pH Titration Experiments Research Articles

Distinguishing normal cells from cancer cells via lysosome-targetable pH biomarkers with benzo[a]phenoxazine skeleton

In this paper, the design of a lysosome-targetable pH probe that has a fluorescent OFF (pH = 4) to ON (pH = 5–6) response is described to identify lysosomes in normal cells. The mechanism of photoinduced electron transfer with a fluorophore-based reaction (FBR-PET) was proposed. Benzo[a]phenoxazines with electro-donating aryl groups were selected, its (2,5-dimethoxyphenyl)imino-, (2-hydroxyphenyl)imino- and (2-hydroxy-5-methoxyphenyl)- imino-derivatives (probes 1a−c) were prepared and their optical responses towards pH were evaluated; their fluorescence pH titration experiments gave regularly changes with the increasing electro-donating abilities at the linked aryl groups, the (2-hydroxy-5-methoxyphenyl)iminobenzo[a]phenoxazine (probe 1c) exhibited a nearly OFF−ON response at 580–800 nm. All probes were reversible, and they showed excellent selectivity toward the proton over other competitive species. Fluorescence confocal images were performed with HeLa, KB cancer cells and V79 normal cells, probes 1a−c are all lysosome-targetable pH probes, and benzo[a]phenoxazine with (2-hydroxy-5-methoxyphenyl)imino-group (probe 1c) has potential applications in selective differentiation of normal cells from cancer cells.

Functional and Photochemical Characterization of a Light-Driven Proton Pump from the Gammaproteobacterium Pantoea vagans.

Photoactive retinal proteins are widely distributed throughout the domains of the microbial world (i.e., bacteria, archaea, and eukarya). Here we describe three retinal proteins belonging to a phylogenetic clade with a unique DTG motif. Light-induced decrease in the environmental pH and its inhibition by carbonyl cyanide m-chlorophenylhydrazone revealed that these retinal proteins function as light-driven outward electrogenic proton pumps. We further characterized one of these proteins, Pantoea vagans rhodopsin (PvR), spectroscopically. Visible spectroscopy and high-performance liquid chromatography revealed that PvR has an absorption maximum at 538 nm with the retinal chromophore predominantly in the all-trans form (>90%) under both dark and light conditions. We estimated the pKa values of the protonated Schiff base of the retinal chromophore and its counterion as approximately 13.5 and 2.1, respectively, by using pH titration experiments, and the photochemical reaction cycle of PvR was measured by time-resolved flash-photolysis in the millisecond timeframe. We observed a blue-shifted and a red-shifted intermediate, which we assigned as M-like and O-like intermediates, respectively. Decay of the M-like intermediate was highly sensitive to environmental pH, suggesting that proton uptake is coupled to decay of the M-like intermediate. From these results, we propose a putative model for the photoreaction of PvR.

Crystal Structures and Coordination Behavior of Aqua- and Cyano-Co(III) Tetradehydrocorrins in the Heme Pocket of Myoglobin.

Myoglobins reconstituted with aqua- and cyano-Co(III) tetradehydrocorrins, rMb(Co(III)(OH2)(TDHC)) and rMb(Co(III)(CN)(TDHC)), respectively, were prepared and investigated as models of a cobalamin-dependent enzyme. The former protein was obtained by oxidation of rMb(Co(II)(TDHC)) with K3[Fe(CN)6]. The cyanide-coordinated Co(III) species in the latter protein was prepared by ligand exchange of rMb(Co(III)(OH2)(TDHC)) with exogenous cyanide upon addition of KCN. The X-ray crystallographic study reveals the hexacoordinated structures of rMb(Co(III)(OH)(TDHC)) and rMb(Co(III)(CN)(TDHC)) at 1.20 and 1.40 Å resolution, respectively. The (13)C NMR chemical shifts of the cyanide in rMb(Co(III)(CN)(TDHC)) were determined to be 108.6 and 110.6 ppm. IR measurements show that the cyanide of rMb(Co(III)(CN)(TDHC)) has a stretching frequency peak at 2151 cm(-1) which is higher than that of cyanocobalamin. The (13)C NMR and IR measurements indicate weaker coordination of the cyanide to Co(III)(TDHC) relative to cobalamin, a vitamin B12 derivative. Thus, the extent of π-back-donation from the cobalt ion to the cyanide ion is lower in rMb(Co(III)(CN)(TDHC)). Furthermore, the pK(1/2) values of rMb(Co(III)(OH2)(TDHC)) and rMb(Co(III)(CN)(TDHC)) were determined by a pH titration experiment to be 3.2 and 5.5, respectively, indicating that the cyanide ligation weakens the Co-N(His93) bond. Theoretical calculations also demonstrate that the axial ligand exchange from water to cyanide elongates the Co-N(axial) bond with a decrease in the bond dissociation energy. Taken together, the cyano-Co(III) tetradehydrocorrin in myoglobin is appropriate for investigation as a structural analogue of methylcobalamin, a key intermediate in methionine synthase reaction.

