Radical Pair Formation Research Articles

Identification of the First Steps in Charge Separation in Bacterial Photosynthetic Reaction Centers of Rhodobacter sphaeroides by Ultrafast Mid-Infrared Spectroscopy: Electron Transfer and Protein Dynamics

Time-resolved visible pump/mid-infrared (mid-IR) probe spectroscopy in the region between 1600 and 1800 cm −1 was used to investigate electron transfer, radical pair relaxation, and protein relaxation at room temperature in the Rhodobacter sphaeroides reaction center (RC). Wild-type RCs both with and without the quinone electron acceptor Q A, were excited at 600 nm (nonselective excitation), 800 nm (direct excitation of the monomeric bacteriochlorophyll (BChl) cofactors), and 860 nm (direct excitation of the dimer of primary donor ( P) BChls ( P L/ P M)). The region between 1600 and 1800 cm −1 encompasses absorption changes associated with carbonyl (C O) stretch vibrational modes of the cofactors and protein. After photoexcitation of the RC the primary electron donor P excited singlet state ( P*) decayed on a timescale of 3.7 ps to the state P + B L − (where B L is the accessory BChl electron acceptor). This is the first report of the mid-IR absorption spectrum of P + B L − ; the difference spectrum indicates that the 9-keto C O stretch of B L is located around 1670–1680 cm −1. After subsequent electron transfer to the bacteriopheophytin H L in ∼1 ps, the state P + H L − was formed. A sequential analysis and simultaneous target analysis of the data showed a relaxation of the P + H L − radical pair on the ∼20 ps timescale, accompanied by a change in the relative ratio of the P L + and P M + bands and by a minor change in the band amplitude at 1640 cm −1 that may be tentatively ascribed to the response of an amide C O to the radical pair formation. We conclude that the drop in free energy associated with the relaxation of P + H L − , is due to an increased localization of the electron hole on the P L half of the dimer and a further consequence is a reduction in the electrical field causing the Stark shift of one or more amide C O oscillators.

The Primary Electron Donor of Photosystem II of the Cyanobacterium Acaryochloris marina Is a Chlorophyll d and the Water Oxidation Is Driven by a Chlorophyll a/Chlorophyll d Heterodimer

We present a theoretical analysis of the flash-induced absorbance difference spectrum assigned to the formation of the secondary radical pair P(+)QA(-) in photosystem II of the chlorophyll d-containing cyanobacterium Acaryochloris marina. An exciton Hamiltonian determined previously for chlorophyll a-containing photosystem II complexes is modified to take into account the occupancy of certain binding sites by chlorophyll d instead of chlorophyll a. Different assignments of the reaction center pigments to chlorophyll a or d from the literature are investigated in the calculation of the absorbance difference spectrum. A quantitative explanation of the experimental data requires one chlorophyll a molecule per reaction center, located at the site of P(D1). The remaining sites are occupied by chlorophyll d and pheophytin a. By far, the lowest site energy is found for the accessory chlorophyll of the D1 branch, Chl(D1), which represents the sink of excitation energy and therefore the primary electron donor. The cationic state is stabilized at the chlorophyll a, which drives the oxidation of water.

Kinetic and Thermodynamic Characterization of CoII−Substrate Radical Pair Formation in Coenzyme B12-Dependent Ethanolamine Ammonia-Lyase in a Cryosolvent System by Using Time-Resolved, Full-Spectrum Continuous-Wave Electron Paramagnetic Resonance Spectroscopy

