Bimolecular processes Electron transfer *A + B A + + B - *A + B A - + B + EA IP *EA *IP LUMO An excited state is a better oxidant and a better reductant than the ground state HOMO X X*
Kinetic of electron transfer: Marcus Model E Energy profile of reactants k el = n N kexp(-dg # /RT) A.A A + A - autoexchange reaction l DG # Energy profile of products Nuclear configuration The meaning of the various terms can be conveniently discussed considering the energy profiles. For simplicity we consider an auto-exchange electron transfer reaction. These curves represent potential energies of reactants and products as a function of a reaction coordinate. This is the combination of internal coordinates (bonds and angles) and of arrangement of solvent molecules surrounding reactants and reagents. At the equilibrium geometry of reactants, products have a very higher energy (l) called reorganization energy Fluctuations around equilibrium geometry can lead reactants to the geometry of the crossing between the two curves. At this geometry reactants and products have the same energy and the electron can be easily transferred: then products relaxed to their equilibrium geometry
E Energy profile of reactants k el = n N kexp(-dg # /RT) A.A A + A - autoexchange reaction l DG # Energy profile of products n N is the nuclear frequency factor of the reaction. It is the weighted mean of the frequencies of the nuclear vibration modes involved in the reaction coordinate. k is the trasmission coefficient of the reaction, that is the probability that the reactants convert into products once they have reached the geometry of the crossing point. Its values vary from 0 to 1. l is the reorganization energy (energy of products at the equilibrium geometry of reactants). DG # is the activation energy, that is the energy difference between the crossing point and the reactant minimum. Nuclear configuration
k el = n N kexp(-dg # /RT) When the electron transfer process involves two different reactants A and B, a free energy variation (DG 0 ) has to be considered E A.B In this case DG # depends on DG 0 : A + B - l DG # DG # = (DG 0 + l) 2 4l DG 0 Nuclear configuration
DG # = (DG 0 + l) 2 4l k el = n N kexp(-dg # /RT) Log k et DG 0 = 0; DG # = l/4 -l < DG 0 < 0, logk et increases when DG 0 decreases: normal -l 0 DG 0 DG 0 = -l; DG # =0 and logk et is maximum: activationless DG 0 < -l; DG # >0 and logk et decreases: inverted
Photosynthesis photosynthesis H 2 O + CO 2 1/n (CH 2 O)n + 3 O 2 respiration DH = + 470 kj/mole Photosynthesis is an example of uphill process and it is energy consuming. For this reason it has a high degree of complexity and a very high organization. In plants, the primary photosynthetical events take place in the membrane of special vesicles inside chloroplasts.
Efficiency in light absorption is fulfilled through the presence of different organic pigments Tetrapyrrolic ring containing Mg 2+ is the light absorbing component (molar extinction coefficient is about 10 5 M -1 cm -1 ). Other pigments allow to cover a broader spectral absorption range Long hydrophobic chain binds chlorophyll to the membrane
Light-harvesting The absorption of photons by the pigments is quick (femtoseconds) and yields to singlet excited state that are very short lived. The major part of chlorophylls (> 98%) act as antenna devices and collect available photons. The absorbed energy is then transferred through consecutive energy transfer reactions to the actual photoreaction center which contain less than 2% of the total chlorophyll content. Energy transfer does not require any movement: many chlorophylls are arranged in spatial proximity with a specific orientation: they are able to funnel the light energy to the reaction centers with 95% of efficiency within 10-100 ps.
Energy-transfer cascade for antenna pigments Energy transfer proceeds via spectral overlap of emission bands of the donor excited species with the absorption bands of the acceptor. So the light harvesting complexes of the photosynthetic membrane feature a spatially as well as spectrally optimized cross-section for photon capture. S 1 12 ps 22 ps 52 ps 530 nm 578 nm 640 nm 660 nm 685 nm S 0 phycoerythrin phycocyanin allo-phycocyanin chlorophyll a Energy-transfer cascade for antenna pigments in light-harvesting complexes of the algae Porphyridium cruetum
Role of magnesium in chlorophyll It contributes to the three dimensional organization of chlorophylls. They are fixed and correctly oriented not only by the bond of their long chain to the membrane, but also through the two axial coordination sites of magnesium. it has proper size, sufficient natural abundance and strong tendency for hexacoordination it is a light atom with a small spin orbit coupling constant. So inter system crossing is inhibited so favouring the energy transfer from the excited singlet states it is not a redox metal and does not interfere in the charge separation steps.
Charge-separation step *P680 Pheo Q A Q B etc P680 +. Pheo -. Q A Q B P680 +. Pheo Q.- A Q B P680 +. Pheo Q A Q.- B and so on
Summarizing, the key aspects for successful charge-separation are: -The arrangement of the components is the basis for the strong preference for charge separation steps instead of charge recombination -The arrangement and the confinement of the components reduces the activation energies for the forward electron transfer and accelerates the electron transfer reaction. - Back electron transfer process falls in the Marcus inverted region, where reaction rate decreases in spite of an increase of DG 0, i.e. in spite of a more favorable equilibrium.
Photophosphorilation process
Ferredoxin-NADP reductase (FNR) 2Fd 2+ red + 2H + + NADP + (in stroma) 2 Fd 3+ ox + NADPH + H +
Dark reaction: Calvin cycle
Water photolysis 2 H 2 O O 2 + 4H + + 4e - Photoexcitation of PSII leaves the chlorophyll a in the reaction center with a deficit of electron. This electron must be obtained from some other reducing agent. The external source of electrons is water OEC is a redox active structure containing manganese as redox species. Oxygen evolving complex
Oxygen evolving complex or water splitting complex Oxygen evolving complex can exist in 5 states: S 0 to S 4. photons trapped by PSII move the complex from one state to the next one. S 4 is unstable and reacts with water to produce O 2 P680 +. accepts an eletron from Tyr. Tyr +. Accepts an electron from manganese ion that changes its oxidation state in OEC. After 4 of these steps OEC takes 4 electrons from the oxidation of two water molecules, releasing O 2. 4 H + are also released in the lumen so contributing to the proton gradient. No intermediates between H 2 O and O 2 are released. 2 H 2 O O 2 + 4H + + 4e - S 0 S1 S 2 S 3 S 4 Mn 3+ Mn 4+ Mn 3+ Mn 4+ Mn 3+ Mn 4+ Mn 4+ Mn 4+ Mn 4+ Mn 4+ Mn 3+ Mn 3+ Mn 3+ Mn 4+ Mn 4+ Mn 4+ Mn 4+ Mn 4+ Mn 4+ Mn 3+ 5+
Role of manganese in the OEC Mn(III) and Mn(IV) oxides or hydroxides were certainly available in sea water under the conditions of developing photosynthesis (3x10 9 years ago) Manganese has a large variety of stable oxidation states (+II, +III, +IV, +VI, +VII) The importance of manganese for the O 2 metabolism is not restricted to photosynthesis.