Degenerate Electron Exchange Research Articles

Temperature dependence of the degenerate electron exchange between guanosine-5′-monophosphate cation and its short-lived radical dication in aqueous solution

Temperature dependence of the degenerate electron exchange between guanosine-5′-monophosphate cation and its short-lived radical dication in aqueous solution

Photoinduced flavin-tryptophan electron transfer across vesicle membranes generates magnetic field sensitive radical pairs

ABSTRACTDue to the photobiology of the flavoproteins DNA photolyase and cryptochrome, electron transfer reactions between flavins and tryptophan are of significant biological relevance. In addition, electron transfer across vesicle membranes has also seen much attention. In this work, we study the electron transfer reaction between flavins and tryptophan across lipid bilayer membranes in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine small unilamellar vesicles using time-resolved optical absorption microspectroscopy and magnetically affected reaction yield spectroscopy. We demonstrate that riboflavin tetrabutyrate is embedded in the vesicle bilayer and can undergo electron transfer with tryptophan molecules in either the inner water pool or the bulk solution. Remarkably, flavin mononucleotide encapsulated in the inner water pool can undergo electron transfer across the vesicle bilayer to generate a magnetically sensitive radical pair with tryptophan molecules located in the bulk solution. The observed kinetics suggest that back electron transfer occurs between radical pairs generated by diffusive reencounter, either in the vesicle surface water or via electron hopping through degenerate electron exchange.

CIDNP study of sensitized photooxidation of S-methylcysteine and S-methylglutathione in aqueous solution

Time-resolved CIDNP (chemically induced dynamic nuclear polarization) was applied to the investigation of the photo-oxidation of S-methylcysteine, Cys(Me), and S-methylglutathione, GS(Me). It was established that the reaction of Cys(Me) with a photosensitizer, triplet excited 4-carboxybenzophenone or 3,3′,4,4′-carboxybenzophenone, occurs via electron transfer from the sulfur atom with the formation of a sulfur-centered cation radical. From the pH-dependence of nuclear polarization it is established that the pKa value of the terminal amino group of this radical is 4.3, which is much lower than that of the initial compound, pKa = 9.0. At pH below 4.3 the sulfur-centered cation radical of Cys(Me) formed as a result of triplet quenching participates in the degenerate electron exchange reaction with the parent molecule resulting in a fast decay of CIDNP in time. At pH above this pKa, CIDNP is constant in time, which is a result of fast decarboxylation of the cyclic radical with a three-electron bond between sulfur and oxygen atoms that is formed from a linear radical with an uncharged amino group. The involvement of this radical was established by the dependence of CIDNP on the external magnetic field ranging from 0.1 to 7 T. For GS(Me), the formation of the cyclic radical the indication of which is time-independent CIDNP correlates with the deprotonation of the terminal amino group of the glutamate residue of GS(Me). The formation of a cyclic radical with a three-electron bond between sulfur and nitrogen atoms is ruled out because of the absence of polarization of the CH-proton in α-position with respect to the amino group, and also by the CIDNP field dependence which is typical for an (S∴O) radical. Thus, the cyclic radical originating from one-electron removal from sulfur of GS(Me) is the 10-membered one with a three-electron bond between the sulfur atom and the oxygen atom of the carboxylic group of the glutamate residue.

Investigations of the Degenerate Intramolecular Charge Exchange in Symmetric Organic Mixed Valence Compounds: Solvent Dynamics of Bis(triarylamine)paracyclophane Redox Systems

Triarylamines are important hole-transport components in optoelectronic devices. Understanding the factors controlling their intra- and intermolecular electron transfer properties is crucial to the application and optimization of organic hole conductors. Here, we report on the degenerate intramolecular electron exchange reactions of several purely organic mixed valence compounds based on the bis(triarylamine)paracyclophane structural unit, which are archetypical molecular wires. Different bridging moieties are compared, and the foremost impact of the solvent environment on the rate of electron transfer is demonstrated. Comparing the rate constants found for many different solvents, we find that surprisingly the electron transfer reaction is limited by the solvent dynamic effect and not strongly impacted by the peculiarities of the bridging moiety, a finding which was not anticipated for this type of long-range, thermally activated intramolecular charge transfer from previous studies. Rate constants are measured by dynamic electron paramagnetic resonance spectroscopy. Our insight was possible using various solvents spanning a wide range of longitudinal relaxation times (0.24 ps ≤ τL ≤ 516 ps) and Pekar factors (0.298 ≤ γ ≤ 0.526).