The formation of the Co(II)-substrate radical pair catalytic intermediate in coenzyme B12 (adenosylcobalamin)-dependent ethanolamine ammonia-lyase (EAL) from Salmonella typhimurium has been studied by using time-resolved continuous-wave electron paramagnetic resonance (EPR) spectroscopy in a cryosolvent system. The 41% v/v DMSO/water cryosolvent allows mixing of holoenzyme and substrate, (S)-2-aminopropanol, at 230 K under conditions of kinetic arrest. Temperature step from 230 to 234-248 K initiates the cleavage of the cobalt-carbon bond and the monoexponential rise (rate constant, k(obs) = tau(obs)(-1)) of the EPR-detected Co(II)-substrate radical pair state. The detection deadtime: tau(obs) ratio is reduced by >10(2), relative to millisecond rapid mixing experiments at ambient temperatures. The EPR spectrum acquisition time is <tau(obs), which allows continuous acquisition of spectra during progress of the reaction. The k(obs) values and Co(II)-substrate radical pair amplitudes are independent of substrate concentration at each temperature. Therefore, the reaction occurs from the enzyme x coenzyme x substrate ternary complex. The constant value of the Co(II)-substrate radical pair amplitude at reaction times >5tau(obs), the approximately 10(2)-fold slower rate of the substrate radical rearrangement reaction relative to k(obs), and the reversible temperature dependence of the amplitude indicate that the Co(II)-substrate radical pair and ternary complex are essentially at equilibrium. The reaction is thus treated as a relaxation to equilibrium by using a linear two-step, three-state mechanism. The intermediate state in this mechanism, the Co(II)-5'-deoxyadenosyl radical pair, is not detected by EPR at signal-to-noise ratios of 10(3), which indicates that the free energy of the Co(II)-5'-deoxyadenosyl radical pair state is >3.3 kcal/mol, relative to the Co(II)-substrate radical pair. Van't Hoff analysis yields DeltaH13 = 10.8 +/- 0.8 kcal/mol and DeltaS13 = 45 +/- 3 cal/mol/K for the transition from the ternary complex to the Co(II)-substrate radical pair state. The free energy difference, DeltaG13, is zero to within one standard deviation over the temperature range 234-248 K. The extrapolated value of DeltaG13 at 298 K is -2.6 +/- 1.2 kcal/mol. The estimated EAL protein-associated contribution to the free energy difference is DeltaG(EAL) = -24 kcal/mol at 240 K, and DeltaH(EAL) = -13 kcal/mol and DeltaS(EAL) = 38 cal/mol/K. The results show that the EAL protein makes both strong enthalpic and entropic contributions to overcome the large, unfavorable cobalt-carbon bond dissociation energy, which biases the reaction in the forward direction of Co-C bond cleavage and Co(II)-substrate radical pair formation.

Low-Temperature Pulsed EPR Study at 34 GHz of the Triplet States of the Primary Electron Donor P865and the Carotenoid in Native and Mutant Bacterial Reaction Centers ofRhodobacter sphaeroides

The photosynthetic charge separation in bacterial reaction centers occurs predominantly along one of two nearly symmetric branches of cofactors. Low-temperature EPR spectra of the triplet states of the chlorophyll and carotenoid pigments in the reaction center of Rhodobacter sphaeroides R-26.1, 2.4.1 and two double-mutants GD(M203)/AW(M260) and LH(M214)/AW(M260) have been recorded at 34 GHz to investigate the relative activities of the "A" and "B" branches. The triplet states are found to derive from radical pair and intersystem crossing mechanisms, and the rates of formation are anisotropic. The former mechanism is operative for Rb. sphaeroides R-26.1, 2.4.1, and mutant GD(M203)/AW(M260) and indicates that A-branch charge separation proceeds at temperatures down to 10 K. The latter mechanism, derived from the spin polarization and operative for mutant LH(M214)/AW(M260), indicates that no long-lived radical pairs are formed upon direct excitation of the primary donor and that virtually no charge separation at the B-branch occurs at low temperatures. When the temperature is raised above 30 K, B-branch charge separation is observed, which is at most 1% of A-branch charge separation. B-branch radical pair formation can be induced at 10 K with low yield by direct excitation of the bacteriopheophytin of the B-branch at 590 nm. The formation of a carotenoid triplet state is observed. The rate of formation depends on the orientation of the reaction center in the magnetic field and is caused by a magnetic field dependence of the oscillation frequency by which the singlet and triplet radical pair precursor states interchange. Combination of these findings with literature data provides strong evidence that the thermally activated transfer step on the B-branch occurs between the primary donor, P865, and the accessory bacteriochlorophyll, whereas this step is barrierless down to 10 K along the A-branch.

