Protonation Equilibria Research Articles

Protonation equilibria of glycine, 1,10-phenanthroline and 2,2-bipyridyl in ethylene glycol–water mixtures

The solute–solvent interactions of glycine, 1,10-phenanthroline and 2,2-bipyridyl have been studied in 0–60% v/v ethylene glycol–water media by a pH metric method. The protonation constants were estimated with the computer program MINIQUAD75. Selection of the best fit chemical model of the protonation equilibria is based on the standard deviation in protonation constants and residual analysis using a sum of squares of residuals in all mass-balance equations. The observed linear variation of protonation constants with the inverse of dielectric constant of the solvent mixture can be attributed to the dominance of the electrostatic forces. The distribution of species, protonation equilibria and effects of influential parameters on the protonation constants are also presented.

Surface Electrostatics and Lipid-Substrate Interactions of Nanopore-Confined Lipid Bilayers

Substrate-supported lipid bilayers serve many purposes: from acting as versatile models of cellular membranes to biotechnological applications including substrate functionalization and stabilizing membrane proteins in functional conformations. While adsorption and subsequent reorganization of phospholipid vesicles on solid substrates were studied in the past, the exact nature of physicochemical interactions between the lipids and substrate surfaces remain largely unknown. Here we employed recently synthesized pH-sensitive spin-labeled phospholipids - derivatives of 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol (PTE) with pH-reporting nitroxides that are covalently attached to the lipid's headgroup - to investigate surface electrostatics of nanotubular lipid bilayers confined in cylindrical nanopores. The lipid nanotubes were formed by self-assembling phospholipids inside ordered nanochannels of anodic aluminum oxide with pore diameters from 60 to 170 nm and diameter-to-pore length ratio of up to 1:1000. 31P NMR confirmed formation of macroscopically aligned lipid nanotubes with just 1-2° mosaic spread from zwitterionic DMPC, anionic DMPG, and their mixtures. Interfacial potentials were measured by carrying out titration experiments and observing the protonation state of the nitroxide tag by EPR. For nanopore-confined DMPC:DMPG (1:1) bilayers the protonation equilibrium was shifted to more acidic values: when the single lipid bilayer was deposited per nanopore the pKa of the nitroxide probe was shifted by (−0.91±0.05) pH units but only by (−0.34±0.05) when three bilayers per nanopore were present. Notably, the nitrogen hyperfine coupling constant for non-protonated nitroxides remained the same in all the samples indicating essentially the same interfacial dielectric environment. Thus, these shifts in pKa must come from changes in the lipid bialyer surface potential that was estimated to increase by 52±3 mV. EPR data on the lipid-substrate interface were combined with differential scanning calorimetry to elucidate effects of pore curvature, surface modification, and binding of antibacterial peptides on lipid-substrate interactions. Supported by DE-FG02-02ER15354.

Triadic Intramolecular Charge Transfer Compound of Tetrathiafulvalene Exhibiting Multicolor Solvatochromism

An A-D-A triad compound comprising a tetrathiafulvalene (TTF) moiety and two pyridyl groups, py-TTF-py (1), has been studied in view of intramolecular charge transfer (ICT). The compound shows a sharp and multicolor change in different solvents and at different pH values, exhibiting good solvatochromism. The results of electronic absorption spectroscopy, cyclic voltammetry, and theoretical calculations confirm that there exists predominantly a monoprotonated A-D-A triad that displays a strong ICT effect, which is tunable as a function of pH. The equilibrium of protonation of 1 has been further studied by means of pH titration, and the result confidently supports the conclusion that only one equivalent H(+) combines with py-TTF-py in dilute solution, even when excessive acid is added. However, unlike the state in dilute solution, the crystal structure of the protonated 1 is a diprotonated A-D-A triad, (1.2H(+)).(CF(3)SO(3)(-))(2).H(2)O (2), that has been structurally characterized.