Theoretical Treatment of Degenerate Electron Exchange and Dimerization in Spin Dynamics of Radical Ion Pairs as Observed by Magnetic Field Effects

In this work we have compared manifestations of degenerate electron exchange (DEE) and dimerization reactions in MARY (magnetically affected reaction yield) spectroscopy and time-resolved magnetic field effects (TR-MFE) of radical ion pairs (RIPs). It is shown that dimerization results in phase and frequency changes of the quantum beats in TR-MFE traces, whereas DEE leads to relaxation-type changes in the spin dynamics of RIPs. Dimerization does not change the width of MARY lines considerably, but leads to new lines corresponding to dimer radical ions. In contrast, DEE substantially changes the MARY line widths. Our treatment provides ways to discriminate between both processes from experimental MARY spectra and TR-MFE kinetics. An experimental example dealing with concentration-dependent broadening of the MARY lines is also discussed.

1H CIDNP study of the kinetics and mechanism of the reversible photoinduced oxidation of tryptophyl-tryptophan dipeptide in aqueous solutions

The pulsed method of chemically induced dynamic nuclear polarization (CIDNP) with the microsecond time resolution was used to study the nuclear polarization kinetics of Trp-Trp (tryptophyl-tryptophan) dipeptide protons in the photoreaction between the dipeptide and 2,2′-dipyridyl in aqueous solutions at pH from 2 to 10. The dependence of the selectivity of the reversible photoinduced oxidation of the dipeptide by 2,2′-dipyridyl in the triplet-excited state was found. When the pH of the solutions was below pKa of the terminal amino group, polarization was observed only for the protons of the C-terminal residue. The threshold effect of protonation of the terminal amino group of the dipeptide on the selectivity of both the deactivation of the 2,2′-dipyridyl triplet state and the degenerate intermolecular electron exchange was revealed. No intramolecular electron migration in the oxidized dipeptide was detected. At the pH exceeding pKa of the terminal amino group of the dipeptide, the quenching of the triplet excited state of 2,2′-dipyridyl is not selective. This is manifested as equal intensities of the nuclear polarization signals of the both residues of tryptophan in the 1H NMR spectrum of the dipeptide containing the CIDNP signals of the geminate reaction products. The rate constant of the degenerate electron exchange and nuclear paramagnetic relaxation times of the protons of the neutral radical and radical cation of the dipeptide were determined by the numerical simulation of the nuclear polarization kinetics.

Deprotonation of Transient Guanosyl Cation Radical Catalyzed by Buffer in Aqueous Solution: TR-CIDNP Study

The reaction of deprotonation of the guanosyl cation radical formed in the photoinduced reaction of guanosine monophospate (GMP) with triplet 2,2′-dipyridyl-d8 is studied in aqueous solution by time-resolved chemically induced dynamic nuclear polarization (TR-CIDNP). In the course of the cyclic photoreaction, spin-polarized products are generated. Their polarization patterns that reflect the properties at the radical stage are analyzed using high-resolution nuclear magnetic resonance. The identification of transient radicals contributing to the polarization kinetics is based on its sensitivity to the degenerate electron exchange reaction of transient radicals with the parent diamagnetic molecules. Degenerate electron exchange is allowed only for the cation radical and manifests itself in the fast decay of the CIDNP signal in time with the rate of decay proportional to the concentration of parent GMP molecules. Because the formation of the neutral transient radical stops the exchange, the deprotonation changes the CIDNP kinetics from a decaying to a growing one. The rate constant of deprotonation, k d, was obtained from modeling of CIDNP kinetics data with taking into consideration the difference of the CIDNP enhancement factors for neutral and cation guanosyl radicals. The value obtained at pH* 5 for k d = 1 × 106 s−1 is consistent with the proton dissociation constant of the radical (pK a = 3.9). The linear dependence of the deprotonation rate on the buffer concentration is revealed for phosphate, formate, and acetate. Deprotonation is catalyzed by the buffer to a degree that depends on the difference in pK a value of the buffer and the guanosyl cation radical in full accordance with Eigen’s model.

