Protonation Equilibria Research Articles

Proton Sensitivity of Luminescent [M(bpy)2(AB)]2+ Complexes and Their Monomethylated Counterparts [M(bpy)2(ABMe)]3+ Where AB Is an Asymmetric Quaterpyridine with a Pendant Bipyridyl Site [M = RuII, OsII

We report the preparation and the electrochemical and spectroscopic properties of the [Ru(bpy)2(ABMe)]3+ and [Os(bpy)2(ABMe)]3+ complexes, where bpy is 2,2‘-bipyridine, AB is bis-chelating 2,2‘:3‘,2‘‘:6‘‘,2‘‘‘ quaterpyridine (which is composed of a bpy-type fragment, A, coordinated to the metal center, and a free bpy-type fragment, B), and Me indicates methylation at a pyridyl N of site B. The electrochemical and photophysical properties (luminescence spectra, emission lifetimes, quantum yields) of the title complexes have been compared with those of the parent species [Ru(bpy)2(AB)]2+ and [Os(bpy)2(AB)]2+ in which the pendant bpy site B of the AB ligand is not methylated. In the methylated complexes a charge-separated energy level is present which involves the oxidized metal center and the reduced methylated center. For energetic reasons, this level cannot be reached from the luminescent metal-to-ligand charge-transfer level and no quenching of the Ru- and Os-based luminescence takes place: the spectroscopic changes observed with respect to the related unmethylated complexes may be understood in terms of perturbation effects. The luminescence properties of the investigated complexes are affected by acidity of the solvent and HCl titration in acetonitrile:water (1:1) solvent for [Ru(bpy)2(AB)]2+ and [Ru(bpy)2(ABMe)]3+ has been employed to investigate acid−base equilibria at B site. In the 4 < pH < 1.5 interval, a deprotonation−protonation equilibrium is observed for [Ru(bpy)2(ABH)]3+, whose excited and ground states exhibit a similar poorly basic character, being pKa* = 2.28, pKa ∼ 1.9, and ΔpKa ∼ 0.4. The luminescence properties of [Ru(bpy)2(ABMe)]3+ do not change at pH > 2: the methyl group protects the free N center of B site from proton access in this pH range. In more acidic solvent, pH < 2, the luminescence of [Ru(bpy)2(ABMe)]3+ is quenched {and so does at pH < 1.5 that of [Ru(bpy)2(ABH)]3+}, because of a protonation process occurring in nonequilibrium conditions with a second order rate constant kpr = 5.3 × 107 M-1 s-1. A discussion is given on the nature of the process occurring at pH < 2.

Kinetics of the p-Aminodiphenylamine Radical in Organic Solution: An Electrochemical and Electron Spin Resonance Study

In studying the reaction mechanism of anodic aniline oxidation the dimer p-aminodiphenylamine (ADPA) was found to be the main intermediate product in acidic solution. It is oxidized in both the free amine and the protonated form. The cyclic voltammograms have been simulated by taking into account an electrochemical reaction of both the protonated and the nonprotonated forms. Thus, it was also found that the ADPA concentration is determined by the protonation equilibrium. The oxidation product N-phenylquinone diimine (PQDI) undergoes a symproportionation reaction with ADPA forming the ADPA•+ cation radical. The equilibrium constant for this reaction of PQDI in dimethyl sulfoxide was determined and found to be independent of the concentration of the protons in solution. The ADPA•+ cation radical decay can be described by a rate equation based on a second-order reaction for the radical dimerization and a pre-equilibrium for the radical formation by symproportionation. The reaction mechanisms of the anodic oxidation of p-aminodiphenylamine and its chemical follow-up reactions are given.