Effect of pH and temperature on carbon-supported manganese oxide oxygen reduction electrocatalysts

Abstract Manganese oxides nanoparticles were chemically deposited on a high area (ca. 300 m2 g−1) carbon black substrate to act as electrocatalysts for oxygen reduction. The morphology and chemistry of the carbon-supported MnOx nanoparticles was characterised by Transmission Electron Microscopy), X-ray Diffraction, and chemical analysis. The oxygen reduction reaction (ORR) catalytic activity was studied in the 7–10 pH range using a rotating disk electrode (RDE). High activity towards oxygen reduction and very good stability in neutral and slightly basic solution were obtained. At low current densities, at 25 °C, MnOx/C displayed a reaction order with respect to OH− ions of −0.5 and Tafel slopes of −0.153 and −0.167 V dec−1 at pH 7 and 10 respectively; showing that the ORR mechanism on MnOx/C is unchanged in the 7–10 pH range. From the data, we propose that the first electrochemical step of the 4-electron ORR mechanism, in the 7–10 pH range, is the quasi equilibrium proton insertion process in MnO2 yielding MnOOH (insoluble in neutral or slightly basic solution). The ORR activity of the MnOx/C materials increased with increasing temperatures from 5 to 40 °C. The 2-electron pathway of oxygen reduction, yielding hydrogen peroxides as intermediates, may however be favoured over the 4-electron O2 reduction at higher temperatures.

Structures, Ionization Equilibria, and Tautomerism of 6-Oxopurines in Solution

6-Oxopurine and its analogues form an important class of biological molecules that include nucleobases and their precursors and are substrates of a wide range of enzymes. Solution structures of purines have been debated in the literature because of the many possible tautomers and protonation states in which they can exist in solution. Substitutions on the pyrimidine and imidazole rings alter tautomerization and protonation equilibria, and as a consequence, the solution compositions and structures of closely related analogues can be significantly different. We have obtained resonance Raman spectra of 6-oxopurines: hypoxanthine, xanthine, their riboside phosphates, guanine monophosphate in the protonated and deprotonated forms with UV excitation at 260 nm. The species present in solution under different pH conditions were identified by isotopic labeling with deuterium as well as by comparison with extensive density functional theoretical calculations. At physiological pH, while N7H and N9H tautomeric forms of hypoxanthine exist in equilibrium, in xanthine, the additional carbonyl group at C2 shifts the equilibrium in favor of the N7H tautomer. The corresponding nucleotide of xanthine, xanthosine monophosphate, on the other hand, is in the anionic form (pK(a) 5.5). We find that Raman spectra show systematic shifts with change in the protonation state and substitution on the ring. In general, deprotonation of the neutral molecule is marked by a downshift in the observed Raman wavenumbers, and protonation is accompanied by an upshift.

Theory of Proton-Coupled Electron Transfer in Energy Conversion Processes

Proton-coupled electron transfer (PCET) reactions play an essential role in a broad range of energy conversion processes, including photosynthesis and respiration. These reactions also form the basis of many types of solar fuel cells and electrochemical devices. Recent advances in the theory of PCET enable the prediction of the impact of system properties on the reaction rates. These predictions may guide the design of more efficient catalysts for energy production, including those based on artificial photosynthesis and solar energy conversion. This Account summarizes the theoretically predicted dependence of PCET rates on system properties and illustrates potential approaches for tuning the reaction rates in chemical systems. A general theoretical formulation for PCET reactions has been developed over the past decade. In this theory, PCET reactions are described in terms of nonadiabatic transitions between the reactant and product electron-proton vibronic states. A series of nonadiabatic rate constant expressions for both homogeneous and electrochemical PCET reactions have been derived in various well-defined limits. Recently this theory has been extended to include the effects of solvent dynamics and to describe ultrafast interfacial PCET. Analysis of the rate constant expressions provides insight into the underlying physical principles of PCET and enables the prediction of the dependence of the rates on the physical properties of the system. Moreover, the kinetic isotope effect, which is the ratio of the rates for hydrogen and deuterium, provides a useful mechanistic probe. Typically the PCET rate will increase as the electronic coupling and temperature increase and as the total reorganization energy and equilibrium proton donor-acceptor distance decrease. The rate constant is predicted to increase as the driving force becomes more negative, rather than exhibit turnover behavior in the inverted region, because excited vibronic product states associated with low free energy barriers and relatively large vibronic couplings become accessible. The physical basis for the experimentally observed pH dependence of PCET reactions has been debated in the literature. When the proton acceptor is a buffer species, the pH dependence may arise from the protonation equilibrium of the buffer. It could also arise from kinetic complexity of competing concerted and sequential PCET reaction pathways. In electrochemical PCET, the heterogeneous rate constants and current densities depend strongly on the overpotential. The change in equilibrium proton donor-acceptor distance upon electron transfer may lead to asymmetries in the Tafel plots and deviations of the transfer coefficient from the standard value of one-half at zero overpotential. Applications of this theory to experimentally studied systems illustrate approaches that can be utilized to tune the PCET rate. For example, the rate can be tuned by changing the pH or using different buffer species as proton acceptors. The rate can also be tuned with site-specific mutagenesis in biological systems or chemical modifications that vary the substituents on the redox species in chemical systems. Understanding the impact of these changes on the PCET rate may assist experimental efforts to enhance energy conversion processes.

