Rate Constant Kq Research Articles

Bimolecular Electron Transfer in Ionic Liquids: Are Reaction Rates Anomalously High?

Steady-state and picosecond time-resolved emission spectroscopy are used to monitor the bimolecular electron transfer reaction between the electron acceptor 9,10-dicyanoanthracene in its S(1) state and the donor N,N-dimethylaniline in a variety of ionic liquids and several conventional solvents. Detailed study of this quenching reaction was undertaken in order to better understand why rates reported for similar diffusion-limited reactions in ionic liquids sometimes appear much higher than expected given the viscous nature of these liquids. Consistent with previous studies, Stern-Volmer analyses of steady-state and lifetime data provide effective quenching rate constants k(q), which are often 10-100-fold larger than simple predictions for diffusion-limited rate constants k(D) in ionic liquids. Similar departures from k(D) are also observed in conventional organic solvents having comparably high viscosities, indicating that this behavior is not unique to ionic liquids. A more complete analysis of the quenching data using a model combining approximate solution of the spherically symmetric diffusion equation with a Marcus-type description of electron transfer reveals the reasons for frequent observation of k(q) ≫ k(D). The primary cause is that the high viscosities typical of ionic liquids emphasize the transient component of diffusion-limited reactions, which renders the interpretation of rate constants derived from Stern-Volmer analyses ambiguous. Using a more appropriate description of the quenching process enables satisfactory fits of data in both ionic liquid and conventional solvents using a single set of physically reasonable electron transfer parameters. Doing so requires diffusion coefficients in ionic liquids to exceed hydrodynamic predictions by significant factors, typically in the range of 3-10. Direct, NMR measurements of solute diffusion confirm this enhanced diffusion in ionic liquids.

Micellar-mediated binding interaction between perylene and dl-phenylalanine: Insights from spectroscopic investigations

The interaction between perylene and dl-phenylalanine (DL-PA) was studied in micellar solution of sodium dodecyl sulfate (SDS) by spectroscopic methods including absorption, FTIR and steady state fluorescence spectroscopy. The fluorescence of perylene is quenched by DL-PA and quenching is in accordance with Stern–Volmer relation. The number of binding sites (n) and binding constant (K) was obtained by fluorescence method. The quenching rate constant (kq) was calculated from the fluorescence lifetime of perylene in SDS and was measured on time resolved fluorimeter (TRF) in the absence of DL-PA. The FTIR spectrum showed the conformational change of DL-PA in the presence of perylene. The analytical applications such as effect of foreign ion substance, effect of SDS were studied hence proposed quenching method is simple, selective and rapid.

Fluorescence Quenching of Tris(2,2′-bipyridine)Ruthenium(II) Dichloride by Certain Organic Dyes

Fluorescence quenching of [Ru(bpy)3]2+ by a series of organic dyes has been investigated by using the steady state fluorescence technique in aqueous medium. The dyes used are anthraquinone dyes: uniblue, acid blue 129, alizarin, alizarin red S and the azo dyes: congo red, sunset yellow, methyl orange, tartrazine, acid orange 63, methyl red and erichrome black T. The quenching of [Ru(bpy)3]2+ was found to obey the Stern-Volmer equation and the corresponding Stern-Volmer plots were linear indicating dynamic quenching. The quenching rate constants (kq) were calculated from the fluorescence data. The mechanism of quenching was discussed on the basis of the quenching rate constants as well as the reduction potential of dyes. The electron transfer mechanism has been proved by the calculation of Gibbs energy changes (ΔGet) by applying the Rehm-Weller equation.

Fluorescence Quenching of Phycoerythrin by Anthraquinone Dyes

Abstract Fluorescence quenching of phycoerythrin (PE) by a series of anthraquinone dyes has been investigated using steady state fluorescence technique in water medium. The dyes used were Uniblue, Acid blue 129, Alizarin and Alizarin red S. The quenching of PE was found to obey the Stern-Volmer equation and the corresponding Stern-Volmer plots were linear indicating the dynamic quenching. The quenching rate constants (k q) were calculated by using fluorescence data. The mechanism of quenching is discussed on the basis of quenching rate constants as well as the reduction potential of dyes. The electron transfer mechanism has been proved by the calculation of free energy change (ΔGet) by applying Rehm-Weller equation.