Characterization of Lanthanide(III) DOTP Complexes: Thermodynamics, Protonation, and Coordination to Alkali Metal Ions

Several solution properties of complexes formed between the trivalent lanthanide ions (LnIII) and the macrocyclic ligand DOTP8-, including stability constants, protonation equilibria, and interactions of the LnDOTP5- complexes with alkali metal ions, have been examined by spectrophotometry, potentiometry, osmometry, and 1H, 31P, and 23Na NMR spectroscopy. Spectrophotometric competition experiments between DOTP and arsenazo III for complexation with the LnIII ions at pH 4 indicate that the thermodynamic stability constants (log KML) of LnDOTP5- range from 27.6 to 29.6 from LaIII to LuIII. The value for LaDOTP5- obtained by colorimetry (27.6) was supported by a competition experiment between DOTP and EDTA monitored by 1H NMR (27.1) and by a potentiometric competition titration between DTPA and DOTP (27.4). Potentiometric titrations of several LnDOTP5- complexes indicated that four protonation steps occur between pH 10 and 2; the protonation constants determined by potentiometry were consistent with 31P shif...

On the chemistry of pyrrole pigments, XCIII: 1,2-bis-(dipyrrinon-9-ylidene)-ethane ? a novelb-homoverdin chromophore

Condensation of a 9-unsubstituted dipyrrinone with glyoxal in dichloromethane yielded a red pigment. It could be identified as 1,2-bis-(dipyrrinon-9-ylidene)-ethane which is an example of the hitherto unknown dehydro-b-homo-verdins. In addition, the corresponding mesobiliverdin-XIIIα could be isolated from the reaction mixture. The mechanistic aspects of this reaction are discussed. Absorption spectra, protonation equilibria, and complexation of the title compound with transition metal ions were investigated.

A novel PC-based gravimetric autotitrator with a multi-solution delivery system

The development of a novel PC-based gravimetric autotitrator is described. A single electronic balance serves as the weight buret for the delivery of three different solutions. It is therefore possible to add titrand and titrant under automated control. Investigation of the standard nickel(II)–glycinate–proton equilibrium system was used to evaluate performance. Keywords: titrator, titration, potentiometry, automated control.

Influence of solution acidity on the adsorption and charge transfer kinetics of parabanic acid reduction

Parabanic acid reduction has been studied in the − 1.4 ⩽ H0 ⩽ 0.9 acidity range by the ac impedance technique. Changes in the adsorption of reactant and products with solution acidity lead to protonation equilibrium constants that differ from their bulk values. The observed pKa shifts were consistent with interfacial dielectric saturation. Variation of the charge transfer parameters with potential and proton activity allows a detailed analysis of the reduction mechanism to be performed in terms of the scheme of squares.

The inner boundary of a neutron-star crust

Abstract The phase boundary between the liquid matter of the interior of a neutron star and the solid matter that comprises the crust is studied. The matter is assumed to consist of electrons and non-relativistic neutrons and protons in beta equilibrium. The characteristic used to identify the phase boundary is the onset of instability of the uniform matter against proton clustering, and it is examined for four interactions either used previously in astrophysical studies — Skyrme 1′, SkM and FPS - or proposed here as an improvement — FPS21. The relationship of this important property of neutron-star matter to the neutron-matter character of each interaction is explored. Characteristic densities associated with proton drip and with the liquid-gas boundary of one- and two-fluid models of the matter are compared with the instability density. Properties of some other effective interactions, relativistic and non-relativistic, are compared with those of the interactions discussed. The desirability of more calculations of neutron-matter properties, and of the utilization of those that do exist when formulsting dense-matter equations of state, is emphasized. For the improved interaction FPS21, the crust fraction of the moment of inertia of a neutron star is estimated.

Reaction in ion-exchangers: characterization of the ethylenediamine protonation equilibrium in a sulfonic membrane devoted to carbon dioxide facilitated transport

A method of characterization of the protonation equilibrium of ethylenediamine (EDA) in a cation-exchange membrane is propounded. This method involves an intermediate reaction of the membrane (initially in the mixed EDAH +EDAH 2 2+ form) with gaseous carbon dioxide then the elution and the titration of the carbamic acid formed. This method is more precise and is preferred to the conversion of the sorbed EDAH + and EDAH 2 2+ into EDA by sodium hydroxide followed by an acidic titration. The selected method allows to calculate accurately the apparent equilibrium constant K ∗ of the protonation of EDA in the membrane and is very useful to characterize the carrier (EDAH +) content of membranes conditioned to achieve facilitated transport of carbon dioxide. The data obtained will be used in an improved analysis of this transport process.

