downstream processes before the crystallization of lactose. The process line includes a pre-treatment and desalination by ED of the industrial waste solution of the lactose mother liquor (LML). The LML was diluted to 25% total solids to overcome hydraulic issues with the ED desalination process. Two different levels of electrical conductivity reduction (70% and 90%) of the LML solutions were applied to decrease the mineral components and organic acids of the LML samples. The ED performance parameters such as ash transfer rate (J), the specific capacity (C_F) of the ED and specific electric energy consumption (E) were determined and the influence of the LML solution on the monopolar ion-exchange membranes has been investigated. A higher degree of desalination is associated with higher electric energy consumption (by 50%) and lower specific capacity (by 40%). A noticeable decrease (by 12.8%) in the resistance of the anion exchange membranes was measured after the trials whereas the resistance of the cation exchange membranes remained practically unchanged. Any deposition of the alkaline earth metals on the membrane surface was not observed.

Stability of Dibromo-Dipyrromethene Complexes Coordinated with B, Zn, and Cd in Solutions of Various Acidities

The spectral luminescent properties of dipyrromethenates halogenated with bromine on both ends of the long axis and coordinated using boron fluoride, zinc, or cadmium in neutral ethanol and acidified with hydrochloric acid solutions were studied. The constants of the acid-base equilibrium of the complexes in the proton-donor solvents in the ground and excited states was determined. The mechanisms of complex protonation were discussed, depending on the structure of the compounds. The electronic structures of the neutral and protonated compounds were modeled and analyzed based on the quantum-chemical method. The structures and spectral-luminescence properties were calculated using the SMD model of ethanol solvent using the TD-DFT theory with the B3LYP functional and the composite def2-SVP/def2-TZVP/def2-TZVPP_ECP basis sets, depending on the atomic number of the elements.

Fully Symmetric Cyclodextrin Polycarboxylates: How to Determine Reliable Protonation Constants from NMR Titration Data

Acid-base properties of cyclodextrins (CDs), persubstituted at C-6 by 3-mercaptopropionic acid, sualphadex (Suα-CD), subetadex (Suβ-CD) and sugammadex (Suγ-CD, the antidote of neuromuscular blocking steroids) were studied by ¹H NMR-pH titrations. For each CD, the severe overlap in protonation steps prevented the calculation of macroscopic pK_a values using the standard data fitting model. Considering the full symmetry of polycarboxylate structures, we reduced the number of unknown NMR parameters in the "Q-fitting" or the novel "equidistant macroscopic" evaluation approaches. These models already provided pK_a values, but some of them proved to be physically unrealistic, deceptively suggesting cooperativity in carboxylate protonations. The latter problem could be circumvented by adapting the microscopic site-binding (cluster expansion) model by Borkovec, which applies pairwise interactivity parameters to quantify the mutual basicity-decreasing effect of carboxylate protonations. Surprisingly, only a single averaged interactivity parameter could be calculated reliably besides the carboxylate 'core' microconstant for each CD derivative. The speciation of protonation isomers hence could not be resolved, but the optimized microscopic basicity parameters could be converted to the following sets of macroscopic pK_a values: 3.84, 4.35, 4.81, 5.31, 5.78, 6.28 for Suα-CD; 3.82, 4.31, 4.73, 5.18, 5.64, 6.06, 6.54 for Suβ-CD and 3.83, 4.28, 4.65, 5.03, 5.43, 5.81, 6.18, 6.64 for Suγ-CD. The pH-dependent charge of these compounds can now be accurately calculated, in support of designing new analytical methods to exploit their charge-dependent molecular recognition such as in cyclodextrin-aided chiral capillary electrophoresis.

Accurate pKa Calculations in Proteins with Reactive Molecular Dynamics Provide Physical Insight Into the Electrostatic Origins of Their Values

Classical molecular dynamics simulations are a versatile tool in the study of biomolecular systems, but they usually rely on a fixed bonding topology, precluding the explicit simulation of chemical reactivity. Certain modifications can permit the modeling of reactions. One such method, multiscale reactive molecular dynamics, makes use of a linear combination approach to describe condensed-phase free energy surfaces of reactive processes of biological interest. Before these simulations can be performed, models of the reactive moieties must first be parametrized using electronic structure data. A recent study demonstrated that gas-phase electronic structure data can be used to derive parameters for glutamate and lysine which reproduce experimental pKa values in both bulk water and the staphylococcal nuclease protein with remarkable accuracy and transferability between the water and protein environments. In this work, we first present a new model for aspartate derived in similar fashion and demonstrate that it too produces accurate pKa values in both bulk and protein contexts. We also describe a modification to the prior methodology, involving refitting some of the classical force field parameters to density functional theory calculations, which improves the transferability of the existing glutamate model. Finally and most importantly, this reactive molecular dynamics approach, based on rigorous statistical mechanics, allows one to specifically analyze the fundamental physical causes for the marked pKa shift of both aspartate and glutamate between bulk water and protein and also to demonstrate that local steric and electrostatic effects largely explain the observed differences.