Acrylonitrile Quenching of Trp Phosphorescence in Proteins: A Probe of the Internal Flexibility of the Globular Fold

Quenching of Trp phosphorescence in proteins by diffusion of solutes of various molecular sizes unveils the frequency-amplitude of structural fluctuations. To cover the sizes gap between O2 and acrylamide, we examined the potential of acrylonitrile to probe conformational flexibility of proteins. The distance dependence of the through-space acrylonitrile quenching rate was determined in a glass at 77 K, with the indole analog 2-(3-indoyl) ethyl phenyl ketone. Intensity and decay kinetics data were fitted to a rate, k(r) =k0 exp[−(r −r0)/re], with an attenuation length re = 0.03 nm and a contact rate k0 = 3.6 × 1010 s−1. At ambient temperature, the bimolecular quenching rate constant (kq) was determined for a series of proteins, appositely selected to test the importance of factors such as the degree of Trp burial and structural rigidity. Relative to kq = 1.9 × 109 M−1s−1 for free Trp in water, in proteins kq ranged from 6.5 × 106 M−1s−1 for superficial sites to 1.3 × 102 M−1s−1 for deep cores. The short-range nature of the interaction and the direct correlation between kq and structural flexibility attest that in the microsecond-second timescale of phosphorescence acrylonitrile readily penetrates even compact protein cores and exhibits significant sensitivity to variations in dynamical structure of the globular fold.

Photoinduced Electron Transfer Occurs between 2-Aminopurine and the DNA Nucleic Acid Monophosphates: Results from Cyclic Voltammetry and Fluorescence Quenching

2-Aminopurine (2AP) is a fluorescent adenine analogue that is useful in part because its substantial fluorescence quantum yield is sensitive to base stacking with native bases in ss- and ds-DNA. However, the degree of quenching is sequence dependent and the mechanism of quenching is still a matter of some debate. Here we show that the most likely quenching mechanism in aqueous solution involves photoinduced electron transfer (PET), as revealed by cyclic voltammetry (CV) performed in aprotic organic solvents. These potentials were used with spectroscopic data to obtain excited-state reduction and oxidation potentials. Stern-Volmer (S-V) experiments using the native base monophosphate nucleotides (NMPs) rGMP, rAMP, rCMP, and dTMP were performed in aqueous solution to obtain quenching rate constants kq. The results suggest that 2AP* can act as either an electron donor or an electron acceptor depending on the particular NMP but that PET proceeds for all NMPs tested.

Studies on the Photoinduced Interaction between Zn(II) Porphyrin and Colloidal TiO2

The interaction of Zn(II) porphyrin (ZnPP) with colloidal TiO2was studied by absorption and fluorescence spectroscopy. The fluorescence emission of ZnPP was quenched by colloidal TiO2upon excitation of its absorption band. The quenching rate constant (kq) is1.24×1011 M−1 s−1. These data indicate that there is an interaction between ZnPP and colloidal TiO2nanoparticle surface. The quenching mechanism is discussed on the basis of the quenching rate constant as well as the reduction potential of the colloidal TiO2. And the mechanism of electron transfer has been confirmed by the calculation of free energy change(ΔGet)by applying Rehm-Weller equation as well as energy level diagram.

PH-Dependent Conformational Changes in Tear Lipocalin by Site-Directed Tryptophan Fluorescence

Tear lipocalin (TL), a major protein of human tears, binds a broad array of endogenous ligands. pH-dependent ligand binding in TL may have functional implications in tears. Previously, conformational selections of the AB and GH loops have been implicated in ligand binding by site-directed tryptophan fluorescence (SDTF). In this study, SDTF was applied to the AB and GH loops to investigate pH-driven conformational changes relevant to ligand binding. Both loops demonstrate significant but distinct conformational rearrangements over a wide pH range. In the low-pH transition, from 7.3 to 3.0, residues of the GH loop exhibit decreased solvent accessibilities. In acrylamide quenching experiments, the average quenching rate constant (k(q), accessibility parameter) of the residues in the GH loop is decreased approximately 38%, from 2.1 x 10(9) to 1.3 x 10(9) M(-1) s(-1). However, despite the significant changes in accessibilities for some residues in the AB loop, the average accessibility per residue remained unchanged (average k(q) = 1.2 M(-1) s(-1)). Accordingly, the low-pH transition induces conformational changes that reshuffle the accessibility profiles of the residues in the AB loop. A significant difference in the titration curves between the holo and apo forms of the W28 mutant suggests that the protonation states of the residues around position 28 modulate conformational switches of the AB loop relevant to ligand binding.

