Electronic structure of model chlorophyll systems

An examination of the ground and excited states of magnesium porphine and pheoporphine and their positive and negative ions using the intermediate neglect of differential overlap/spectroscopic method is reported. It is found that rather large changes in the Mg out-of-plane coordinate can be attained at very little cost in energy. These geometric changes alone can be used to explain the large range of spectroscopic shifts observed in the visible band of chlorophyll that are site dependent. These calculations also show a porphyrin to Mg charge transfer excitation at about 20,000 cm^?1, which, if populated and allowed to relax, becomes the lowest excited singlet. This suggests that the initial photochemical event in photosynthesis may be excitation to the visible Q band followed by internal conversion to the charge transfer state accompanied by movement of the Mg atom out of the plane of the ring. This movement could result in intermolecular charge transfer, and relaxation of the Mg atom back to the plane of the ring. The nature of the positive and negative ions of these model chlorophylls is discussed in light of their role as the electron donor and their potential role as initial electron acceptor in photosynthesis.     

Monte Carlo simulation of the diabatic free energy curves for a dissociative electron transfer reaction in a polar solvent

In the present work we have carried out a Monte Carlo simulation of a dissociative electron transfer reaction in a polar solvent. In particular, we have chosen as a very simple model the electrochemical reduction of hydrogen fluoride to give a hydrogen atom and a fluoride anion in a dipolar solvent. From a classical point of view, the electron transfer occurs at the intersection region S* of the diabatic potential hypersurfaces H_pp and H_ss, corresponding to the precursor and successor complexes, respectively. We have evaluated both diabatic surfaces using potential functions that have been built up with ab initio methods by us. For each of the obtained configurations the parameter ΔE = H_ss – H_pp has been calculated. This parameter is then used as the reaction coordinate for obtaining the diabatic free energy curves of the reaction. Because the activation energy is high, a suitable mapping potential along with the statistical perturbation theory is employed to force the system to evolve toward the intersection region S*. A total of 68,340,000 configurations have been generated. The main conclusion of this article is that Marcus' relationship seems to fail for this kind of inner-sphere processes. © 1992 by John Wiley & Sons, Inc.     

CHEMILUMINESCENCE IN HYDROCARBON OXIDATION IN SOLUTION. A QUANTITATIVE STUDY OF THE EXCITATION AND EMISSION STEPS†

Abstract— Chemiluminescence in the visible region during liquid-phase hydrocarbon oxidation is excited by peroxy radical disproportionation; a carbonyl compound, P, in the triplet state is the emitter. Several types of energy transfer from P to acceptors are considered. These provide valuable information (lifetimes, rate constants, emission yields) relevant to triplet state molecules. The excitation yields are estimated making use of this information, the absolute chemiluminescence intensities and the reaction rates. Electronic excitation of P, vibrational excitation of ground state P and reverse decomposition of an intermediate complex into initial peroxy radicals are considered as competing processes strongly dependent on transformation of chemical energy into vibrations.     

Theoretical investigation of the rates of electron transfer processes Q−I + QII → QI + Q−II and Q−I + Q−II → QI + Q2−II in photosynthesis