Cysteine Oxidation in Proteins: Structure, Biophysics, and Simulation

Cysteine side chains can exist in distinct oxidation states depending on the pH and redox potential of the environment, and cysteine oxidation plays important yet complex regulatory roles. Compared with the effects of post-translational modifications such as phosphorylation, the effects of oxidation of cysteine to sulfenic, sulfinic, and sulfonic acid on protein structure and function remain relatively poorly characterized. We present an analysis of the role of cysteine reactivity as a regulatory factor in proteins, emphasizing the interplay between electrostatics and redox potential as key determinants of the resulting oxidation state. A review of current computational approaches suggests underdeveloped areas of research for studying cysteine reactivity through molecular simulations.

Water Orientation at the Anatase TiO2 Nanoparticle Interface: A Probe of Surface pKa Values

The acid-base properties of surfaces significantly influence catalytic and (photo)electrochemical processes. Estimation of acid dissociation constants (pKa values) for colloids is commonly performed through electroanalytical techniques or spectroscopic methods employing label molecules. Here, we show that polarimetric angle-resolved second harmonic scattering (AR-SHS) can be used as an all-optical, label-free probe of colloid surface pKa values. We apply AR-SHS to dispersions of 100 nm anatase TiO2 particles to extract surface potential and surface susceptibility, a measure of interfacial water orientation, as a function of pH. The surface potential follows changes in surface charge density, while the interfacial water orientation inverts at pH ∼4.8, ∼6, and ∼7.6. As the variation in bulk pH modifies the populations of Ti-OH2+, Ti-OH, and Ti-O- interfacial groups, a change in water orientation reports on the ratio of protonated/deprotonated species. Such observation allows for pKa evaluation from plots of surface susceptibility versus pH. A Nerstian trend in the surface potential is additionally demonstrated.

Comparative photo-oxidative degradation of etodolac, febuxostat and imatinib mesylate by UV-C/H2O2 and UV-C/S2O82- processes: Modeling, treatment optimization and biodegradability enhancement

The pharmaceutical contamination in aquatic environment has arisen increasing concern due to its potentially chronic toxicity. In recent years, HO° and SO₄°^- based advanced oxidation processes (AOPs) have been widely applied in water and wastewater treatments due to their highly efficiency on contaminant removal. Here, the response surface modeling (RSM) was used to investigate the degradation of three typical pharmaceuticals (i.e., etodolac (ETD), febuxostat (FBU) and imatinib mesylate (IMT)) by UV/H₂O₂ and UV/S₂O₈^2- processes. Based on the multiple regression analysis on full factorial design matrix and calculated reaction rate constants, the RSM was built. The experimental rate constants under optimal conditions were quite close to those obtained from the model, implying the good fit of the RSM. In addition, the RSM results indicated that UV/S₂O₈^2- process was less sensitive to pH in comparison to the UV/H₂O₂ process on target contaminant removal. Finally, it showed that UV/S₂O₈^2- process was superior to the UV/H₂O₂ process to on the enhancement of target contaminant biodegradability.

Dielectric heating of highly corrosive and oxidizing reagents on a hybrid glass microfiber-polymer centrifugal microfluidic device