Direct Comparison of Chiral Substituent Effects for Viologens in Stereoselective Photoinduced Electron-Transfer Reactions with Zinc Myoglobin

Abstract In order to understand the detailed mechanisms of stereoselective photoinduced electron-transfer (ET) reactions of zinc-substituted myoglobin (ZnMb) with optically active viologens by flash photolysis, we prepared new optically active agents, such as (R,R)- and (S,S)-1,1′-bis(3-hydroxy-2-methylpropyl)-4,4′-bipyridinediylium dichloride ([BMV]Cl2). The photo-excited triplet state of ZnMb, 3(ZnMb)*, was successfully quenched by (R,R)- and (S,S)-[BMV]2+ ions to form the radical pair of ZnMb cation (ZnMb•+) and (R,R)- and (S,S)-[BMV]•+, followed by a thermal back ET reaction to the ground state. The ratio of the rate constants (kq) for ET quenching at 25 °C, kq(R,R)/kq(S,S) = 1.1, indicates that the (R,R)-[BMV]2+ preferentially quenches 3(ZnMb)*. The selectivity of the rate constants (kb) for the back ET from (R,R)- and (S,S)-[BMV]•+ to ZnMb•+ at 25 °C was kb(R,R)/kb(S,S) = 0.83. The highest stereoselectivity of 1.3 for [BMV]•+ was found at low temperature (10 °C) in the thermal back ET reaction, where ΔΔH≠(R–S) = 6.2 ± 1.9 kJ mol−1 and ΔΔS≠(R–S) = 19 ± 2 J mol−1 K−1 were obtained. The structural differences between [BMV]2+ and the other viologens imply that the bulky aromatic substituent seems to be more important than the distance from the chiral center to the viologen moiety in order to enhance the ET stereoselectivity.

Oxygen Reduction Kinetics and Transport Properties of (Ba,Sr)(Co,Fe)O3-δ and Related SOFC Cathode Materials

Oxygen surface reduction kinetics is studied with the help of impedance spectra measured on dense thin film microelectrodes of (Ba1-xSrx)(Co1-yFey)O3-δ (BSCF) and related SOFC cathode materials. Effective oxygen exchange rate constants kq, calculated from area-specific surface resistances Rs extracted from the impedance spectra, increase in the sequence of LSM (La1-xSrxMnO3+/-δ), LSCF (La1-xSrxCo1-yFeyO3-δ) and BSCF. The influence of bulk properties, such as electronic conductivity and Goldschmidt tolerance factor, on kq is investigated to supply information for identifying the rate-determining step. Possible roles of oxygen vacancies in the oxygen surface incorporation reaction are discussed.

Kinetic Study of Singlet-Oxygen Quenching by Caffeic Acid and Related Phenols

Abstract The second-order rate constants (kQ) of singlet-oxygen quenching by caffeic acid and related phenols were measured in ethanol and toluene. Caffeic acid derivatives deactivate singlet-oxygen largely by physical quenching at kQ = 106–107 M−1 s−1 through the charge-transfer transition state.

Diffusion-mediated electron transfer between cations in ionic solutions monitored by fluorescence quenching

The diffusion-mediated quenching rate constant kq associated with photoinduced electron transfer (ET) reaction between cations in aqueous solutions containing inert electrolyte was theoretically studied by using the modified Tachiya–Murata theory [J. Phys. Chem. 96 (1992) 8441]. The ET was assumed to occur not only at the contact distance R between the reactants but over a range of distances r>R. The calculated kq and its dependence on the ionic strength are in reasonable agreement with experimental values obtained from the single-photon counting measurements on the system 9-anthrylmethylammonium cation donor (D+)–methylviologen cation acceptor (A2+).