A theoretical investigation on the rates of electron-transfer processes Q⁻_I + Q_II → Q⁻_I + Q⁻_II and Q⁻_I + Q⁻_II → Q_I + Q²⁻_II was carried out by using the Marcus theory of long-range electron transfer in solution. The molecular reorganizational parameter λ, the free-energy change ΔG⁰ for the overall reaction, and the electronic matrix element H_DA for these two processes were calculated from the INDO-optimized geometries of molecules Q_I, Q_II, and histidine. Q_I and Q_II are plastoquinones (PQ) which are hydrogen-bonded to a histidine each, and the two histidines may or may not be coordinated to a Fe²⁺ ion. The plastoquinone representing Q_I is additionally flanked by two peptide fragments. Each of the species (Pep)₂Q_I · His and His · Q_II has been considered to be immersed in a dielectric continuum that represents the surrounding molecules and protein folds. INDO calculations confirm the standard reduction potential for the first process (calculated 0.127 V; observed 0.13 V) and predict a midpoint potential of 0.174 V for the second process at 300 K at pH 7 (experimental value remains uncertain but is known to be close to 0.13 V). The plastoquinone fragment carries almost all the net charge (about 95.7%) in [PQ · His]⁻ and the net charge in [PQH · His]⁻. The electron is transferred effectively from the plastoquinone part of [(Pep)₂Q_I · His]⁻ to the plastoquinone moiety of Q_II · His in the first step and to the plastoquinone fragment of HisH⁺ · Q⁻_II in the second step. Therefore, we made use of the formula for the rate of through-space electron transfer from Q_I to Q_II (and to Q⁻_II). The plastoquinones are, of course, electronically coupled to histidines, and the transfer is, in reality, through the molecular bridge consisting of histidines and also Fe²⁺. The through-bridge effect is inherent in our calculation of ΔG⁰, H_DA, and the reorganization parameter λ. We investigated the correlation between half-times for the transfer and (D⁻¹_op− D⁻¹_s), where D_op and D_s are, respectively, optical and static dielectric constants of the condensed phase in the vicinity of the plastoquinones. We found that with reasonable values of D_op (2.6) and D_s (8.5) the experimental rates are adequately explained in terms of transfers from the plastoquinone moiety of Q_I to that of Q_II. The t_1/2 values calculated for the two processes are 247 and 472 μs in the absence of Fe²⁺ and 134 and 181 μs in the presence of Fe²⁺. These are in good agreement with the observed values which are ≈ 100 and ≈ 200 μs when Fe²⁺ is present in the matrix and which are known to be almost twice as large when the Fe²⁺ is evicted from the matrix. The present work also shows that the Marcus-Hush theory of long-range electron transfers can be successfully applied to the investigation of processes occurring in a semirigid condensed phase like the thylakoid membrane region. © 1997 John Wiley & Sons, Inc.     

Photoisomerization of a Maleonitrile‐Type Salen Schiff Base and Its Application in Fine‐Tuning Infinite Coordination Polymers

Strategically designed salen ligand 2,3‐bis[4‐(di‐p‐tolylamino)‐2‐hydroxybenzylideneamino]maleonitrile ( 1 ), which has pronounced excited‐state charge‐transfer properties, shows a previously unrecognized form of photoisomerization. On electronic excitation (denoted by an asterisk), 1Z *→ 1E isomerization takes place by rotation about the C2? C3 bond, which takes on single‐bond character due to the charge‐transfer reaction. The isomerization takes place nonadiabatically from the excited‐state ( 1Z ) to the ground‐state ( 1E ) potential‐energy surface in the singlet manifold; 1Z and 1E are neither thermally inconvertible at ambient temperature (25–30 °C), nor does photoinduced reverse 1E *→ 1Z (or 1Z *) isomerization occur. Isomers 1Z and 1E show very different coordination chemistry towards a Zn^II precursor. More prominent coordination chemistry is evidenced by a derivative of 1 bearing a carboxyl group, namely, N,N′‐dicyanoethenebis(salicylideneimine)dicarboxylic acid ( 2 ). Applying 2Z and its photoinduced isomer 2E as building blocks, we then demonstrate remarkable differences in morphology (sphere‐ and needlelike nanostructure, respectively) of their infinite coordination polymers with Zn^II.     

Molecular orbital study of the Q–e scheme in free radical copolymerization

Molecular orbital calculations were used to study free radical polymerization. Calculations show that the monomer is activated during the reaction and the pi bond becomes a diradical. The radical on the carbon that is about to form the new bond is called the e radical in this article. The other is the Q radical. For different monomers it is shown indirectly that changes in the energies of formation of the Q and e radicals are related to changes in the Q and e terms in the empirical Q–e scheme of Alfrey and Price. The polar effect in the Q–e scheme involves the e-radical, unpaired electron density. Specifically, the Q–e sum (e_x + e_y) is correlated with the e radical spin density. Also the e term is correlated with the electron density on the unsubstituted carbon of the monomer. The relationship of the Q radical to the adjacent substituent is shown by correlating ln Q values with the energy of addition of a hydrogen atom to a monomer. These relationships give theoretical meaning to the Q–e terms and allow calculation of Q and e values from molecular orbital properties for small monomers.     