Many assays necessitate the use of highly concentrated acids, powerful oxidizing agents, or a combination of the two. Although microfluidic devices offer vast potential for rapid analytical interrogation at the point-of-need (PON), they cannot escape the fundamental requirement for reagent compatibility. Worse, many innovative protocols have been developed that would represent a significant improvement to current field-forward practices within their respective disciplines, but adoption falters due to chemical incompatibility with challenging reagents. Polymeric centrifugal microfluidic devices meet many of the needs for accommodating complex chemical or biochemical protocols in a multiplexed and automatable format. Yet, they also struggle to accommodate highly reactive chemical components long term. In this work, we report on a simple and inexpensive reagent storage strategy that bypasses the typical complexity involved with integration of liquid reagents on microfluidic devices. Moreover, we demonstrate microdevice compatibility and operation after six months of corrosive reagent storage as well as post dielectric heating. This new strategy allows for storage of multiple highly corrosive and oxidative reagents simultaneously, enhancing the possibilities for multistep assay integration at the PON for a diverse array of applications. Successful detection after one week of corrosive reagent storage of an illicit drug and neurotransmitter metabolite, for forensic and clinical applications, is demonstrated. Furthermore, environmental sample preparation via microwave-assisted wet acid digestion is performed on-disc and integrated with downstream detection. Quantitative detection of a heavy metal in soil is achieved by way of on-disc calibration and found to be accurate within 2.4% compared to a gold standard reference (ICP-OES).

Lower degree of dissociation of pyruvic acid at water surfaces than in bulk

Understanding the acid/base behavior of environmentally relevant organic acids is of key relevance for accurate climate modelling. Here we investigate the effect of pH on the (de)protonation state of pyruvic acid at the air-water interface and in bulk by using the analytical techniques surface-specific vibrational sum frequency generation and attenuated total reflection spectroscopy. To provide a molecular interpretation of the observed behavior, simulations are carried out using a free energy perturbation approach in combination with electronic structure-based molecular dynamics. In both the experimental and theoretical results we observe that the protonated form of pyruvic acid is preferred at the air-water interface. The increased proton affinity is the result of the specific microsolvation at the interface.

High CT-Fluorophore Featuring a Basic Moiety into D-A Chain as a pKa Probe

The determination of acidity represents a significant challenge within fluorometry, and no effective strategy has been developed successfully yet. It is attributed to the fact that acidity tends to be enhanced upon excitation, giving, in general, an overestimation of the ionization constant, pK_a. Herein, we developed a strategy for pK_a estimation of Brønsted acids in solution through fluorometry by using a convenient pK_a probe, N¹-aryl-7-methoxy-2-(trifluoromethyl)benzo[b][1,8]naphthyridin-4(1H)-one. It allowed us to obtain a linear log K_SV versus pK_a correlation derived from the selective quenching response of the probe by an interaction with different Brønsted acids. The key points of N¹-aryl-7-methoxy-2-(trifluoromethyl)benzo[b][1,8]naphthyridin-4(1H)-one as a pK_a probe were (i) the location of a weak basic moiety in the donor-acceptor chain of the fluorophore, which favors a selective quenching of the intramolecular charge-transfer process according to the acidity of acid, and (ii) the high CT character upon excitation that promotes higher quenching magnitudes and favors a wider pK_a range (19.5pK_a) for the log K_SV versus pK_a correlation. Other key principles were to delimit the study to pure proton transfer and nonfluorescent acids, which allowed restricting the quenching response to a process dependent mainly on the acid-base equilibrium. All these findings open a new perspective as a proof of concept to design effective fluorescent pK_a probes.

Prediction of acid pKa values in the solvent acetone based on COSMO-RS

In this contribution we extent the use of the conductor-like screening model for realistic solvation (COSMO-RS) to the prediction of pK_a values in acetone, a commonly used dipolar aprotic solvent. For this, we calculated the Gibbs free energy of dissociation of 120 organic acids (nine acrylic acids, 87 benzoic acids, nine phenols, and 15 benzenesulfonamides) using COSMO-RS at the two levels BP-TZVP and BP-TZVPD-FINE and determined the parameters for a linear free energy relation for the p K a prediction by performing linear fits to experimental values. Our results suggest that the data set dissects into two groups, with the phenols being different from the other three subsets. The acrylic and benzoic acids and the sulfonamides can be treated together and yield an excellent linear correlation ( r 2 > 0.95 ) with an RMSD of only ~0.3. The slope is found to be significantly smaller than the theoretical value ( 1 / RT ln 10 ), only 45% of it, which is in accordance with findings in the literature. The phenols, however, while similarly well correlated in their own subset with an RMSD of 1.7-1.9, exhibit a slope larger than one. We discuss both a higher uncertainty in the reference values as well as physical origins as possible reasons.