Photo-CIDNP Study of Transient Radicals of Met-Gly and Gly-Met Peptides in Aqueous Solution at Variable pH

Time-resolved chemically induced dynamic nuclear polarization (CIDNP) was applied to the investigation of the photo-oxidation of two sulfur containing peptides, glycylmethionine (Gly-Met) and methionylglycine (Met-Gly). It was established that the reaction of Gly-Met with a photosensitizer, triplet 4-carboxybenzophenone, occurs via electron transfer from the sulfur atom and also from the terminal amino group in its uncharged state. The latter process leads to the formation of nuclear polarization of the alpha-protons of the glycine residue. The sulfur-centered cation radical of Gly-Met formed as a result of triplet quenching participates in the degenerate electron exchange reaction with the parent molecule. The rate constant of this reaction obtained from a simulation of the CIDNP kinetics is 2x10(8) M(-1) s(-1). Two channels of triplet quenching were also found for the Met-Gly peptide at pH values above the pKa of the terminal amino group: electron transfer from the amino group and from the sulfur atom. On the basis of the analysis of the CIDNP spectra and kinetics, it was found that at pH below pKa of the terminal amino group photo-oxidation of Met-Gly leads to the formation of an open-chain S-centered cation radical, which releases a proton from its N-terminal amino group to form a five-membered cyclic radical structure with a three electron bond between the S and N atoms. The rate constant of deprotonation obtained to be 1.8x10(5) s(-1) is in agreement with the pKa=4.7 of the S-centered radical of Met-Gly determined from the pH dependence of nuclear polarization. At pH>pKa, the aminium radicals formed in both peptides as a result of electron transfer from the lone pair of N-terminal amino group undergo deprotonation to the neutral aminyl radical on the submicrosecond time scale. The involvement of the different radicals was confirmed by the dependence of CIDNP on the external magnetic field ranging from 0.1 T to 7 T.

Degenerate electron exchange reaction involving n-nonane radical cation in solution

Solvent radical cations (RCs) are the primary positive charge carriers that emerge upon ionizing irradiation of alkane solutions. Conductivity measurements of irradiated alkanes show that the rate and mechanism of solvent RC (hole) transport can differ radically in different types of alkanes [1, 2]. For example, the solvent RC mobility in cyclohexane is two orders of magnitude higher than the mobility of molecular ions. Such a high mobility is associated with fast degenerate electron exchange (DEE) between the solvent molecules and the holes:

Time-resolved CIDNP as a probe of 2,2′-dipyridyl radical anion complexation with β-cyclodextrin

The photochemical reaction of 2,2′-dipyridyl (DP) with 5-sulfosalicylic acid (SSA) was studied using time-resolved CIDNP in homogeneous and β-cyclodextrin (β-CD) aqueous solutions. The degenerate electron exchange rate constant of DP radical anion with diamagnetic precursor was estimated as 1.1 × 10 8 M −1 s −1 from CIDNP kinetic in homogeneous solution. It was found that DP-β-CD inclusion complex formation alter the kinetics of DP polarization. For β-CD solution the drastic decreasing of nuclear relaxation time of DP radical anion was found comparing to the homogeneous solution.

Degenerate Electron Exchange Reaction of n-Alkane Radical Cations in Solution

The degenerate electron exchange (DEE) reaction involving radical cations (RCs) of n-nonane, n-dodecane, and n-hexadecane in n-hexane solution was studied over the temperature range 253-313 K using the method of time-resolved magnetic field effect in recombination fluorescence of spin-correlated radical ion pairs. In the dilute solutions the rate constant of DEE was found to be 200 times slower than the diffusion limit. Using n-nonane as an example, we showed that two reasons are responsible for the low value of the RC self-exchange rate: (1) conformational variability of molecules and RCs and (2) the activation barrier of DEE reaction. The calculations of the reaction enthalpy performed by the B3LYP/6-31G(d) method indicated that electron transfer can be effective only upon collision of RC with a neutral molecule either in the all-trans conformation or in the conformation differing from the latter by rotation of the end ethyl fragment. The activation barrier of the DEE reaction was estimated using the reorganization energy of the internal degrees of freedom calculated at the B3LYP level and was found to be about 6 kcal/mol. A possible influence of the interaction between RC and a neutral molecule in an encounter complex on DEE rate constant is also discussed.

A MARY study of radical anions of fluorinated benzenes

The paper reports the results of two studies centred on the properties of radical anions of polyfluorinated benzenes in liquid dodecane solutions at room temperature using level crossing (MARY) spectroscopy of spin-correlated radical ion pairs generated in nonpolar solutions by ionizing radiation. The first study takes advantage of the large, nearly two orders of magnitude, span of hyperfine couplings in the series of otherwise similar radical anions to probe into the peculiarities of degenerate electron exchange reaction for system with large hyperfine couplings. The second study provided the first observation of radical anion of 1,3,5-trifluorobenzene and yielded a hyperfine coupling constant of 7.2–7.5 mT with three equivalent fluorine nuclei and coherent lifetime of the radical anion about 20 ns at room temperature, which has thus far precluded its observation by other techniques.