A quantum chemical study of interchain hopping model of negatively charged solitons in polyacetylene

Phonon-assisted interchain hopping of negatively charged solitons in polyacetylene has been studied using a local chemical reaction model CH + CH₄ → CH₄ + CH. Quantum chemical characteristics of the electron transfer process have been analyzed in terms of the dynamic electron density and the mutual polarization moment. The CH stretching vibrational motion of CH₄, which is a local model of the sp³ defect, has been found to play a significant role for the electron transfer. The excitation of the corresponding vibrational mode of the sp³ defect would promote the interchain hopping of the charged soliton. The electron transfer process has also been studied in terms of the “regional” density functional theory. It has been shown that the driving force of the electron transfer is represented by the regional chemical potentials.     

The new theory of electron transfer. Thermodynamic potential profiles in the inverted and superverted regions

In a previous paper (S Fletcher, J Solid State Electrochem 11:965, 2007) a non-Marcus theory of electron transfer was developed, with results applicable to the normal region of thermodynamic driving forces. In the present paper, the theory is extended to highly exergonic reactions (the inverted region) and to highly endergonic reactions (the superverted region). The results are presented mathematically and in the form of Gibbs energy profiles plotted against a charge fluctuation reaction coordinate. The new theory utilizes the concept of donor and acceptor “supermolecules,” which consist of conventional donor and acceptor species plus their associated ionic atmospheres. The key findings are as follows. (1) In the inverted region, donor supermolecules are positively charged both before and after the electron transfer event. (2) In the normal region, donor supermolecules change polarity from negative to positive during the electron transfer event. (3) In the superverted region, the donor supermolecule is negatively charged both before and after the electron transfer event. This overall pattern of events makes it possible for polar solvents to catalyse electron transfer in the inverted and superverted regions. Because this new effect is predicted only by the present theory and not by the Marcus theory, it provides a clear means of distinguishing between them.     

Ultrafast spectroscopy studies on the mechanism of electron transfer and energy conversion in the isolated pseudo ginseng, water hyacinth and spinach chloroplasts

The spectroscopy characteristics and the fluorescence lifetime for the chloroplasts isolated from the pseudo ginseng, water hyacinth and spinach plant leaves have been studied by absorption spectra, low temperature steady-state fluorescence spectroscopy and single photon counting measurement under the same conditions and by the same methods. The similarity of the absorption spectra for the chloroplasts at room temperature suggests that different plants can efficiently absorb light of the same wavelength. The fluorescence decays in PS II measured at the natural QA state for the chloroplasts have been fitted by a three-exponential kinetic model. The three fluorescence lifetimes are 30, 274 and 805 ps for the pseudo ginseng chloroplast; 138, 521 and 1494 ps for the water hyacinth chloroplast; 197, 465 and 1459 ps for the spinach chloroplast, respectively. The slow lifetime fluorescence component is assigned to a collection of associated light harvesting Chl a/b proteins, the fast lifetime component to the react     

Theoretical study of electric field‐induced intramolecular electron transfer in donor–acceptor pairs via the rigid space and their suitability as molecular electronic devices

The geometries of two reaction systems have been optimized under the constraint of C_2ν symmetry, using the UHF/6‐31G method. The potential energy surfaces (PES) of the two systems in different external electric field have been constructed using a linear reaction coordinate. It is concluded that the reorganization energies and electron transfer matrix elements for both systems are almost independent of the external electric field. However, the standard Gibbs energy difference changes remarkably with the change of the external electric field. Hence, the applied electric field leads to the variation of rate constant of electron transfer reaction. The threshold field, where the electron transfer becomes barrier free, is obtained to be 0.0015 a.u. for the anion system 1, and 0.00097 a.u. for the anion system 2. The threshold field for modified system 1, in which the hydrogen atoms linking to benzene rings are replaced by fluorine atom, is 0.0018 a.u. The calculations show that the best way to adjust the threshold field is to adjust the dipole moment of the reaction system by changing the length of the bridge. As the rate constant in field‐free case is taken into account, neither reaction systems could be used as molecular electronic device. But if the bridge consists of three or four HCTDs, the rate constant and threshold field will satisfy the practical demand. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

A theoretical investigation on the regioselectivity of the [3+2] cycloaddition of nitrile oxide and N-vinylpyrrole

Theoretical calculations were performed on the [3+2] cycloaddition (32CA) reaction of nitrile oxide and N-vinylpyrrole. The regiochemistry of the reaction has been studied based on potential energy surface analysis and global and local reactivity indices of the reactants. The global electron density transfer (GEDT) calculations at the possible transition states revealed that this cycloaddition has a nearly non-polar character. The ELF topological analyses of the selected structures involved in the intrinsic reaction coordinate (IRC) of TS1a suggests that this 32CA reaction takes place through a two-stage one-step mechanism.     