Characterization of nanoparticles-based vaccines for COVID-19

Several vaccines against COVID-19 use nanoparticles to protect the antigen cargo (either proteins or nucleic acids), increase the immunogenicity and ultimately the efficacy. The characterization of these nanomedicines is challenging due to their intrinsic complexity and requires the use of multidisciplinary techniques and competencies. The accurate characterization of nanovaccines can be conceptualized as a combination of physicochemical, immunological and toxicological assays. This will help to address key challenges in the preclinical characterization, will guide the rapid development of safe and effective vaccines for current and future health crises, and will streamline the regulatory process.

Surface processing for iron-based degradable alloys: A preliminary study on the importance of acid pickling

The formation of a heterogeneous oxidized layer, also called scale, on metallic surfaces is widely recognized as a rapid manufacturing event for metals and their alloys. Partial or total removal of the scale represents a mandatory integrated step for the industrial fabrication processes of medical devices. For biodegradable metals, acid pickling has already been reported as a preliminary surface preparation given further processes, such as electropolishing. Unfortunately, biodegradable medical prototypes presented discrepancies concerning acid pickling studies based on samples with less complex geometry (e.g., non-uniform scale removal and rougher surface). Indeed, this translational knowledge lacks a detailed investigation on this process, deep characterization of treated surfaces properties, as well as a comprehensive discussion of the involved mechanisms. In this study, the effects of different acidic media (HCl, HNO3, H3PO4, CH3COOH, H2SO4 and HF), maintained at different temperatures (21 and 60 °C) for various exposition time (15-240 s), on the chemical composition and surface properties of a Fe-13Mn-1.2C biodegradable alloy were investigated. Changes in mass loss, morphology and wettability evidenced the combined effect of temperature and time for all conditions. Pickling in HCl and HF solutions favor mass loss (0.03-0.1 g/cm2) and effectively remove the initial scale.

Determination of proton dissociation constants (pKa) of hydroxyl groups of 2,5-dihydroxy-1,4-benzoquinone (DHBQ) by UV-Vis, fluorescence and ATR-FTIR spectroscopy

The dissociation constant is an important physicochemical parameter of amolecule. The protonation state of a molecule reflects its reactivity, solubility or ability to chemically interact with other molecules. In the present study, dissociation constants (pK_a) values of 2,5-dihydroxy-1,4-benzoquinone (DHBQ) were determined by UV-Vis, fluorescence and ATR-FTIR spectroscopy at 25 °C. The resulting pK_a values for DHBQ were 2.95 and 5.25. We have also experimentally found out that the monoanionic form (HBQ^-) provides weak fluorescence in the pH range of about 3-6. This allowed us to determine not only the pK_a in the ground but also the excited state of the molecule (pK_a1* = 4.38 andpK_a2* = 5.27).

Low Voltage Voltammetry Probes Proton Dissociation Equilibria of Amino Acids and Peptides

Platinum-catalyzed electrochemical reduction of dissociable protons at low potentials was used to investigate proton dissociation equilibria of freely diffusing and peptide-incorporated charged amino acids. We first demonstrate with five charged essential amino acids and their analogs that the electrochemically induced deprotonation of each amino acid occurs at distinct formal reduction potential. Moreover, the observed direct reduction for all the charged species, excluding arginine, occurs at low potentials suitable for investigation under aqueous conditions (-0.4 to -0.9 V vs Ag/AgCl). The direct proton reduction was resolved via deconvolution of the observed differential pulse voltammogram (DPV) from background hydronium reduction and water electrolysis. A linear correlation was found between the formal reduction potentials and the pK_a values of the dissociable protons hosted by various molecular moieties in the amino acids and their analogs and further verified with tripeptides. DPV of poly(l-lysine) decamer (Lys₁₀) distinctively resolved the pK_a values of the amino groups in the side chains and N-terminus, at a resolution not possible by conventional acid-base titration. This work demonstrates selective electrochemical titration of dissociable protons in charged amino acids in the free state and as residues in biomolecules, as well as the utility of DPV to indirectly interrogate local electrostatic environments that are essential to the stability and function of biomolecules.

Calculations of pKa Values for a Series of Naturally Occurring Modified Nucleobases

Modified nucleobases are found in functionally important regions of RNA and are often responsible for essential structural roles. Many of these nucleobase modifications are dynamically regulated in nature, with each modification having a different biological role in RNA. Despite the high abundance of modifications, many of their characteristics are still poorly understood. One important property of a nucleobase is its pK_a value, which has been widely studied for unmodified nucleobases, but not for the modified versions. In this study, the pK_a values of modified nucleobases were determined by performing ab initio quantum mechanical calculations using a B3LYP density functional with the 6-31+G(d,p) basis set and a combination of implicit-explicit solvation systems. This method, which was previously employed to determine the pK_a values of unmodified nucleobases, is applicable to a variety of modified nucleobases. Comparisons of the pK_a values of modified nucleobases give insight into their structural and energetic impacts within nucleic acids.