Photoinduced Electron Transfer between Triphenylpyrylium Ion (TPP+) and Certain Phenols

Abstract The photoinduced interaction between triphenylpyrylium ion (TPP+) and certain phenols were studied in acetonitrile medium by using absorption, steady state and time resolved fluorescence spectroscopic methods. The quenchers used were phenol, o-aminophenol, o-cresol, o-hydroxyphenol, o-chlorophenol, salicylic acid and salicylaldehyde. Linear Stern-Volmer plots were obtained from both steady state and time resolved measurements and also the calculated values of bimolecular quenching rate constants (k q) were matched well. The free energy change (ΔG et) has been evaluated by using Rehm-Weller equation for the confirmation of electron transfer from phenols to TPP+ ion.

A Homogeneous System for the Photogeneration of Hydrogen from Water Based on a Platinum(II) Terpyridyl Acetylide Chromophore and a Molecular Cobalt Catalyst

The complex [Co(dmgH)2pyCl]2+ (1, dmgH = dimethylglyoximate, py = pridine) has been used as a molecular catalyst for visible light driven hydrogen production in the presence of [Pt(tolylterpyridine)(phenylacetylide)]+ (3) as a photosensitizer and triethanolamine (TEOA) as a sacrificial reducing agent. Complex 3 is quenched oxidatively by [Co(dmgH)pyCl]2+ (1) with a rate constant kq of 1.27 x 10(9) M(-1) s(-1). Photogeneration of H2 is only seen when 1 + 3 + TEOA are all present. H2 production is maximized for this system at pH 8.5 and declines to very low levels at pH < 7 and pH > 12. Irradiation of the reaction solution initially containing 1.61 x 10(-2) M TEOA, 1.11 x 10(-5) M of 3, and 1.99 x 10(-4) M of Co catalyst 1 in MeCN/water (3:2 v/v) at pH = 8.5 for 10 h with lambda > 410 nm yields 400 turnovers of H2. When TEOA is 0.27 M, approximately 1000 turnovers are obtained after 10 h of irradiation. Spectroscopic study of the photolyses solutions suggests that H2 formation proceeds via Co(I) and protonation to form Co(III) hydride species.

Fluorescence Quenching of Meso-Tetrakis(p-Sulfonatophenyl)Porphyrin (TSPP) by certain Organic Dyes

Abstract The fluorescence quenching of meso-tetrakis (p-sulfonatophenyl) porphyrin (TSPP) by certain anthraquinone and azo dyes has been investigated using steady state fluorescence technique. The dyes used were Uniblue-A, Acid blue 129, Alizarin, Alizarin red S, Erichrome Black T, Tartrazine, Methyl orange, Methyl red, Acid orange 63 and Congo red. The quenching was found to obey Stern-Volmer equation and the corresponding Stern-Volmer plots were linear. The quenching rate constant (k q ) is in the order of 0.19–6.08×1012 M− 1s− 1. This study reveals effective quenching of TSPP by organic dyes.

A Study on the Fluorescence Quenching of Eosin by certain Organic Dyes

Abstract The fluorescence quenching of Eosin by certain dyes has been investigated using steady state technique. The dyes used are Uniblue-A, Acid blue 129, Alizarin, Alizarin red S, Erichrome Black T, Tartrazine, Methyl orange, Methyl red, Acid orange 63, Congo red. The quenching was found to obey the Stern-Volmer equation and the corresponding Stern-Volmer plots were linear in the range of quencher concentration used [0−5×10− 5] M. The quenching rate constant (k q ) is in the range of 0.85–26.65×1012 M− 1s− 1. The quenching mechanism is discussed on the basis of the quenching rate constants as well as the reduction potential of the dyes. Rehm-Weller equation is applied for the calculation of free energy change (ΔGet).

Photosensitization of Colloidal TiO2 with ZnTPP and Pyrene

The interaction of Zinc(II) tetraphenyl porphyrin (ZnTPP) and pyrene with colloidal TiO2 was studied by absorption and fluorescence spectroscopy. Upon excitation of its absorption band, the fluorescence emission of ZnTPP and pyrene was quenched by colloidal TiO2. The bimolecular quenching rate constant (kq ) is 4.09 × 1011 M−1s−1 for ZnTPP and 2.09 × 109 M−1s−1 for pyrene. ZnTPP and pyrene adsorbed on colloidal TiO2 can participate in the sensitization process by injecting electrons from their excited states into the conduction band of TiO2. The mechanism is discussed on the basis of free energy change (ΔGet) obtained from Rehm-Weller equation considering quenching rate constants and conduction band potential of colloidal TiO2.