Studies of organotin(IV)-orthoquinone systems

The primary process in the reaction of hexaphenylditin with various substituted orthoquinones (Q) is shown to involve attack by the quinone at a phenyl ligand. The intermediate thus formed decomposes to yield Ph₃Sn(SQ^·), where S(Q^·−) is the corresponding semiquinonate. Rearrangement of these species in solution gives rise to biradicals, while intramolecular electron transfer may lead to the formation and precipitation of Ph₂Sn(CAT), where CAT²⁻ is the corresponding substituted catecholate. The identification of these processes depends in part on electron paramagnetic resonance spectroscopy. The reaction of Ph₃SnCl or Ph₂SnCl₂ with Na(TBSQ^·) (TBSQ^·−=3,5-di-tert-butyl-orthobenzosemiquinonate) results in the formation of Ph₂Sn(TBSQ^·), which can undergo redistribution and intramolecular electron transfer, so that the solution chemistry of these latter systems is similar to that of the products of the Sn₂Ph₆+Q reaction.     

Destabilization of the Oxygen Evolving Complex of Photosystem II by Al3+

The inhibitory effect of Al³⁺ on photosynthetic electron transport was investigated in isolated thylakoid membranes of spinach. A combination of oxygen evolution, chlorophyll fluorescence induction (FI) and decay and thermoluminescence measurements have been used to characterize photosystem II (PSII) electron transport in the presence of this toxic metal cation. Our results show that below 3 mm , Al³⁺ already caused a destabilization of the Mn₄O₅Ca cluster of the oxygen evolving complex (OEC). At these concentrations, an increase in the relative amplitude of the first phase (OJ) of FI curve and retardation of the fluorescence decay kinetics following excitation with a single turnover flash were also observed. A transmembrane structural modification of PSII polypeptides due to the interaction of Al³⁺ at the OEC is proposed to retard electron transfer between the quinones Q_A and Q_B. Above 3 mm , Al³⁺ strongly retarded fluorescence induction and significantly reduced F_v/F_m together with the maximal amplitude of chlorophyll fluorescence induced by a single turnover flash. This chlorophyll fluorescence quenching was attributed to the formation of P680⁺ due to inhibition of electron transfer between tyrosine 161 of D1 subunit and P680.     

Order,disorder, and geometrical information indices of molecules

Structural features of various molecular systems with symmetry of point groups ranging fromC ₁ to the icosahedral symmetry are analyzed in the framework of the model suggested previously for the evaluation of order and disorder in the arrangement of atoms in a molecule based on the equationQ = I -P/3n (whereQ is the index of order, andP is the number of independent coordinates needed to fix ann-atomic molecule in the Cartesian coordinate system). TheQ value depends on various structural parameters of the molecule: the number of atoms in it, the symmetry, the dimensionality, and the number of structural degrees of freedom. The disorder indexP/3n = 1 -Q correlates with Shannon's entropy of information, andQ correlates with negentropy or excess information; this makes it possible to useP/3n as a new geometrical information molecular index that is obtained by a nonprobabilistic method. Analysis of the relationship between order and chaos in molecular systems, as well as of the specific order indexq =Q/n, makes it possible to identify both general and specific features of molecules.Translated from Izvestiya Akadernii Nauk. Seriya Khimicheskaya, No. 8, pp. 191219-121927, August, 1996.     

LASER PHOTOLYSIS STUDIES OF QUINONE REDUCTION BY CHLOROPHYLL a IN ALCOHOL SOLUTION

Upon laser photolysis of chlorophyll-quinone solutions in ethanol, transients due to the chlorophyll triplet state (C_t), the chlorophyll cation radical (C⁺) and the semiquinone radical (Q^-) can be observed. The rise of Q^- parallels the decay of C_t. demonstrating the precursor role of the triplet. The decay of C⁺ is second order, consistent with reverse electron transfer, and has a rate constant which is independent of quinone potential, and an activation energy of 14kJ/mol due mainly to the temperature dependence of solvent viscosity. Triplet quenching and C⁺ yield are found to decrease with decreasing quinone potential.     