Strong Hydrogen Bond between Glutamic Acid 46 and Chromophore Leads to the Intermediate Spectral Form and Excited State Proton Transfer in the Y42F Mutant of the Photoreceptor Photoactive Yellow Protein

In the Y42F mutant of photoactive yellow protein (PYP) the photoreceptor is in an equilibrium between two dark states, the yellow and intermediate spectral forms, absorbing at 457 and 390 nm, respectively. The nature of this equilibrium and the light-induced protonation and structural changes in the two spectral forms were characterized by transient absorption, fluorescence, FTIR, and pH indicator dye experiments. In the yellow form, the oxygen of the deprotonated p-hydroxycinnamoyl chromophore is linked by a strong low-barrier hydrogen bond to the protonated carboxyl group of Glu46 and by a weaker one to Thr50. Using FTIR, we find that the band due to the carbonyl of the protonated side chain of Glu46 is shifted from 1736 cm(-1) in wild type to 1724 cm(-1) in the yellow form of Y42F, implying a stronger hydrogen bond with the deprotonated chromophore in Y42F. The FTIR data suggest moreover that in the intermediate spectral form the chromophore is protonated and Glu46 deprotonated. Flash spectroscopy (50 ns-10 s) shows that the photocycles of the two forms are essentially the same except for a transition around 5 mus that has opposite signs in the two forms and is due to the chemical relaxation between the two dark states. The two cycles are coupled, likely by excited state proton transfer. The Y42F cycle differs from wild type by the occurrence of a new intermediate with protonated chromophore between the usual I(1) and I(2) intermediates which we call I(1)H (370 nm). Transient fluorescence measurements indicate that in I(1)H the chromophore retains the orientation it had in I(1). Transient proton uptake occurs with a time constant of 230 mus and a stoichiometry of 1. No proton uptake was associated however with the formation of the I(1)H intermediate and the relaxation of the yellow/intermediate equilibrium. These protonation changes of the chromophore thus occur intramolecularly. The chromophore-Glu46 hydrogen bond in Y42F is shorter than in wild type, since the adjacent chromophore-Y42 hydrogen bond is replaced by a longer one with Thr50. This facilitates proton transfer from Glu46 to the chromophore in the dark by lowering the barrier, leading to the protonation equilibrium and causing the rapid light-induced proton transfer which couples the cycles.

Acid–Base Equilibrium of Dodecyldimethyl‐Amine‐N‐Oxide Micelles in Water–Butanol Binary at 298 K

AbstractDodecyldimethyl‐amine‐N‐oxide (DDAO) is a zwitterionic surfactant that can be protonated in water to form DDAOH+ if the pH of the medium is appropriate. We proposed, in a previous study, an experimental approach to determine directly the ratio of ionization of the surfactant in the micelle and in the bulk, at a given acidity level, without the need of a presupposed model of protonic exchange. We also showed that the relations of Thermodynamics impose a relationship between the critical micelle concentration (cmc) and the level of protonation of DDAO in the micelle and in the bulk. In order to appreciate the general nature of our approach, we have now modified the protonic exchange equilibrium between DDAO and DDAOH+ by introducing butanol into the solution. We studied three mixed media (2, 4 and 6% alcohol by volume). For all these mixtures, we validated our experimental and theoretical approaches. Moreover, for all the hydroalcoholic mixtures considered and all pH values, the presence of butanol stabilizes the micelle leading to a decrease of the cmc. Similar to pure water (Lair et al. in Langmuir, 20:8490–8495, 2004), we showed that, in these media, the protonic exchange equilibrium between DDAOH+ and DDAO in their micellized forms cannot be characterized by an unique equilibrium constant independent of the pH. However, as the butanol content of the medium increases, the variations in apparent pK with pH decrease, indicating that the behaviour of the acid–base pair in the micelle is classical and corresponds to a stoichiometric proton exchange.