The pH and Potential Dependence of Pb-Catalyzed Electrochemical CO2 Reduction to Methyl Formate in a Dual Methanol/Water Electrolyte

The conversion of waste CO₂ to value-added chemicals through electrochemical reduction is a promising technology for mitigating climate change while simultaneously providing economic opportunities. The use of non-aqueous solvents like methanol allows for higher CO₂ availability and novel products. In this work, the electrochemistry of CO₂ reduction in acidic methanol catholyte at a Pb working electrode was investigated while using a separate aqueous anolyte to promote a sustainable water oxidation half-reaction. The selectivity among methyl formate (a product unique to reduction of CO₂ in methanol), formic acid, and formate was critically dependent on the catholyte pH, with higher pH conditions leading to formate and low pH favoring methyl formate. The potential dependence of the product distribution in acidic catholyte was also investigated, with a faradaic efficiency for methyl formate as high as 75 % measured at -2.0 V vs. Ag/AgCl.

Capillary electrophoresis applied for the determination of acidity constants and limiting electrophoretic mobilities of ionizable herbicides including glyphosate and its metabolites and for their simultaneous separation

Thermodynamic acidity constants and limiting ionic mobilities were determined for polyprotic non-chromophore analytes using capillary electrophoresis with capacitively coupled contactless conductivity detection. It was not necessary to work with buffers of identical ionic strength as ionic strength effects on effective electrophoretic mobilities were corrected by modeling during data evaluation (software AnglerFish). The mobility data from capillary electrophoresis coupled to conductivity detection were determined in the pH range from 1.25 to 12.02 with a high resolution (36 pH steps). With this strategy, thermodynamic acidity constants and limiting ionic mobilities for various acidic herbicides were determined, sometimes for the first time. The model analytes included glyphosate, its metabolites, and its acetylated derivates (aminomethyl phosphonic acid, glyoxylic acid, sarcosine, glycine, N-acetyl glyphosate, N-acetyl aminomethyl phosphonic acid, hydroxymethyl phosphonic acid). The obtained data were used in simulations to optimize separations by capillary electrophoresis. Simulations correlated very well to experimental results. With the new method, the separation of glyphosate from interfering components like phosphate in beer samples was possible.

Role of entropy in determining the phase behavior of protein solutions induced by multivalent ions

Recent experiments have reported lower critical solution temperature (LCST) phase behavior of aqueous solutions of proteins induced by multivalent ions, where the solution phase separates upon heating. This phenomenon is linked to complex hydration effects that result in a net entropy gain upon phase separation. To decipher the underlying molecular mechanism, we use all-atom molecular dynamics simulations along with the two-phase thermodynamic method for entropy calculation. Based on simulations of a single BSA protein in various salt solutions (NaCl, CaCl₂, MgCl₂, and YCl₃) at temperatures (T) ranging 283-323 K, we find that the cation-protein binding affinity increases with T, reflecting its thermodynamic driving force to be entropic in origin. We show that in the cation binding process, many tightly bound water molecules from the solvation shells of a cation and the protein are released to the bulk, resulting in entropy gain. To rationalize the LCST behavior, we calculate the ζ-potential that shows charge inversion of the protein for solutions containing multivalent ions. The ζ-potential increases with T. Performing simulations of two BSA proteins, we demonstrate that the protein-protein binding is mediated by multiple cation bridges and involves similar dehydration effects that cause a large entropy gain which more than compensates for rotational and translational entropy losses of the proteins. Thus, the LCST behavior is entropy-driven, but the associated solvation effects are markedly different from hydrophobic hydration. Our findings have direct implications for tuning the phase behavior of biological and soft-matter systems, e.g., protein condensation and crystallization.

Chlorination-induced pKa decrease: an improved strategy to design ratiometric hypochlorite fluorescent probes with ideally high selectivity

Based on the ClO−-triggered chlorination reaction and the subsequent pKa decrease, a two-photon ratiometric ClO− fluorescent probe was designed.