Charge Separation and Energy Transfer in the Photosystem II Core Complex Studied by Femtosecond Midinfrared Spectroscopy

The core of photosystem II (PSII) of green plants contains the reaction center (RC) proteins D1D2-cytb559 and two core antennas CP43 and CP47. We have used time-resolved visible pump/midinfrared probe spectroscopy in the region between 1600 and 1800 cm −1 to study the energy transfer and charge separation events within PSII cores. The absorption difference spectra in the region of the keto and ester chlorophyll modes show spectral evolution with time constants of 3 ps, 27 ps, 200 ps, and 2 ns. Comparison of infrared (IR) difference spectra obtained for the isolated antennas CP43 and CP47 and the D1D2-RC with those measured for the PSII core allowed us to identify the features specific for each of the PSII core components. From the presence of the CP43 and CP47 specific features in the spectra up to time delays of 20–30 ps, we conclude that the main part of the energy transfer from the antennas to the RC occurs on this timescale. Direct excitation of the pigments in the RC evolution associated difference spectra to radical pair formation of P D 1 + Phe o D 1 − on the same timescale as multi-excitation annihilation and excited state equilibration within the antennas CP43 and CP47, which occur within ∼1–3 ps. The formation of the earlier radical pair Ch l D 1 + Phe o D 1 − , as identified in isolated D1D2 complexes with time-resolved mid-IR spectroscopy is not observed in the current data, probably because of its relatively low concentration. Relaxation of the state P D 1 + Phe o D 1 − , caused by a drop in free energy, occurs in 200 ps in closed cores. We conclude that the kinetic model proposed earlier for the energy and electron transfer dynamics within the D1D2-RC, plus two slowly energy-transferring antennas C43 and CP47 explain the complex excited state and charge separation dynamics in the PSII core very well. We further show that the time-resolved IR-difference spectrum of P D 1 + Phe o D 1 − as observed in PSII cores is virtually identical to that observed in the isolated D1D2-RC complex of PSII, demonstrating that the local structure of the primary reactants has remained intact in the isolated D1D2 complex.

Both chlorophylls a and d are essential for the photochemistry in photosystem II of the cyanobacteria, Acaryochloris marina

We have measured the flash-induced absorbance difference spectrum attributed to the formation of the secondary radical pair, P +Q −, between 270 nm and 1000 nm at 77 K in photosystem II of the chlorophyll d containing cyanobacterium, Acaryochloris marina. Despite the high level of chlorophyll d present, the flash-induced absorption difference spectrum of an approximately 2 ms decay component shows a number of features which are typical of the difference spectrum seen in oxygenic photosynthetic organisms containing no chlorophyll d. The spectral shape in the near-UV indicates that a plastoquinone is the secondary acceptor molecule (Q A). The strong C-550 change at 543 nm confirms previous reports that pheophytin a is the primary electron acceptor. The bleach at 435 nm and increase in absorption at 820 nm indicates that the positive charge is stabilized on a chlorophyll a molecule. In addition a strong electrochromic band shift, centred at 723 nm, has been observed. It is assigned to a shift of the Qy band of the neighbouring accessory chlorophyll d, Chl D1. It seems highly likely that it accepts excitation energy from the chlorophyll d containing antenna. We therefore propose that primary charge separation is initiated from this chlorophyll d molecule and functions as the primary electron donor. Despite its lower excited state energy (0.1 V less), as compared to chlorophyll a, this chlorophyll d molecule is capable of driving the plastoquinone oxidoreductase activity of photosystem II. However, chlorophyll a is used to stabilize the positive charge and ultimately to drive water oxidation.

Metalation of 9‐Oxabicyclo[3.3.1]nonadiene – the Deprotonation Path to Bridgehead Olefins