Photoinduced Electron and H-atom Transfer Reactions of Xanthone by Laser Flash Photolysis

The property of the lowest excited triplet states of xanthone in acetonitrile was investigated using time-resolved laser flash photolysis at 355 nm. The transient absorption spectra and the quenching rate constants (kq) of the excited xanthone with several amines were determined. Good correlation between lgkq and the driving force of the reactions suggests the electron transfer mechanism, except aniline and 3-nitroaniline (3-NO2-A) which showed energy transfer mechanism. With the appearance of ketyl radical, hydrogen atom transfer also happened between xanthone and dimethyl-p-toluidine, 3,5,N,N-tetramethylaniline, N,N-dimethylaniline, and triethylamine. Therefore, both electron transfer and H-atom transfer occured in these systems. Great discrepancies of kq values were discovered in H-atom abstraction reactions for alcohols and phenols, which can be explained by different abstraction mechanisms. The quenching rate constants between xanthone and alcohols correlate well with the -C-H bonding energy of alcohols.

Interaction of Co(II) with Bovine Serum Albumin under UV C Irradiation

AbstractThe interaction of Co(II) with BSA under UV C (253.7 nm) irradiation under physiological conditions has been studied by UV‐vis spectrum, ultraviolet second‐derivative spectroscopy and fluorescence spectrum. The quenching rate constant kq and the association constant Ka were calculated according to Stern‐Volmer equation based on the quenching of the fluorescence of BSA by Co(II).

The Long-Lived Triplet Excited State of an Elongated Ketoprofen Derivative and Its Interactions with Amino Acids and Nucleosides

The aim of the present work was to find a ketoprofen (KP) equivalent suitable for time-resolved studies on the interactions of its KP-like triplet state with biomolecules or their simple building blocks, under physiologically relevant conditions. Such a compound should fulfill the following requirements: (i) it should be soluble in aqueous media; (ii) its triplet lifetime should be longer than that of KP, ideally in the microsecond range; and (iii) its photodecarboxylation should be slow enough to avoid interference in the time-resolved studies associated with formation of photoproducts. Here, the glycine derivative of ketoprofen (KPGly) has been found to fulfill all the above requirements. In a first stage, the attention has been focused on the photophysical and photochemical properties of KPGly, and then on its excited-state interactions with key amino acids and nucleosides. In acetonitrile, the typical benzophenone-like triplet-triplet absorption (3KPGly) with lambda(max) at 520 nm and a lifetime of 5.3 micros was observed. This value is very close to that of 3KP (5.6 micros) obtained under the same conditions. In methanol, the 3KPGly features were also close to those of 3KP with detection of a short-lived triplet state that evolves to give a ketyl radical. By contrast with the behavior of KP, in deaerated aqueous solutions at pH = 7.4, the transient detected in the case of KPGly displayed two bands at lambda(max) at 330 and 520 nm, very similar to those observed in acetonitrile solution but with a lifetime of 7.5 micros at 520 nm. Hence, it was assigned to the KPGly triplet. In the case of KP, efficient decarboxylation occurs in the subnanosecond time scale, via intramolecular electron transfer. This process gives rise to a detectable carbanion intermediate (lifetime approximately 250 ns) and prevents detection of the shorter-lived 3KP signal. In a second stage, the attention has been focused on the excited-state interactions between 3KPGly and amino acids or nucleosides; for this purpose, 2'-deoxyguanosine (dGuo), thymidine (Thd), tryptophan (Trp), and tyrosine (Tyr) have been chosen as photosensitization targets. In general, efficient quenching (rate constant kq > 109 M(-1) x s(-1)) was observed; it was attributed for dGuo, Tyr, and Trp to a photochemical reaction involving initial electron transfer from the biological target to 3KPGly, followed by proton transfer from the amino acid or the nucleoside radical cation to KPGly-*. As a matter of fact, ketyl radical together with guanosinyl, tyrosinyl, or tryptophanyl radicals were detected; this supports the proposed mechanism. The results with Thd were somewhat different, as the efficient 3KPGly quenching was ascribed to oxetane formation by a Paterno Büchi photocycloaddition.