ESR-Investigations on the Dynamic Solvent Effects of Degenerate Electron Exchange Reactions. Part I: Cyanobenzenes

Summary The rates of degenerate electron exchange (electron self-exchange) of various cyanobenzenes have been measured by EPR line broadening technique in nine different solvents at room temperature. The molecules studied comprise besides benzene-1,2-dicarbonitrile, benzene-1,4-dicarbonitrile and benzene-1,2,4,5-tetracarbonitrile, the two isomeric tricyanobenzenes, benzene-1,2,3-tricarbonitrile and benzene-1,2,4-tricarbonitrile, the anion radicals of which have not been characterized before. The experimentally observed rates vary from 4.5 × 108 to 44.0 × 108 M−1 s−1 and show the pronounced dependence on the longitudinal relaxation times, τL, of the solvents. The solvent dynamical effect so manifested is confirmed with remarkable clarity using solvents spanning a wide range of τL-values, which comprise acetonitrile (0.2 ps) and o-dichlorobenzene (6.0 ps) at its extremes. The rate constants are compared with Marcus theory using the continuum model (CM) and the mean spherical approximation (MSA) for the outer sphere reorganization energies and Nelson’s method for the inner sphere reorganization energies. Furthermore, an estimation of the resonance splitting energies, V RP, is given based on the experimental rates.

ESR and ENDOR investigations of the degenerate electron exchange reactions of various viologens in solution. Solvent dynamical effects

The temperature dependences of the rates of the degenerate electron transfer of various viologens (1,1′-di(hydrocarbyl)-4,4′-bipyridinium salts) are measured in seven different solvents by means of electron spin resonance (ESR) line broadening. Rates vary between 1.7·108 and 1.1·109 M−1s−1 at room temperature and clearly show a solvent dynamical effect, which is inferred from the dependence of the rate constants on the longitudinal relaxation time of the solvent. Activation energies ranging from 5.3 to 24.4 kJ mol−1 are found. For the first time, hyperfine coupling constants are reported for the radical cations of the hydroxyethyl viologen and the amino viologen based on both continuous-wave ESR and electron-nuclear double resonance spectroscopy. Furthermore, the temperature and the solvent dependence of the hyperfine coupling constants of the methyl viologen radical cation are reported.

Longitudinal Electron Spin Relaxation Induced by Degenerate Electron Exchange as Studied by Time-Resolved Magnetic Field Effects

T(1) paramagnetic relaxation of radical ions induced by degenerate electron exchange (DEE) reactions is studied theoretically and experimentally. Our theoretical analysis shows that T(1) relaxation time is well described by the Redfield theory at arbitrary values of the characteristic DEE time tau. Longitudinal relaxation of norbornane (NB) radical cation is studied by means of the time-resolved magnetic field effects (TR-MFE) technique; the rate constant of DEE involving NB(*+) radical cation and NB neutral molecule is obtained. Advantages of the TR-MFE technique and its potential for measuring the short DEE times are discussed in detail.

Time-Resolved Chemically Induced Dynamic Nuclear Polarization Studies of Structure and Reactivity of Methionine Radical Cations in Aqueous Solution as a Function of pH

Using time-resolved chemically induced dynamic nuclear polarization (CIDNP) techniques, we have studied the mechanism of the photoreactions of triplet excited 4-carboxybenzophenone (CBP) with l-methionine (Met) and 3-(methylthio)propylamine (MTPA) in aqueous solution and the details of the formation of CIDNP at pH from 6.7 to 13.6. At a pH below the pKa of the nitrogen atom of Met, the CIDNP is strongly affected by degenerate electron exchange between the S-S cationic radical dimer and the zwitterionic form of Met with the rate constant kex = 3.4 x 10(8) s(-1) providing an exhaustive explanation of the pH dependence of steady-state CIDNP that was previously interpreted as a manifestation of fast interconversion among three different methionine radical species (Goez, M.; Rozwadowski, J. J. Phys. Chem. A 1998, 102, 7945-7953). By analyzing the polarization of different protons formed in geminate recombination as a function of the pH, we obtained the branching ratio between two reaction pathways for oxidative quenching of (T)CBP via electron transfer from the sulfur and nitrogen atoms of Met and MTPA. Nuclear spin-lattice relaxation times were determined in the dimeric cation radical of Met (T1,S = 8.5 micros). In the cyclic radical cation of MTPA with a three-electron two-center S-N bond, the estimated paramagnetic relaxation is comparatively slow for all protons. Fast deprotonation of the primary aminium radical cation of MTPA and Met in strongly basic solution takes place on the submicrosecond time scale leading to efficient formation of CIDNP in the neutral aminyl radical.