AbstractA new route to bridgehead olefins based on a deprotonation reaction has been established. The deprotonation of 9‐oxabicyclo[3.3.1]nonadiene (1) with tBuLi/TMEDA occurs selectively in the allylic position. Whilst 1 has four allyl positions, only one bridgehead proton was removed, as demonstrated by quenching with Me3SnCl, Me3SiOTf, and Me3PbCl. With a threefold excess of deprotonating agent and subsequent treatment with Me3SnCl, three stannylated derivatives – 3, 4a, and 4b – were obtained. Compound 3 is the starting diene 1 substituted at the bridgehead position 1 in the skeleton, and when smaller quantities of deprotonating agent were used it was the only stannylated derivative of 1 formed. Compounds 4a and 4b had each undergone a shift of one double bond to give bridgehead olefins exo‐ and endo‐stannylated at position 3 in the skeleton. The common intermediate precursor of compounds 3, 4a, and 4b was a bridgehead allyl anion, which can also be generated from 3 and could be observed directly by 6Li and 13C NMR spectroscopy. After quenching of the anion with Me3SiOTf, and Me3PbCl, two silylated and plumbylated derivatives of 1 were obtained in each case: the bridgehead‐substituted compounds and the 3‐exo‐substituted bridgehead olefins. The stannylated bridgehead olefins 4a and 4b rearranged slowly to give 3 in what is formally a 1,3‐stannatropic shift reaction. The kinetics of the reactions are zero‐order in 4a and 4b, respectively, pointing to intermediate light‐promoted homolytic cleavage of the Sn–C bond and the formation of a tight radical pair.(© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2006)

Kinetics and mechanism of electron transfer in intact photosystem II and in the isolated reaction center: Pheophytin is the primary electron acceptor

The mechanism and kinetics of electron transfer in isolated D1/D2-cyt(b559) photosystem (PS) II reaction centers (RCs) and in intact PSII cores have been studied by femtosecond transient absorption and kinetic compartment modeling. For intact PSII, a component of approximately 1.5 ps reflects the dominant energy-trapping kinetics from the antenna by the RC. A 5.5-ps component reflects the apparent lifetime of primary charge separation, which is faster by a factor of 8-12 than assumed so far. The 35-ps component represents the apparent lifetime of formation of a secondary radical pair, and the approximately 200-ps component represents the electron transfer to the Q(A) acceptor. In isolated RCs, the apparent lifetimes of primary and secondary charge separation are approximately 3 and 11 ps, respectively. It is shown (i) that pheophytin is reduced in the first step, and (ii) that the rate constants of electron transfer in the RC are identical for PSII cores and for isolated RCs. We interpret the first electron transfer step as electron donation from the primary electron donor Chl(acc D1). Thus, this mechanism, suggested earlier for isolated RCs at cryogenic temperatures, is also operative in intact PSII cores and in isolated RCs at ambient temperature. The effective rate constant of primary electron transfer from the equilibrated RC* excited state is 170-180 ns(-1), and the rate constant of secondary electron transfer is 120-130 ns(-1).

The glutathione peroxidase homologous gene Gpxh in Chlamydomonas reinhardtii is upregulated by singlet oxygen produced in photosystem II

The expression of the glutathione peroxidase homologous gene Gpxh, known to be specifically induced by the formation of singlet oxygen (1O2), was analyzed in cells of Chlamydomonas reinhardtii exposed to environmental conditions causing photoinhibition. Illumination with high light intensities, leading to an increased formation of 1O2 in photosystem II, continuously induced the expression of Gpxh in cell for at least 2 h. Phenolic herbicides like dinoterb, raise the rate of 1O2 formation by increasing the probability of charge recombination in photosystem II via the formation of the primary radical pair and thereby 3P680 formation (Fufezan C et al. 2002, FEBS Letters 532, 407-410). In the presence of dinoterb the light-induced loss of the D1 protein in C. reinhardtii was increased and the high light-induced Gpxh expression was further stimulated. DCMU, a urea-type herbicide, causing reduced 1O2 generation in photosystem II, protected the D1 protein slightly against degradation and downregulated the expression of the Gpxh gene compared to untreated cells exposed to high light intensities. This indicates that the Gpxh expression is induced by 1O2 under environment conditions causing photoinhibition.