Kinetic and Thermodynamic Barriers to Chlorine Transfer between Amines in Aqueous Solution

Third-order rate constants for the acid-catalyzed reversible reaction of N-chlorotaurine with benzylamine and dimethylamine were determined in water at 25 degrees C and I = 0.5 (NaClO4). The reaction with benzylamine shows inverse solvent deuterium isotope effects of kH/kD=0.57 and 0.47 in the forward and reverse directions, respectively. These isotope effects, together with the absence of detectable general acid catalysis for this reaction, provide evidence for a stepwise mechanism involving fast equilibrium protonation of N-chlorotaurine followed by rate-determining chlorine transfer from the protonated chloramine to benzylamine. The observation of strong catalysis by general acids of the reaction of dimethylamine with N-chlorotaurine suggests a change to a concerted mechanism with proton and chlorine transfer occurring in a single step. This change in mechanism is enforced by the absence of a significant lifetime for protonated chlorotaurine in contact with this strongly nucleophilic amine. The kinetic and thermodynamic parameters for the reaction between protonated chlorotaurine and benzylamine are used to estimate a Marcus intrinsic reaction barrier of deltaG(0)+/+ =4.1 kcal/mol for chlorine transfer between amines. Comparison of this intrinsic barrier with those reported previously for bromine transfer between carbanions points to the existence of certain similarities between halogen and proton transfer reactions.

Effect of environment on enzymatic action: His-64 … asp-32 proton transfer in subtilisin

CNDO/FORCE and electrostatic potential calculations were performed to study the preferred position of the His-64 … Asp-32 bridge proton in a serine protease, subtilisin. The energy difference between neutral and zwitterionic forms was approximated by the sum of two terms. The first term is the geometry-optimized CNDO/2 energy of the His-64 … Asp-32 diad, the second one comes from the environment which includes the neighboring amino-acid residues, the first hydration shell, and the counterion. Numerical results suggest that environmental effects tend to stabilize the zwitterionic form, as confirmed by Bachovchin and Roberts with nuclear magnetic resonance studies. The effect of the counterion seems to be important. Further factors influencing the protonation equilibrium are discussed.

A Chemical System that Mimics Decoding Operations

The chemical information stored in equilibrium mixtures of molecular species is larger than the sum of information carried by the individual molecules. Protonation equilibria in dilute dichloromethane solution of a shape-persistent macrocycle bearing two 2,2'-bipyridine units and two Coumarin 2 moieties (see figure) can be exploited to mimic decoding operations.

Analysis of kinetic isotope effects for proton-coupled electron transfer reactions.

A series of rate constant expressions for nonadiabatic proton-coupled electron transfer (PCET) reactions are analyzed and compared. The approximations underlying each expression are enumerated, and the regimes of validity for each expression are illustrated by calculations on model systems. In addition, the kinetic isotope effects (KIEs) for a series of model PCET reactions are analyzed to elucidate the fundamental physical principles dictating the magnitude of the KIE and the dependence of the KIE on the physical properties of the system, including temperature, reorganization energy, driving force, equilibrium proton donor-acceptor distance, and effective frequency of the proton donor-acceptor mode. These calculations lead to three physical insights that are directly relevant to experimental data. First, these calculations provide an explanation for a decrease in the KIE as the proton donor-acceptor distance increases, even though typically the KIE will increase with increasing equilibrium proton donor-acceptor distance if all other parameters remain fixed. Often the proton donor-acceptor frequency decreases as the proton donor-acceptor distance increases, and these two effects impact the KIE in opposite directions, so either trend could be observed. Second, these calculations provide an explanation for an increase in the KIE as the temperature increases, even though typically the KIE will decrease with increasing temperature if all other parameters remain fixed. The combination of a rigid hydrogen bond, which corresponds to a high proton donor-acceptor frequency, and low solvent polarity, which corresponds to small solvent reorganization energy, allows the KIE to either increase or decrease with temperature, depending on the other properties of the system. Third, these calculations provide insight into the dependence of the rate constant and KIE on the driving force, which has been studied experimentally for a wide range of PCET systems. The rate constant increases as the driving force becomes more negative because excited vibronic product states associated with low free energy barriers and relatively large vibronic couplings become accessible. The ln[KIE] has a maximum near zero driving force and decreases significantly as the driving force becomes more positive or negative because the contributions from excited vibronic states increase as the reaction becomes more asymmetric, and contributions from excited vibronic states decrease the KIE. These calculations and analyses lead to experimentally testable predictions of trends in the KIEs for PCET systems.