Ratiometric fluorescence detection of Hg(II) in aqueous solutions at physiological pH and live cells with a chemosensor based on tyrosine

A fluorescent chemosensor (Pyr-Tyr) based on tyrosine showed highly selective and sensitive ratiometric response to Hg(II) among 14 metal ions in buffered aqueous solutions. The emission intensity ratio at 480 and 383nm increased significantly from 0 to about 1.5 with the increase of Hg(II) concentrations (0–1equiv.) and the ratiometric response to Hg(II) was not interfered by other metal ions such as Cu(II), Cd(II), and Ag(I). The detection limit of the chemosensor for Hg(II) was calculated to be 12±1nM (2.4ppb). Pyr-Tyr exhibited ratiometric responses to Hg(II) in a wide range of pH values (pH 4.5–11.5) and the chemosensor was demonstrated to detect intracellular Hg(II) in HeLa cells. The binding mode of the chemosensor with Hg(II) was investigated by 1H NMR titration and pH titration experiments, and ESI mass spectrometry.

A ratiometric fluorescent detection of Zn(II) in aqueous solutions using pyrene-appended histidine

A fluorescent chemosensor, Py-His, based on histidine was easily synthesized in solid phase synthesis. Py-His displayed a highly sensitive ratiometric response to Zn(II) with potent binding affinity (Ka=1.17×1013M−2) in aqueous solutions. The detection limit of Py-His for Zn(II) was calculated as 80.8nM. Moreover, Py-His distinguished Zn(II) and Hg(II) by different ratiometric response type; the chemosensor showed a more enhanced increase of excimer emission intensity to Zn(II) than Hg(II). Upon addition of Ag(I) and Cu(II), Py-His showed a turn-off response mainly due to the quenching effect of these metal ions. The binding stoichiometry (2:1 or 1:1) of Py-His to target metal ions played a critical role in the fluorescent response type (ratiometric and turn off response) to target metal ions. The role of imidazole group of Py-His for ratiometric detection of Zn(II) was proposed by pH titration experiments.

A new peptidyl fluorescent chemosensors for the selective detection of mercury ions based on tetrapeptide

A novel peptidyl chemosensor (PySO2-His-Gly-Gly-Lys(PySO2)-NH2, 1) was synthesized by incorporation of two pyrene (Py) fluorophores into the tetrapeptide using sulfonamide group. Compound 1 exhibited selective fluorescence response towards Hg(II) over the other metal ions in aqueous buffered solutions. Furthermore, 1 with the potent binding affinity (Kd=120nM) for Hg(II) detected Hg(II) without interference of other metal ions such as Ag(I), Cu(II), Cd(II), and Pb(II). The binding mode of 1 with Hg(II) was investigated by UV absorbance spectroscopy, 1H NMR titration experiment, and pH titration experiment. The addition of Hg(II) induced a significant decrease in both excimer and monomer emissions of the pyrene fluorescence. Hg(II) interacted with the sulfonamide groups and the imidazole group of His in the peptidyl chemosensor and then two pyrene fluorophores were close to each other in the peptide. The decrease of both excimer and monomer emission was mainly due to the excimer/monomer emission change by dimerization of two pyrene fluorophores and a quenching effect of Hg(II).