Time-Resolved Measurements of the Photolysis and Recombination of Adenosylcobalamin Bound to Glutamate Mutase

Femtosecond to nanosecond transient absorption spectroscopy is used to investigate the photolysis of 5'-deoxyadenosylcobalamin (coenzyme B12, AdoCbl) bound to glutamate mutase. The photochemistry of AdoCbl is found to be inherently dependent upon the environment of the cofactor. Excitation of AdoCbl bound to glutamate mutase results in formation of a metal-to-ligand charge transfer intermediate state which decays to form cob(II)alamin with a time constant of 105 ps. This observation is in contrast to earlier measurements in water where the photohomolysis proceeds through an intermediate state in which the axial dimethylbenzimidazole ligand appears to have dissociated, and measurements in ethylene glycol where prompt bond homolysis is observed (Yoder, L. M.; Cole, A. G.; Walker, L. A., II; Sension, R. J. J. Phys. Chem. B 2001, 105, 12180-12188). The quantum yield for formation of stable radical pairs in the enzyme is found to be phi = 0.05 +/- 0.03, and the resulting intrinsic rate constants for geminate recombination and "cage escape" are 1.0 +/- 0.1 and 0.05 +/- 0.03 ns(-1), respectively. The rate constant for geminate recombination is 30% less than that observed for AdoCbl in water or ethylene glycol. This reduction is insufficient to account for the 10(12)-fold increase in the homolysis rate observed when substrate is bound to the protein. Finally, the protein provides a cage to prevent diffusive loss of the adenosyl radical; however, the ultimate yield for long-lived radicals is determined by the evolution from a singlet to a triplet radical pair as proposed for AdoCbl in ethylene glycol.

Pathways and Timescales of Primary Charge Separation in the Photosystem II Reaction Center as Revealed by a Simultaneous Fit of Time-Resolved Fluorescence and Transient Absorption

We model the dynamics of energy transfer and primary charge separation in isolated photosystem II (PSII) reaction centers. Different exciton models with specific site energies of the six core pigments and two peripheral chlorophylls (Chls) in combination with different charge transfer schemes have been compared using a simultaneous fit of the absorption, linear dichroism, circular dichroism, steady-state fluorescence, transient absorption upon different excitation wavelengths, and time-resolved fluorescence. To obtain a quantitative fit of the data we use the modified Redfield theory, with the experimental spectral density including coupling to low-frequency phonons and 48 high-frequency vibrations. The best fit has been obtained with a model implying that the final charge separation occurs via an intermediate state with charge separation within the special pair (RP 1). This state is weakly dipole-allowed, due to mixing with the exciton states, and can be populated directly or via 100-fs energy transfer from the core-pigments. The RP 1 and next two radical pairs with the electron transfer to the accessory Chl (RP 2) and to the pheophytin (RP 3) are characterized by increased electron-phonon coupling and energetic disorder. In the RP 3 state, the hole is delocalized within the special pair, with a predominant localization at the inactive-branch Chl. The intrinsic time constants of electron transfer between the three radical pairs vary from subpicoseconds to several picoseconds (depending on the realization of the disorder). The equilibration between RP 1 and RP 2 is reached within 5 ps at room temperature. During the 5–100-ps period the equilibrated core pigments and radical pairs RP 1 and RP 2 are slowly populated from peripheral chlorophylls and depopulated due to the formation of the third radical pair, RP 3. The effective time constant of the RP 3 formation is 7.5 ps. The calculated dynamics of the pheophytin absorption at 545 nm displays an instantaneous bleach (30% of the total amplitude) followed by a slow increase of the bleaching amplitude with time constants of 15 and 12 ps for blue (662 nm) and red (695 nm) excitation, respectively.

Two Different Types of Light-Dependent Responses to Magnetic Fields in Birds

A model of magnetoreception proposes that the avian magnetic compass is based on a radical pair mechanism, with photon absorption leading to the formation of radical pairs. Analyzing the predicted light dependency by testing migratory birds under monochromatic lights, we found that the responses of birds change with increasing intensity. The analysis of the orientation of European robins under 502 nm turquoise light revealed two types of responses depending on light intensity: under a quantal flux of 8.10(15) quanta m(-2) s(-1), the birds showed normal migratory orientation in spring as well as in autumn, relying on their inclination compass. Under brighter light of 54.10(15) quanta m(-2) s(-1), however, they showed a "fixed" tendency toward north that did not undergo the seasonal change and proved to be based on magnetic polarity, not involving the inclination compass. When birds were exposed to a weak oscillating field, which specifically interferes with radical pair processes, the inclination compass response was disrupted, whereas the response to magnetic polarity remained unaffected. These findings indicate that the normal inclination compass used for migratory orientation is based on a radical-pair mechanism, whereas the fixed direction represents a novel type of light-dependent orientation based on a mechanism of a different nature.