Protonation and acid catalysed hydrolysis of nitrosoaryl ethers

Studies of the equilibrium protonation and kinetics of acid catalysed hydrolysis of 4-nitrosoanisole, 4-nitrosophenyl phenyl ether and related compounds in dilute aqueous acid and in concentrated aqueous trifluoroacetic acid are reported. Hydrolysis is remarkably facile and occurs by nucleophilic attack of water at the ring carbon bearing the alkoxy or aryloxy substituent of the protonated nitroso aromatic. Direct and indirect pKa determinations for nitroso protonation are reported.

V-NMR investigation on the formation of peroxo vanadium complexes in aqueous solution: some novel observations

Depending on the experimental conditions either monoperoxo or diperoxo vanadium complexes are formed in acid aqueous solutions by addition of H2O2 to NH4VO3. The 51V-NMR spectra of both species have been registered at different pH values. While the signal of the monoperoxo derivative is unchanged in the pH range 0–4, the chemical shift of the diperoxo species shows a continuous variation in the same interval thus suggesting the occurrence of a protonation equilibrium leading to a neutral diperoxo vanadium complex. In the presence of bidentate ligands L, e.g., picolinic or pyrazinic acid, under otherwise identical experimental conditions, complexed peroxo vanadium species are formed. The association equilibria can be conveniently studied by 51V-NMR spectroscopy. Thus, different species, e.g., VO(O2)L, [VO(O2)L2]−, [VO(O2)2L]2− as a function of the pH has been detected and the corresponding association constants have been measured.

51V-NMR investigation on the formation of peroxo vanadium complexes in aqueous solution: Some novel observations

Depending on the experimental conditions either monoperoxo or diperoxo vanadium complexes are formed in acid aqueous solutions by addition of H 2O 2 to NH 4VO 3. The 51V-NMR spectra of both species have been registered at different pH values. While the signal of the monoperoxo derivative is unchanged in the pH range 0–4, the chemical shift of the diperoxo species shows a continuous variation in the same interval thus suggesting the occurrence of a protonation equilibrium leading to a neutral diperoxo vanadium complex. In the presence of bidentate ligands L, e.g., picolinic or pyrazinic acid, under otherwise identical experimental conditions, complexed peroxo vanadium species are formed. The association equilibria can be conveniently studied by 51V-NMR spectroscopy. Thus, different species, e.g., VO(O 2)L, [VO(O 2)L 2] −, [VO(O 2) 2L] 2− as a function of the pH has been detected and the corresponding association constants have been measured.

STEADY‐STATE FLUORESCENCE METHOD FOR EVALUATING EXCITED STATE PROTON REACTIONS: APPLICATION TO FLUORESCEIN

Abstract Fluorescein is a complex fluorophore in the sense that it displays four prototropic forms (cation, neutral, monoanion and dianion) in the pH range 1–9. In experiments with fluorescein‐labeled proteins we have sometimes observed complex nanosecond emission kinetics, which could be due to conversion of the excited monoanion into the excited dianion through an excited state proton exchange with a proton acceptor in the labeled protein. However, the literature is ambiguous on whether this possible excited state proton reaction of fluorescein does occur in practice. In this article we describe a general steady‐state fluorescence method for evaluating excited state proton reactions of simple as well as complex pH‐sensitive fluorophores and apply it to evaluate excited state proton reactions of fluorescein. The method depends on finding a buffer that can serve as an excited state proton donor‐acceptor but does not significantly perturb ground state proton equilibrium and especially does not form ground (or excited state complexes) with the fluorophore. Our results show that the excited monoanion‐dianion proton reaction of fluorescein does occur in the presence of phosphate buffer, which serves as a proton donor‐acceptor that does not significantly perturb ground state proton equilibria. The reaction becomes detectable at phosphate buffer concentrations greater than 20 mM and the reaction efficiency increases with increase in phosphate buffer concentrations. The reaction is most clearly demonstrated by adding phosphate buffer to a solution of fluorescein at constant pH 5.9 with preferential excitation of the monoanion. Under these conditions, the excited monoanion converts to the dianion during its lifetime. The conversion is detected experimentally as an increase in dianion and decrease in monoanion fluorescence intensities with increase in phosphate buffer concentration. The absorption spectrum is not significantly perturbed by the increase in phosphate buffer concentration. To quantitate the reaction, we have recorded titration graphs of fluorescence intensity versus pH for fluorescein solutions at low (5 mM) and high buffer (1 M) concentrations with preferential excitation of the monoanion and preferential detection of the dianion emission. We have also developed theoretical expressions that relate fluorescence intensity to pH in terms of the concentration of the four prototrophic forms of fluorescein, extinction coefficients, fluorescence efficiencies and ground and excited state pKa. The theoretical expressions give very good fits to the experimental data and allow evaluation of fundamental parameters such as pKa and fluorescence efficiencies. The analysis of the experimental data shows that the excited monoanion‐dianion reaction does not significantly occur at 5 mM phosphate buffer concentration. However, at 1 M buffer concentration the reaction is sufficiently fast that it practically achieves equilibrium during the lifetimes of the excited fluorescein monoanion and dianion. The pKa* of the excited monoanion‐dianion proton reaction is around 6.3. The results and methods presented here should be useful in the development and testing of pH‐sensitive labeling fluorophores and fluorescent indicators.