P-Nitrophenolate: A Probe for Determining Acid Strength in Ionic Liquids

In order to obtain an acidity scale in room-temperature ionic liquid solutions, the protonation equilibrium of sodium p-nitrophenolate in [bm(2)im][NTf(2)] and in [bmpyrr][NTf(2)], at 298 K, has been studied by means of spectrophotometric titration. Carboxylic acids differing in both structure and in water solution strength have been used. Data collected indicate that in the analyzed ionic liquid solutions carboxylic acids are less dissociated than in water solution. Furthermore, by virtue of a mutual interaction, the studied equilibrium seems to be affected by both the nature of the ionic liquids and the carboxylic acids.

The Cu(II) complex of a C-lipophilized 13aneN4 macrocycle with an additional protonable amino group as micellar anion receptor

Three 13aneN4 macrocyclic ligands have been prepared bearing a -CH(2)NHR side arm (R = H, n-C(5)H(11), n-C(10)H(21)) on a carbon atom. When Cu(2+) is complexed in the macrocyclic ring, the amino group of the side arm undergoes an acid-base protonation equilibrium but it is not able to coordinate apically the metal cation even when it is deprotonated. The Cu(2+) complex with the ligand bearing the longest appended aliphatic chain is fully confined inside Triton X-100 micelles, and its ability to bind and sequestrate a series of anions inside micelles has been studied at two different pH values, i.e. both with protonated and neutral side-arm amino group. The favourable role played by the protonated amino group in the side arm has been demonstrated.

The reduction of doxorubicin at a mercury electrode and monitoring its interaction with DNA using constant current chronopotentiometry

In this report, voltammetry with linear scan and chronopotentiometric stripping (CPS) with constant current were used for the analysis of doxorubicin (DOX) at a hanging mercury drop electrode (HMDE). CPS was used for the study of DOX in situ electrochemical reduction in adsorbed state and for ex situ (adsorptive transfer) analysis of the drug. For the first time, CPS was used to study the reversible reduction of the DOX quinine moiety at –0.45 V (vs Ag|AgCl|3 M KCl) as well as electrode processes giving rise to an irreversible signal around –1.45 V at the HMDE in 0.2 M acetate or Britton–Robinson buffers at different pH values. The dependence of the latter signal on pH revealed involvement of protonation equilibria; however, neither CV nor CPS data confirmed the catalytic character of the electrode reaction previously suggested by other authors. The CPS method was also applied to monitor the DOX interaction with double- (ds) and single-stranded (ss) DNA. In the presence of dsDNA, more pronounced changes in DOX signal intensity were observed, in agreement with a strong intercalation of the DOX redox centre into the DNA double helix.

The 3-amino-derivative of γ-cyclodextrin as chiral selector of Dns-amino acids in electrokinetic chromatography

The enantioseparation of the enantiomeric pairs of 10 Dns derivatives of α-amino acids was successfully carried out by using for the first time the 3-amino derivative of the γ-cyclodextrin. The effects of pH and selector concentration on the migration times and the resolutions of analytes were studied in detail. 3-Deoxy-3-amino-2(S),3(R)-γ-cyclodextrin (GCD3AM) shows very good chiral recognition ability even at very low concentrations at all the three investigated values of pH, as shown by the very large values of selectivity and resolution towards several pairs of amino acids. The role played by the cavity, the substitution site and the protonation equilibria on the observed properties of chiral selectivity, on varying the specific amino acid involved, is discussed.