Cover Picture: Probing Molecular Wires: Synthesis, Structural, and Electronic Study of Donor–Acceptor Assemblies Exhibiting Long‐Range Electron Transfer (Chem. Eur. J. 16/2005)

The rational design of linear donor–acceptor arrays by integrating fully conjugated oligomers with a well‐defined length and constitution is reported by N. Martín, D. M. Guldi et al. on page 4819 ff. Photoinduced electron transfer (shown schematically in the cover picture) over distances of up to 50 Å with formation of the respective radical pairs is demonstrated. The strong electron coupling between the donor and acceptor moieties through the π‐conjugated oligomer is particularly important for the observed wire‐like behavior.

Inverse Temperature Dependence of Chain Transfer Rate Constant for a Chromium Metalloradical in Polymerization of MMA

Mayo plots have been used to determine the rate constant ktr for chain transfer catalysis by (C5Ph5)Cr(CO)3• of the AIBN-initiated polymerization of MMA from 60 to 80 °C. The value of ktr decreases with temperature, implying that abstraction of H• from the chain-carrying radical by (C5Ph5)Cr(CO)3• does not occur in a single step and suggesting the associative formation of a caged radical pair. No CIDNP is observed for the reaction of (C5Ph5)Cr(CO)3H with MMA.

Chlorophyll a Fluorescence Rise Induced by High Light Illumination of Dark-adapted Plant Tissue Studied by Means of a Model of Photosystem II and Considering Photosystem II Heterogeneity

Chlorophyll a fluorescence rise (FLR) measured in vivo in dark-adapted plant tissue immediately after the onset of high light continuous illumination shows complex O–K–J–I–P transient. The steps typically appear at about 400 μs (K), 2 ms (J), 30 ms (I), and 200 – 500 ms (P) and a transient decrease of fluorescence to local minima (dips D) can be observed after the K, J, and I steps. As the FLR reflects a function of photosystem II (PSII) and to more understand the FLR, a PSII reactions model was formulated comprising equilibrium of excited states among all light harvesting and reaction centre pigments and P680, reversible radical pair formation and the donor and acceptor side functions. Such a formulated model is the most detailed and complex model of PSII reactions used so far for simulations of the FLR. By varying of selected model parameters (rate constants and initial conditions) several conclusions can be made as for the origin of and changes in shape of the theoretical FLR and compare them with in-literature-reported results. For homogeneous population of PSII and using standard in-literature-reported values of the model parameters, the simulated FLR is characterized by reaching the minimal fluorescence F 0 at about 3 ns after the illumination is switched on lasting to about 1 μs, followed by fluorescence rise to a plateau located at about 2 ms and subsequent fluorescence rise to a global maximum that is reached at about 60 ms. Varying of the values of rate constants of fast processes that can compete for utilization of the excited states with fluorescence emission does not change qualitatively the shape of the FLR. However, primary photochemistry of PSII (the charge separation, recombination and stabilization), non-radiative loss of excited states in light harvesting antennae and excited states quenching by oxidized plastoquisnone (PQ) molecules from the PQ pool seem to be the main factors controlling the maximum quantum yield of PSII photochemistry as expressed by the F V / F M ratio. The appearance of the plateau at about 2 ms in the FLR is affected by several factors: the height of the plateau in the FLR increases when the fluorescence quenching by oxidized P680 + is not considered in the simulations or when the electron transfer from Q A − to Q B (−) is slowed down whereas the height of the plateau decreases and its position is shifted to shorter times when OEC is initially in higher S state. The plateau at about 2 ms is changed into the local fluorescence maximum followed by a dip when the fluorescence quenching by oxidized PQ molecules or the charge recombination between P680 + and Q A − is not considered in the simulations or when all OEC is initially in the S 0 state or when the S -state transitions of OEC are slowed down. Slowing down of the S -state transitions of OEC as well as of the electron transfer from Q A − to Q B (−) also causes a decrease of maximal fluorescence level. In the case of full inhibition of the S -state transitions of OEC as well as in the case of full inhibition of the electron donation to P680 + by Y Z , the local fluorescence maximum becomes the global fluorescence maximum. Assuming homogeneous PSII population, theoretical FLR curve that only far resembles experimentally measured O–J–I–P transient at room temperature can be simulated when slowly reducing PQ pool is considered. Assuming heterogeneous PSII population (i.e. the α / β and the Q B -reducing/ Q B -non-reducing heterogeneity and heterogeneity in size of the PQ pool and rate of its reduction) enables to simulate the FLR with two steps between minimal and maximal fluorescence whose relative heights are in agreement with the experiments but not their time positions. A cause of this discrepancy is discussed as well as different approaches to the definition of fluorescence signal during the FLR.