Facile synthesis of gold core–polymer shell responsive particles

The free adsorption of an end-functionalised weak polybase, poly dimethylaminoethyl methacrylate (pDMAEMA), on the surface of colloidal gold nanoparticles (AuNPs) as a route to produce a responsive core–shell nanoparticle is explored here. Optimal conditions for the physisorption of the polymeric chains onto the colloidal nanoparticles are explored. A dense coverage is facilitated by rapidly mixing the well solvated pH responsive homopolymer, at low pH, into a relatively poor solvent environment, at higher pH, containing a stable dispersion of charge-stabilised gold nanoparticles. The rapid pH change causes the polymer chains to concurrently collapse and adsorb onto the gold nanoparticles. In order to achieve sterically stable, monodisperse and responsive core shell nanoparticles, a crucial factor is the pH difference of the systems prior to their mixing. Once adsorbed, end-functional thiol groups on the adsorbed polymer chains can form more permanent covalent attachments with the core particles. Dynamic light scattering coupled with mobility data of pH titration experiments show that the core–shell particles exhibit a responsive character consistent with the observed potentiometric titration data of the polymer. The same particles demonstrate reversible aggregation when cycled between pH extremes. This is confirmed by shifts in the SPR peak of the corresponding UV–Vis absorption profile. The ease and flexibility of this strategy for core–shell particle production, coupled with the stability and responsiveness of the product, make this a promising colloidal coating mechanism.

Structural and spectral characterizations of C.I. Disperse Blue 148 having a new crystalline form

Single-crystal structures of the best high-temperature trichromatic blue azo dye C.I. Disperse Blue 148 and its diazonium component 3-amino-5-nitro-[2,1]- benzisothiazole are described herein. C.I. Disperse Blue 148 exhibits an essentially coplanar molecular structure and a dimeric packing mode between adjacent phenyl and benzisothiazole rings. Particularly, PXRD measurement reveals the existence of a new crystalline form of C.I. Disperse Blue 148 having the highest melting point and the best thermal stability, which is obviously different from all the known α, β, and γ forms. A one-phase recrystallization method, which can guarantee the complete transformation, is used to prepare the new crystalline form instead of the conventional heating of an aqueous suspension of α form. Furthermore, TGA–DSC and pH titration experiments have been carried out to reveal the thermal and pH stabilities of this heterocyclic dye. To our knowledge, this is the first structural example on benzisothiazole involved azo dyes.

Electrochemical water splitting by layered and 3D cross-linked manganese oxides: correlating structural motifs and catalytic activity

Manganese based precious metal-free electrocatalysts for the oxygen evolution reaction (OER) are promising materials for energy storage systems based on dark or photo-coupled water electrolysis, because they are active, inexpensive and of low toxicity. In this work, atomic scale structure–activity relationships of two different nano-structured manganese oxides, MnOx, are established using a combination of X-ray absorption, diffraction and electrochemistry. Prepared by chemical symproportionation (s-MnOx) and impregnation (i-MnOx), the s-MnOx catalyst consisted of a layered structure similar to δ-MnO2 while the i-MnOx catalyst displayed a mixture of tunnelled, 3D cross-linked β- and defective γ-MnO2 structures. During electrocatalytic oxygen evolution the structural motifs of both MnOx remain largely unchanged, but the oxidation state of Mn increases from 3.5 to 3.9–4. Kinetic parameters of the electrocatalytic oxygen evolution reaction were extracted using Tafel slope analysis and pH titration experiment, and the role of the protons abstracted was analyzed. The study reveals fundamental differences of general importance in the catalytic activity between layered and cross-linked structures. The exclusive presence of di-μ-oxo-bridged Mn ions in the layered structure is coupled to a pronounced redox and charge capacity behaviour. This ensured efficient use of surface and bulk active sites, and resulted in a relatively large Tafel slope. Consequently, the intrinsic OER activity is especially high in s-MnOx. In contrast, 3D cross-linked structures with both mono- and di-μ-oxo-bridged Mn ions resulted in lower intrinsic activity but smaller Tafel slope, and thus favourable activity at technological water-splitting rates. The insights from this comparative study will provide guidance in the structural design and optimization of other non precious metal oxide OER catalysts.