Protonation Study and Mechanism of Hydrolysis of 2-[(p-Methoxyphenylazo]-1,3,4-thiadiazole in Aqueous Sulfuric Acid at 60 .degree.C

2-[(p-Methoxyphenyl)azo]-1,3,4-thiadiazole has been found to be associated with a single protolytic equilibrium, its pK a value for the protonation equilibrium has been evaluated, and the structure of the protonated species has been determined. The compound hydrolyzes from the monopronated substrate by the A-S E 2 mechanism of the S N Ar type

Aqueous Chemistry of Labile Oxovanadates: Relevance to Biological Studies

Abstract The aqueous chemistry of vanadate (vanadium(V)) is very complex since many protonation equilibria and oligomerization equilibria are occurring simultaneously. Vanadate monomer (V1), dimer (V2), tetramer (V4) and pentamer (V5) are exchanging with each other on a millisecond time scale so that none of these species can be isolated for aqueous biological studies. Measuring the effects of each anion on an enzyme must be carried out in an equilibrium mixture containing the other vanadate oligomers. Defining conditions to measure the effects of oxovanadates is non-trivial, since vanadate interacts with buffers and other assay components. Information concerning the simple aqueous chemistry of vanadate with various ligands is thus necessary to ensure that the vanadate is free to interact with an enzyme or a protein. Experimental approaches, which take into account the aqueous chemistry of labile oxovanadates, to biological studies are described here. For this purpose, the interactions of oxovanadates wit...

Protonation Equilibrium of Ellipticine Bound to the Energy-Transducing Membrane of Mitochondria

The relationship between the protonation state of ellipticine (5,11-dimethyl-6H-pyrido[4,3-b]carbazole), an antitumoral alkaloid, and the transmembrane proton electrochemical gradient associated with mitochondrial membranes has been investigated. The binding parameters of ellipticine to mitochondria are independent of either the transmembrane ΔpH or membrane potential, which are both components of protongradient (association constant=1.5×10 6 M -1 ; maximal binding ratio=one bound ellipticine per 25 phospholipids). Since the apparent pK of bound ellipticine is 7.1, its two protonation states can be detected from their specific fluorescence emission spectra near physiological pH

A Chromophore-Quencher Complex Incorporating a Photoredox-Active Ligand

A photoredox-active ligand (CL), the chromophore-quencher complexes Cu(CL)(CF 3 SO 3 ) 2 and Cu(CL)-(H 2 O) 2 (CF 3 SO 3 )z, where CL=4,7-di-butoxy- 1,3-bis((3'-pyridylmethyl)imino)isoindoline, and the reference complexes Cu(L)(H 2 O)(CF 3 SO 3 ) 2 , where L=1,3-bis((3'-pyridylmethyl)imino)isoindoline, and Zn(CL)(H 2 O) 2 -(ClO 4 ) 2 have been synthesized and characterized. The ligand CL is subject to a protonation equilibrium with ground state pK a =8.99±0.06 and an approximate excited-state pK a =10.5±0.5

Internal Electron Transfer in Cytochrome c Oxidase Is Coupled to the Protonation of a Group Close to the Bimetallic Site