Photochemical Formation and Reaction of Radical Pairs from NH3−F2 Complexes Isolated in Solid Argon

Photoinduced reactions in binary NH3−F2 complexes in solid argon are studied by FTIR spectroscopy. Analysis of the IR spectra and kinetics of product formation shows that the main photochemical channel at 355 nm is the formation of [H2N•−HF···F•] radical pairs; the relative initial yield of this product is 0.55. We believe that this is the first observation of this mechanism of radical pair formation (i.e., by escape of one of the F atoms from the initial cage containing the complex). The other major product channels form FH2N−HF and NH−HF−HF complexes in approximately equal amounts. Isotopic substitution experiments using 15NH3 and ND3 aided the assignments of the infrared bands of NH−HF−HF, which is identified for the first time. Thermal recombination of the [H2N•−HF···F•] radical pairs at 20 K yields only the addition product FH2N−HF. However, photoinduced recombination of [H2N•−HF···F•] radical pairs at 633, 532, 355, and 266 nm gives predominantly NH−HF−HF complexes. The relative yield of this secondary photolysis product increases with increasing energy of photoexcitation and is close to unity at λ ≤ 355 nm.

SO 4−–SO 3− radical pair formation in Ce doped and Ce, U co-doped K 3Na(SO 4) 2: EPR evidence and its role in TSL

Thermally stimulated luminescence (TSL) and electron paramagnetic resonance (EPR) studies were carried out on cerium doped and cerium–uranium co-doped K 3Na(SO 4) 2 samples after γ-irradiation. Three glow peaks around 352, 415 and 475 K were observed and their spectral characteristics have shown that Ce 3+ and UO 2 2+ act as the emission centres in K 3Na(SO 4) 2:Ce and K 3Na(SO 4) 2:Ce, U, respectively. In Ce–U co-doped sample, energy transfer from cerium to uranium takes place. The commonly occurring radiation-induced centres in sulphates, viz SO 3 − and SO 4 − were observed by EPR and SO 4 − radical ion was found to take part in the TSL emission at 415 K . The hitherto unknown information, however, is the formation of SO 4 −–SO 3 − radical pair creating deep traps in these lattices, apparently assisted by the dopants. This is the first observation of such radical pair formation leading to the identification of deep traps in this lattice. The radical pair, (SO 3 −–SO 4 −) which is stable up to 970 K , decreases the intensity of the peak at 415 K due to the depletion of SO 4 − centres.

Evidence for HOOO radicals in the formation of alkyl hydrotrioxides (ROOOH) and hydrogen trioxide (HOOOH) in the ozonation of C-H bonds in hydrocarbons.