Electrokinetic characterization of the Al 2O 3 ceramic MF membrane by streaming potential measurements

The alumina microfiltration(MF) membrane was modified with nanocrystalline TiO 2 in the pore wall. The electrokinetic properties of unmodified and modified alumina membranes were characterized by streaming potential measurements during tangential filtration at various conditions of pH, ionic concentration and ionic species in solutions. The modified membrane reveals a more acidic characteristic. The influence of pH value on the streaming potential of both membranes is explained by the shifting of the proton equilibrium that occurs at the surface of the membrane. The modification of the alumina membrane with TiO 2 leads to the decrease of its isoelectric point (IEP) from 6.1 to 4.0 when filtered with NaCl solution. The ionic concentration and electrolyte species also have influence on the streaming potential of both membranes. The reversal of the streaming potential sign and the change of the isoelectric point of the membrane when filtered with CaCl 2 and Na 2SO 4 solutions show specific adsorption of Ca 2+ and SO 4 2- ions onto the surface.

Thermodynamics and kinetics of indole oligomerization: Preliminary results in aqueous sulfuric acid

AbstractReaction rates and equilibrium constants of indole dimerization and trimerization in aqueous sulfuric acid at 298 K are reported. The equilibrium of oligomerization is attained in about 4–5 h, and formation of oligomers with more than three monomeric unit is not observed. The equilibrium of formation of the indole dimer is influenced by the protonation equilibrium of indole, which means the pKIH values of indole strongly influences equilibria and kinetics of the whole process. In the evaluation of the kinetic constants, the pKIH values of indole have been taken into account; in this way, the kinetic constant of formation of the dimer (kD) results almost four order of magnitude larger than that of the trimer (kT), suggesting a higher electrophilicity of the 3H‐indolium cations with respect to the protonated dimer (which is an aliphatic ammonium salt). Further indole addition to the trimer, which is a protonated 2‐alkyl‐aniline, does not occur, since the anilinium ion is ineffective as an electrophile. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 107–112, 2009

Complexes of Silver with Histidine and Imidazole Investigated by the Calorimetric and Potentiometric Methods

The stability constants and enthalpies of formation of silver (I) complexes with histidine and imidazole were determined. Two types of complexes were found: AgL and AgL+2 . The second is much more stable that is evidenced by the higher stability constants and enthalpies of formation. For the two systems, the respective constants and enthalpies are very close one to another that suggests similar complexation mechanisms. Most probably, the linear complexes through the N-3 pyridinic atoms are being formed. The calorimetric and potentiometric methods gave virtually the same results for the system silver + imidazole. However, the potentiometric method failed for the histidine system. The molecule of histidine has three protonation spots, two of these probably do not take part in the complexation but their protonation equilibria may shift upon joining the ions of silver and influence the pH of the system in that way.

Shape-Persistent Macrocycles Functionalised with Coumarin Dyes: Acid-Controlled Energy- and Electron-Transfer Processes

We have investigated the spectroscopic properties (absorption spectra, emission spectra, emission lifetimes) of three triads in CH(2)Cl(2): C2-M-C2, C343-M-C343, and C2-M-C343, in which M is a shape-persistent macrocyclic hexagonal backbone composed of two 2,2'-bipyridine (bpy) units embedded in opposing sides, and C2 and C343 are coumarin 2 and coumarin 343, respectively. All the components are strongly fluorescent species (Phi=0.90, 0.79, and 0.93 for M, C2, and C343, respectively, as established by investigating suitable model compounds). In each triad excitation of M leads to almost quantitative energy transfer to the lowest coumarin-localised excited state. Upon addition of acid, the two bpy units of the M component undergo independent protonation leading to monoprotonated (e.g., C2-MH(+)-C2) and diprotonated (e.g., C2-M2 H(+)-C2) species. Further addition of acid leads to protonation of the coumarin component so that each triad is involved in four protonation equilibria. Protonation causes strong (and reversible, upon addition of base) changes in the absorption and fluorescence properties of the triads because of inversion of the excited-state order and/or the occurrence of electron-transfer quenching processes.