Absorbance changes following CO dissociation by flash photolysis from mixed-valence cytochrome oxidase have been followed in the Soret and alpha regions. Apart from CO dissociation and recombination, three kinetic phases with rate constants in the range 10(5)-10(3) s-1 at pH 7.5 can be resolved in both spectral regions. The slowest one of these phases, which had earlier only been observed in the alpha region, has now been detected in the Soret region by the use of a low CO concentration to slow down the recombination reaction. This phase had been assigned to a structural change, but a kinetic difference spectrum demonstrates that it represents electron transfer from cytochrome a3 to cytochrome a. A kinetic deuterium isotope effect of 2-3 at pH 7.5 suggests that it involves proton transfer as well. The temperature dependence of the reaction gives an Arrhenius activation energy of 42 kJ.mol-1. The reaction is faster at low pH, and the equilibrium is shifted toward cytochrome a as the pH is raised. The rate and equilibrium changes can be described as involving acid-base groups with pKa values of approximately 7.7 and 8.7, respectively. The kinetic results can be simulated on the basis of a model in which one acid-base group interacts with cytochrome a3, so that its pKa drops on oxidation of this center. The group is in proton equilibrium with the solvent via a proton pathway, suggested to be a proton channel. The rate of a shift in the redox equilibrium between the two cytochromes reaches a high limit at low pH, where the channel is saturated with protons.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction between dibucaine and pig erythrocyte membranes as studied by NOESY experiments in 1H-NMR spectroscopy. Which form of dibucaine interacts more strongly, cationic or uncharged?

The interaction between amine local anesthetic dibucaine and pig erythrocyte membranes has been studied by 1H-NMR spectroscopy. Two-dimensional NOESY spectra were observed to obtain the conformations of cationic and uncharged forms of dibucaine. The NMR spectra were measured at pH 7.4, and the temperature was raised (318-348 K) to increase the concentration of the uncharged form of dibucaine, taking the temperature dependence of the pKa value of dibucaine into consideration. The dibucaine in a buffered solution showed the presence of two kinds of distinctly different species; one is assignable to the cationic form and the other to the uncharged form of dibucaine, suggesting that the protonation equilibrium between the two forms is slow in the presently employed experimental condition. The uncharged dibucaine showed well-defined NOE cross-peaks in the NOESY spectra of the solution containing no erythrocyte membranes, suggesting that its conformation is relatively fixed. Interestingly, however, it was only the cationic dibucaine that showed NOE cross-peaks when the solution contained the membranes, and experiments were performed at a much shorter mixing time for the buildup of NOEs, suggesting that it appeared only the cationic form of dibucaine is interacting with the membranes. It was concluded that the uncharged form of dibucaine, which was produced by raising the temperature, formed micelles in a buffered solution. Thus formed micelles didn't interact with membranes owing to the repulsive forces between the structured water surrounding the micelles and those at the surface of the membranes. This conclusion could be a promising reason why the cationic local anesthetics are much more active than their uncharged counterparts in blocking nerve conduction.

NMR study of the configuration and protonation equilibria of a pyrazolotriazole azomethine dye

1 H NMR and NOE spectroscopy have been used to investigate the ground state configuration and proton equilibria of a 7H-pyrazolo [5,1-c][1,2,4]triazole phenylamino azomethine dye. NOE experiments show that the dye exists in the syn configuration at room temperature. Two protonation equilibria can be identified in CDCl3–CF3CO2H at 1.6 ± 0.2 and 0.0 ± 0.2 –log [acid] units. The first equilibrium involves protonation of the azomethine nitrogen and leads to changes in the NMR spectrum which are interpreted as arising from increasing deviation from planarity about the azomethine bond because of steric interactions. The second equilibrium involves protonation of the phenylamino nitrogen.

Reversible active site protonation and electron-transfer properties of Achromobacter cycloclastes pseudoazurin: comparisons with other type 1 copper proteins

Kinetic and NMR studies on the copper(I) form of A. cycloclasters pseudoazurin provide evidence for an active site protonation (and dissociation) equilibrium process involving His81, pKa 4.9 (average); the three type 1 copper proteins now known to exhibit this behaviour have just two amino acids separating the ligating Cys and His residues in the C-terminal region.