A Monte Carlo simulation of free energy relationships for the electron transfer reaction between Fe+ and Fe2+ in water

A free energy barrier ΔF^≠ = 174.2 kJ/mol for the self-exchange electron transfer reaction model Fe⁺/Fe²⁺ in water has been calculated by combining Monte Carlo simulations and the statistical perturbation theory. We have shown that, even for those electron transfer reactions that present a very high free energy barrier of activation, the free energy curve behaves parabolically versus the reaction coordinate, which justifies the quadratic expression for the activation free energy done by Marcus.     

Dramatic Influence of an Anionic Donor on the Oxygen‐Atom Transfer Reactivity of a MnV–Oxo Complex

Addition of an anionic donor to an Mn^V(O) porphyrinoid complex causes a dramatic increase in 2‐electron oxygen‐atom‐transfer (OAT) chemistry. The 6‐coordinate [Mn^V(O)(TBP₈Cz)(CN)]^? was generated from addition of Bu₄N⁺CN^? to the 5‐coordinate Mn^V(O) precursor. The cyanide‐ligated complex was characterized for the first time by Mn K‐edge X‐ray absorption spectroscopy (XAS) and gives Mn?O=1.53 Å, Mn?CN=2.21 Å. In combination with computational studies these distances were shown to correlate with a singlet ground state. Reaction of the CN^? complex with thioethers results in OAT to give the corresponding sulfoxide and a 2e^?‐reduced Mn^III(CN)^? complex. Kinetic measurements reveal a dramatic rate enhancement for OAT of approximately 24 000‐fold versus the same reaction for the parent 5‐coordinate complex. An Eyring analysis gives ΔH^≠=14 kcal mol^?1, ΔS^≠=?10 cal mol^?1 K^?1. Computational studies fully support the structures, spin states, and relative reactivity of the 5‐ and 6‐coordinate Mn^V(O) complexes.     

HETEROGENEITY OF THE QUINONE ELECTRON ACCEPTOR SYSTEM IN BACTERIAL REACTION CENTERS

Abstract— In reaction centers from Rhodopseudomonas viridis, biphasicity of the charge recombination kinetics between P⁺, the primary electron donor, and Q_A and Q_B^-, the primary and secondary quinone electron acceptors, respectively, have been analyzed by the flash-induced absorption change technique. We have studied the effect of quinone environment modifications on the ratio of the two phases for the P⁺Q_A- ([A_fast/A_slow]^a) and P⁺Q_B^- ([A_fast/A_slow]^b) charge recombination processes. In reaction centers from Rps. viridis reconstituted in phosphatidylcholine liposomes a notable influence of the nature of the Q_B pocket occupancy was observed on (A_fast/A_slow)^a. This ratio is much affected by the presence of o-phenanthroline compared to reaction centers with an empty Q_B pocket or with terbutryn present. Because o-phenanthroline was proposed to hydrogen bind His^L190, whereas terbutryn does not, we suggest that a His^LI90-Fe-His^M217 (the equivalent to His^LI90 in the Q_A pocket) “wire” may be involved in the existence of the two conformational states associated with the two phases of charge recombination. In chromat-ophores from the T₁ (Ser^L223→ Ala; Arg^L217→ His) and T₄ (Tyr^L222→ Phe) mutants no modification of the (A_fast/A_slow)^a ratio is detected, whereas the (A_fast/A_slow)^b ratios are substantially modified compared to the wild type (WT). In the T₃ mutant (Phe^L216→ Ser; Val^M263→ Phe [4.1 Å apart from Q_A]), (A_fast/A_slow)^a is notably changed compared to the WT. Our data show that any modification in the close protein environment of the quinones and/or of the His^L190 and His^M217 affects the equilibrium between the two reaction center states. This is consistent with the existence of two reaction center states from Rps. viridis, associated with two different conformations of the quinones-histidines-iron system. This “wire” allows both quinone protein pockets to interact over quite long distances.     

Quantum dynamics calculation of reaction probability for H + Cl2 → HCl + Cl

We present in this paper a time-dependent quantum wave packet calculation of the initial state selected reaction probability for H + Cl₂ based on the GHNS potential energy surface with total angular momentumJ = 0. The effects of the translational, vibrational and rotational excitation of Cl₂ on the reaction probability have been investigated. In a broad region of the translational energy, the rotational excitation enhances the reaction probability while the vibrational excitation depresses the reaction probability. The theoretical results agree well with the fact that it is an early down-hill reaction.