Critical Role of Asp227 in the Photocycle of Proteorhodopsin

The photocycle of the proton acceptor complex mutant D227N of the bacterial retinal protein proteorhodopsin is investigated employing steady state pH-titration experiments in the UV-visible range as well as femtosecond-pump-probe spectroscopy and flash photolysis in the visible spectral range. The evaluation of the pH-dependent spectra showed that the neutralization of the charge at position 227 has a remarkable influence on the ground state properties of the protein. Both the pK(a) values of the primary proton acceptor and of the Schiff base are considerably decreased. Femtosecond-time-resolved measurements demonstrate that the general S(1) deactivation pathway; that is, the K-state formation is preserved in the D227N mutant. However, the pH-dependence of the reaction rate is lost by the substitution of Asp227 with an asparagine. Also no significant kinetic differences are observed upon deuteration. This is explained by the lack of a strongly hydrogen-bonded water in the vicinity of Asp97, Asp227, and the Schiff base or a change in the hydrogen bonding of it (Ikeda et al. (2007) Biochemistry 46, 5365-5373). The flash photolysis measurements prove a considerably elongated photocycle with pronounced pH-dependence. Interestingly, at pH 9 the M-state is visible until the end of the reaction cycle, leading to the conclusion that the mutation does not only lower the pK(a) of the Schiff base in the unphotolyzed ground state but also prevents an efficient reprotonation reaction.

Azo-hydrazone tautomerism observed from UV-vis spectra by pH control and metal-ion complexation for two heterocyclic disperse yellow dyes

The azo-hydrazone tautomerism of two pyridine-2,6-dione based Disperse Yellow dyes has been achieved by pH control and metal-ion complexation, respectively, which is evidenced by UV-visible spectra using pH-titration, (1)H NMR and X-ray single-crystal diffraction techniques for two dyes and one neutral dinuclear dye-metal complex. pH-titration experiments under strong and weak acidic conditions (HCl and HOAc) as well as strong and weak alkaline conditions (NaOH and ammonia) demonstrate that there is an equilibrium between the azo (HL(1-A) and HL(2-A)) and hydrazone (HL(1-H) and HL(2-H)) tautomers for two dyes in solution but the hydrazone form is dominant under conventional conditions. The hydrazone proton is also observed in the (1)H NMR spectra of HL(1-H) and HL(2-H) which can be verified by the hydrogen-deuterium exchange and the presence of cooperative six-membered intramolecular hydrogen rings involving the hydrazone proton in their X-ray single-crystal structures. Moreover, the azo-hydrazone tautomerism is evidenced by the formation of a novel neutral dinuclear dye-metal complex Cu(2)(L(2-A))(4), where all the ligands are in the azo form and two types of coordination modes are present for four L(2-A) ligands. Namely, the side two ligands serve as the bidentate capping ligands, while the middle ones act as the quadridentate bridging ligands linking adjacent Cu(II) centers in a reverse fashion.

Evaluation of the pKa values and ionization sequence of bumetanide using 1H and 13C NMR and UV spectroscopy

AbstractNon‐thiazide loop diuretics such as bumetanide and furosemide are well‐established medicaments, with varying substituents that alter their chemical properties in ways that affect biological parameters, including biodistribution, efficacy, and safety. However, literature data supporting the assignment of pKa values and ionization sequence of bumetanide are limited. The present study summarizes available literature data and then characterizes nuclear magnetic resonance (NMR) and ultraviolet (UV) spectral changes over a range of pH values to delineate the apparent sequence of deprotonations. Three macroscopic pKa values, 1.44, 3.74, and 10.37, were determined in water via pH titration experiments monitored by 1H NMR. Both 1H and 13C analyses support assignment of the lowest pKa value to the anilinium ion, as described for another diuretic, piretanide. However, UV data suggest that initial deprotonation of the benzoic acid competes to some extent with deprotonation of the anilinium ion, such that both pathways may operate simultaneously in an aqueous environment. Thus, the observed pKa values likely represent the macroscopic averages of the two competing deprotonation sequences. Calculated pKa values for bumetanide, piretanide, and model compounds are contrary to the deprotonation sequence we and others have determined experimentally, which is not surprising given the complicated factors driving the acid–base properties of these molecules. Drug Dev Res 72: 416–426, 2011. © 2011 Wiley‐Liss, Inc.