Low-temperature ozonation of cumene (1a) in acetone, methyl acetate, and tert-butyl methyl ether at -70 degrees C produced the corresponding hydrotrioxide, C(6)H(5)C(CH(3))(2)OOOH (2a), along with hydrogen trioxide, HOOOH. Ozonation of triphenylmethane (1b), however, produced only triphenylmethyl hydrotrioxide, (C(6)H(5))(3)COOOH (2b). These observations, together with the previously reported experimental evidence, seem to support the "radical" mechanism for the first step of the ozonation of the C-H bonds in hydrocarbons, i.e., the formation of the caged radical pair (R(**)OOOH), which allows both (a) collapse of the radical pair to ROOOH and (b) the abstraction of the hydrogen atom from alkyl radical R(*) by HOOO(*) to form HOOOH. The B3LYP/6-311++G(d,p) (ZPE) calculations revealed that HOOO radicals are considerably stabilized by forming intermolecularly hydrogen-bonded complexes with acetone (BE = 8.55 kcal/mol) and dimethyl ether (7.04 kcal/mol). This type of interaction appears to be crucial for the relatively fast reactions (and the formation of the polyoxides in relatively high yields) in these solvents, as compared to the ozonations run in nonbasic solvents. However, HOOO radicals appear to be not stable enough to abstract hydrogen atoms outside the solvent cage, as indicated by the absence of HOOOH among the products in the ozonolysis of triphenylmethane. The decomposition of alkyl hydrotrioxides 2a and 2b involves a homolytic cleavage of the RO-OOH bond with subsequent "in cage" reactions of the corresponding radicals, while the decomposition of HOOOH is most likely predominantly a "pericyclic" process involving one or more molecules of water acting as a bifunctional catalyst to produce water and singlet oxygen (Delta(1)O(2)).

15N Chemically Induced Dynamic Nuclear Polarization during Reaction of N-Acetyl-[formula omitted]-tyrosine with the Nitrating Systems Nitrite/Hydrogen Peroxide/Horseradish Peroxidase and Nitrite/Hypochloric Acid

During reaction of N-acetyl-l-tyrosine with the nitrating system 15NO−2/H2O2/HRP at pH 7.5, emission is observed in the 15N NMR signals of 3-nitro-N-acetyl-l-tyrosine and the 1-nitrocyclohexa-2,5-dien-4-one derivative of N-acetyl-l-tyrosine. The occurrence of 15N CIDNP proves the formation of radical pairs. The nitration products are formed by recombination of 15NO•2 and N-acetyl-l-tyrosinyl radicals Tyrac•. The 15N NMR signals of 15NO−2 and of 15NO−3 show enhanced absorption. The 15N CIDNP effects are built up in radical pairs [15NO•2, Tyrac•]F formed by diffusive encounters of the radicals. Nuclear polarization is transferred into 15NO−2 and 15NO−3 by disproportionation reactions of 15NO•2 and by H abstraction of 15NO•2 from N-acetyl-l-tyrosine. If N-acetyl-l-tyrosine is not present, 15NO−3 does not show 15N CIDNP, indicating that peroxynitrite is not formed as an intermediate during the reaction. Peroxynitrate is not formed either, which is indicated by the absence of 15N CIDNP during reaction or a 15N NMR signal after reaction. Emission also appears in nitrated tyrosine residues of bovine serum albumin during reaction of albumin with the 15NO−2/H2O2/HRP system. Analogous but smaller effects are observed during reaction of the nitrating system 15NO−2/HOCl with N-acetyl-l-tyrosine at pH 7.5, proving the radical character of this reaction, too. The 15N CIDNP effects are weak because of the low yield of nitration products.

A mixed fullerene-ferrocene thermotropic liquid crystal: synthesis, liquid-crystalline properties, supramolecular organization and photoinduced electron transfer.

Grafting of a ferrocene-containing liquid-crystalline malonate derivative to C60 led to the mixed fullerene-ferrocene material 1 which gave rise to a smectic A phase. Cholesterol was used as liquid-crystalline promoter. X-ray diffraction experiments and volumetric measurements indicated that 1 is organized in double layered structures. The corresponding supramolecular organization within the mesomorphic lamellar phase is characterized by a microsegregation of the different units (ferrocene, fullerene, and cholesterol) in distinct sublayers. In such a smectic A phase, C60 imposes the arrangement of the other molecular moieties. Photophysical studies revealed that electron transfer occurs from the donor ferrocene to the electron accepting fullerene. The formation of a long-lived radical pair, with lifetimes of the order of several hundred nanoseconds, was confirmed by time-resolved spectrometry, especially in the near infrared region, in which the radical anion of the fullerene moiety displays its characteristic fingerprint absorption.