The effect of signs of hyperfine coupling constant on MARY spectra affected by degenerate electron exchange

It is demonstrated that the shape of MAgnetic field effect on Reaction Yield (MARY) spectra depends substantially on the relative signs of the hyperfine coupling constants (HFC) of the constituents of the transient radical pair. The effect is exemplified in terms of the pyrene/benzene-1,4-dicarbonitrile system, the spin dynamics of which are calculated with rigorous account of all HFCs and the peculiarities caused by a degenerate electron exchange reaction. It is shown that MARY spectra calculated using the same sign for all HFCs and those obtained using the signs as predicted from ab initio calculations differ significantly with respect to the amplitude of the low magnetic field feature.

Application of the encounter theory to the description of degenerate electron exchange reactions: spin kinetics at low magnetic fields

Consistent theoretical treatment of degenerate electron exchange reaction in the liquid phase within the framework of the encounter theory has been performed. Finite duration of the reactant encounter and reaction reversibility have been taken into account. Applicability range of the widely used phenomenological approach has been established, encounter theory equations for the radical ions having large hyperfine couplings have been obtained. Case of a radical ion with one magnetic nucleus has been studied in detail. Theoretical treatment of degenerate electron exchange reaction within the framework of the semiclassical approximation corresponding to a large number of magnetic nuclei has been carried out.

Intramolecular Electron Transfer in Tryptophan−Tyrosine Peptide in Photoinduced Reaction in Aqueous Solution

The formation of chemically induced dynamic nuclear polarization (1H-CIDNP) and its time evolution in course of the photochemical reactions of 2,2‘-dipyridyl in its triplet state with the tryptophan−tyrosine dipeptide and with a 1:1 mixture of N-acetyl tryptophan and N-acetyl tyrosine is studied in acid (pH 3.0), neutral (pH 7.4), and basic (pH 11.4) solution. Time resolution on the microsecond scale allows us to establish the full reaction scheme and to determine all relevant transient intermediates. The stationary value of spin polarization is found to depend strongly on secondary reaction steps and does only in part reflect the primary process of triplet quenching. Degenerate electron exchange, spin relaxation in paramagnetic intermediates, and radical pair reactions in the bulk are identified from the polarization kinetics. In particular, the increase of tyrosine quenching efficiency and the influence of intramolecular electron transfer from the tyrosine moiety to the transient tryptophan radical are revealed when the amino acids are linked in the dipeptide in comparison with the case that they are separated in the 1:1 mixture. The strong effect of linkage on the amplitude and the kinetics of spin polarization is discussed with regard to application of this method to studying structure and folding kinetics of proteins.

Time-resolved CIDNP and laser flash photolysis study of the photoreaction between triplet 2,2′-dipyridyl and guanosine-5′-monophosphate in water

Laser flash photolysis and time-resolved CIDNP have been applied to the investigation of the kinetics and the mechanism of the photoreaction between triplet 2,2′-dipyridyl (DP) and guanosine-5′-monophosphate (GMP) over a wide pH range in aqueous solution. The pH dependence of the rate constant kq of quenching the triplet dipyridyl by the nucleotide has been measured. Upon pH titration, four pairs of the reacting species contribute to the observed value of kq: pH < 2.4, TDPH+ and GH+; 2.4 < pH < 5.8, TDPH+ and G; 5.8 < pH < 9.4, TDP and G; and pH > 9.4, TDP and G(−H)−, with the corresponding quenching rate constants k1 = 1.3 × 109 M−1 s−1, k2 = 2.7 × 109 M−1 s−1, k3 = 1.6 × 108 M−1 s−1, k4 = 1.1 × 109 M−1 s−1. Based on LFP and CIDNP data, the established mechanism of the quenching reaction is hydrogen atom transfer in neutral solution (5.8 < pH < 9.4), and electron transfer in all other pH regions. Kinetic CIDNP measurements reveal that in acidic and basic solutions the CIDNP kinetics for GMP is determined by the degenerate electron exchange between the GMP radical and its parent molecule with the rate constants 1.3 × 108 M−1 s−1 (acidic conditions) and 4.0 × 107 M−1 s−1 (basic conditions). The nuclear paramagnetic relaxation time for the proton H8 of GMP, T1 = 20 ± 5 μs, obtained from the simulations of the CIDNP kinetics, is found to be independent of the protonation state of the radical.