Conformational Properties of Polyglutamine Sequences Under Denaturing Conditions

A number of diseases are caused by polyglutamine sequences in proteins such as Huntington's disease, spinocerebellar ataxia and many more. A better insight into the behavior of polyglutamine sequences is essential for understanding their role in diseases. Therefore, it is worth studying the behavior of polyglutamine sequences in one of the early steps of protein folding, i.e. loop formation. To this end, we are looking into the thermodynamics and kinetics of loop formation of polyglutamine sequences using engineered yeast iso-1-cytochrome c, which forms a loop between His(-2) and the heme in the denatured state. The His(-2) is part of a five amino acid N-terminal extension which is not found in eukaryotic cytochrome c and readily accepts homopolymeric inserts. All the inserts are between His(-2) and Ala(-1). Five variants were made in such a way that each variant contains 1 to 5 sets of the sequence QQQQQK. The lysine is present in order to maintain solubility. Stability of these variants was measured by CD spectroscopy and is compared with the previously reported polyalanine inserts [Tzul, F. O and Bowler, B. E. Proc. Natl. Acad. Sci. USA, 107, 11364-9 (2010)]. pH titration experiments were performed in 3 M and 6 M guanidine-HCl to measure equilibrium His-heme loop formation with all variants. The scaling exponents for loop formation are compared with those of polyalanine inserts in 3 M and 6 M guanidine-HCl, respectively. Kinetics of loop formation and loop breakage for polyglutamine variants were performed by pH jump methods in both 3 M and 6 M GdnHCl. The kinetics are compared with the previously reported data for polyalanine sequences and with data for His-heme loop formation in the denatured state of Rhodopseudomonas palustris cytochrome c'.

PH and salt effects on the aggregation behaviour of star polymer with G1 polyamidoamine core and terminal amphiphilic blocks

In this study, the pH and salt effects on the aggregation behaviour of star polymer, with G1 polyamidoamine as the core and poly(propylene oxide)-block-poly(ethylene oxide) as the terminal branches, were investigated. The results from pH and turbidimetric titration experiments showed that with the pH value increasing from 2.0 to 10.0, the solution transforms from transparent to turbid at pK a = 5.5 and eventually presents phase separation at the pH value above 7.5. As revealed by dynamic light scattering and transmission electron microscopy measurements, the size of the aggregates of star polymer becomes bigger with the increase in the pH value or in the presence of salt. It was found that the extent of the salt effect on the aggregation behaviour follows the sequence: C6H5COONa>Na2SO4 > NaCl.

.BETA.-Citryl-L-glutamate Is an Endogenous Iron Chelator That Occurs Naturally in the Developing Brain

The compound beta-citryl-L-glutamate (beta-CG) was initially isolated from developing brains, while it has also been found in high concentrations in testes and eyes. However, its functional roles are unclear. To evaluate its coordination with metal ions, we performed pH titration experiments. The stability constant, logbeta(pqr) for M(p)(beta-CG)(q)H(r) was calculated from pH titration data, which showed that beta-CG forms relatively strong complexes with Fe(III), Cu(II), Fe(II) and Zn(II). beta-CG was also found able to solubilize Fe more effectively from Fe(OH)(2) than from Fe(OH)(3). Therefore, we examined the effects of beta-CG on Fe-dependent reactive oxygen species (ROS)-generating systems, as well as the potential ROS-scavenging activities of beta-CG and metal ion-(beta-CG) complexes. beta-CG inhibited the Fe-dependent degradation of deoxyribose and Fe-dependent damage to DNA or plasmid DNA in a dose-dependent manner, whereas it had no effect on Cu-mediated DNA damage. In addition, thermodynamic data showed that beta-CG in a physiological pH solution is an Fe(II) chelator rather than an Fe(III) chelator. Taken together, these findings suggest that beta-CG is an endogenous low molecular weight Fe chelator.

Titration_DB: Storage and analysis of NMR‐monitored protein pH titration curves

NMR-monitored pH titration experiments are routinely used to measure site-specific protein pKa values. Accurate experimental pKa values are essential in dissecting enzyme catalysis, in studying the pH-dependence of protein stability and ligand binding, in benchmarking pKa prediction algorithms, and ultimately in understanding electrostatic effects in proteins. However, due to the complex ways in which pH-dependent electrostatic and structural changes manifest themselves in NMR spectra, reported apparent pKa values are often dependent on the way that NMR pH-titration curves are analyzed. It is therefore important to retain the raw NMR spectroscopic data to allow for documentation and possible re-interpretation. We have constructed a database of primary NMR pH-titration data, which is accessible via a web interface. Here, we report statistics of the database contents and analyze the data with a global perspective to provide guidelines on best practice for fitting NMR titration curves. Titration_DB is available at http://enzyme.ucd.ie/Titration_DB. Proteins 2010. (c) 2009 Wiley-Liss, Inc.

Control of Periplasmic Interdomain Thiol:Disulfide Exchange in the Transmembrane Oxidoreductase DsbD

The bacterial protein DsbD transfers reductant from the cytoplasm to the otherwise oxidizing environment of the periplasm. This reducing power is required for several essential pathways, including disulfide bond formation and cytochrome c maturation. DsbD includes a transmembrane domain (tmDsbD) flanked by two globular periplasmic domains (nDsbD/cDsbD); each contains a cysteine pair involved in electron transfer via a disulfide exchange cascade. The final step in the cascade involves reduction of the Cys103-Cys109 disulfide of nDsbD by Cys461 of cDsbD. Here we show that a complex between the globular periplasmic domains is trapped in vivo only when both are linked by tmDsbD. We have found previously (Mavridou, D. A., Stevens, J. M., Ferguson, S. J., & Redfield, C. (2007) J. Mol. Biol.370 ,643 -65817544440) that the attacking cysteine (Cys461) in isolated cDsbD has a high pKa value (10.5) that makes this thiol relatively unreactive toward the target disulfide in nDsbD. Here we show using NMR that active-site pKa values change significantly when cDsbD forms a complex with nDsbD. This modulation of pKa values is critical for the specificity and function of cDsbD. Uncomplexed cDsbD is a poor nucleophile, allowing it to avoid nonspecific reoxidation; however, in complex with nDsbD, the nucleophilicity of cDsbD increases permitting reductant transfer. The observation of significant changes in active-site pKa values upon complex formation has wider implications for understanding reactivity in thiol:disulfide oxidoreductases.

Solution NMR Structure of Selenium-binding Protein from Methanococcus vannielii

Selenium is an important nutrient. The lack of selenium will suppress expression of various enzymes that will lead to cell abnormality and diseases. However, high concentrations of free selenium are toxic to the cell because it adversely affects numerous cell metabolic pathways. In Methanococcus vannielii, selenium transport in the cell is established by the selenium-binding protein, SeBP. SeBP sequesters selenium during transport, thus regulating the level of free selenium in the cell, and delivers it specifically to the selenophosphate synthase enzyme. In solution, SeBP is an oligomer of 8.8-kDa subunits. It is a symmetric pentamer. The solution structure of SeBP was determined by NMR spectroscopy. Each subunit of SeBP is composed of an alpha-helix on top of a 4-stranded twisted beta-sheet. The stability of the five subunits stems mainly from hydrophobic interactions and supplemented by hydrogen bond interactions. The loop containing Cys(59) has been shown to be important for selenium binding, is flexible, and adopts multiple conformations. However, the cysteine accessibility is restricted in the structure, reducing the possibility of the binding of free selenium readily. Therefore, a different selenium precursor or other factors might be needed to facilitate opening of this loop to expose Cys(59) for selenium binding.

Effect of solvent influence on J-aggregate of tetra- p-hydroxyphenylporphyrin (THPP) under different pH

In this paper, tetra- p-hydroxyphenylporphyrin (THPP) is dissolved in different solvents (chloroform, ethanol and N, N-dimethylformamide(DMF), Solvent effect is closely related to the nature and extent of solute–solvent interactions. Solvents have great influence on spectral characteristic and physicochemical processes of THPP. It is commonly acknowledged that solvent-dependent spectral shifts can arise from either general or specific solvent effects in the electronic transitions of solutes. For porphyrin, the solute–solvent interaction promotes conformational distortion of porphyrin plane in solution. Based on observed photophysical behavior in different polar solvent, the porphyrin can be grouped into “normal” and “perturbed” categories. The distortion of porphyrin plane causes redistribution of electron density, which changes the acid and basic properties of pyrrolic nitrogen atoms in the center of porphyrin ring. Furthermore, the solvent effects influence aggregate properties of porphyrin in solution. We confirm the results by the pH-titration experiment in the three solvent and investigate the difference of structure between J-aggregate and monomer by the Raman.

Hydronium and hydroxide at the interface between water and hydrophobic media

The behavior of hydronium and hydroxide ions at the water/alkane, water/vapor, and water/rigid wall interfaces was investigated by means of molecular dynamics simulations. All these interfaces exhibit a strong affinity for hydronium, which is in agreement with spectroscopic and low pH zeta-potential measurements. Except for the water/rigid wall interface, which strongly structures water and weakly attracts OH(-), none of the other investigated interfaces shows an appreciable accumulation of hydroxide. This computational result is at odds with the interpretation of higher pH zeta-potential and titration experiments, however, it is supported by surface selective spectroscopies of the surface of water and hydroxide solutions.