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3
Photosynthesis
Light dependent part Dark part Calvine cycle
synthesis energy reach molecules adenosine triphosphate (ATP) Nicotinamide adenine dinucleotide phosphate(NADPH)
fuel production with CO2 fixation Fuel: carbohydrate(CH2O) Source of carbon: CO2 RUBisCO enzyme
6 CO2 + 6 C5H10O5 + 12 ATP + 12 NADPH + 12 H2O →
6 C6H12O6 + 12 ADP + 12 Pi + 12 NADP+ + 6 H2O
ribose
adenine
nicotinamide
ribose
7
( )
( ) SOCHCOSH
OOCHCOOH
h
h
2222
2222
+→+
+→+
ν
ν
Source of the electrons:
H2O - oxygenic photosynthesis (plants, algae and cyanobacteria)
H2S - anoxygenic photosynthesis (green sulfur, purple bacterias).
Byproducts: oxygen and sulfur
Energy gained by light excitation is used to run reactions that require an input of free energy
8
Chloroplast ~5 µm long
The space separation allows coexistence of different in nature processes like oxidation – reduction which generate proton gradient between lumen and stroma space hence proton-motive force for ATP synthesis.
9
QA, QB, PQ- plastoquinone PQH2- plastoquinol Ph- pheophytin, chlorophyll with no Mg at.
Cyt b6- cytochrome complex, transport of electron from plastoquinol (PQH2) to plastocyanin (CU+1/+2) (PC) A0- monomer chlorophyll A1- quinon (vit. K1) FX- 4Fe-4S Centrum Fd- ferredoxin
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12 H2O → 12 [H2] + 6 O2
4 H2O + 3 ADP + 3 Pi → 4 H+ + 4 e- + O2 + 3 ATP + 2 H2O
4 H+ + 4 e- + 2 NADP+ → 2 NADPH + 2 H+
12
In photosystem II, the missing electron, by light excitation that runs reaction chain, is compensated by the electron taken from water decomposition, with oxygen evolution. Next photons adsorption, hence exited electrons are pumped photosystem I cycle and missing electron comes from phosystem II.
14
Photosynthetic Unit (PSU)
light harvesting proteins (LH): LH1, LH2 and reaction center (RC) consisting of the photosystem II and photosystem I
The sunlight of wavelength between 400-700 nm is captured by light harvesting complex.
Energy transfer LH II→LH I→ RC
A. Damjanovic, I. Kosztin, U. Kleinekathöfer, K. Schulten, Phys. Rev. E 65(2002)031919
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Green Plants chlorophyll a and b (substituted tetrapyrol with helated Mg+2 ion) - light adsorption max. 680 nm carotinoids (protection against photo-damaged by oxidation) - light adsorption max. 500 nm Bacterias Bacteriochlorophyll (BChl) - light adsorption max. 960 nm Bacteriopheophytin (BPh) - light adsorption max. 960 nm Cyanobacteria, Red Algae contain large assemblies called Phycobilisomes - light adsorption 470-650 nm green and yellow light that penetrates their ecological niche
16
photons are absorbed by the light-harvesting complexes excitons (electron – hole pairs) are transferred to
the RC charge (electron-hole) separation take place
Energy transfer LH II→LH I→ RC
a) Förster mechanism induced dipole - induced dipole interaction 5-10 nm B850-B800 (18Å)
b) Dexter mechanism hopping of light-generated excitons conductive molecules to be in van der Waals contact
carotenoids and bacteriochlorophyl
A. Damjanovic, I. Kosztin, U. Kleinekathöfer, K. Schulten, Phys. Rev. E 65(2002)031919
LH II
Excitation energy transfer (EET)
17
Water decomposition in PS II
H2O H+
H+
Pheo
Q A
Q B
P680
D1 D2
TyrZ
Mn4Ca
Stroma
Lumen
CP43
CP47
O2
Photon
n x Chl + m x Car
Fe
plants, algae cyanobacteria Metaloprotein oxygen evolving complex (OEC) water oxidizing complex (WOC) Mn4O4:Ca
−+•
−+
++→
++→
eHOHOHeHOOH h
2
24
2 442 ν
Ca Mn O
the most efficient anodic “electrolysis” system known loss of four electrons and four protons from two water molecules formation of oxygen-oxygen bond elimination of one electron at the time leads to formation of a high-energy hydroxyl radical
Complexity of the water splitting is reflected by the fact that even plants find this task difficult: under ambient sunlight in the chloroplasts, the OEC must be resynthesised every half an hour. OEC suffer from the oxygen that it has produced.
28
What can we mimic from nature?
Light absorption Employ enzyme Active center of enzyme Transition metal catalyst
Holy Grail. We want an efficient and long-lived system for splitting water to H2 and 02 with
light in the terrestrial (AM1.5) solar spectrum at an intensity of one sun. For a practical
system, an energy efficiency of at least 10% appears to be necessary. Acc. Chem. Res. 1995,28, 141-145
sunlight + available abundant raw materials (water, carbon dioxide) → converted to oxygen and the reduced organic species that serve as food and fuel.
How to involve light in operation of biocatalytic system?
Nature 414, 589-590 (2001)
Antennas system Semiconductor electrolyte
30
Artificial photosynthesis
Artificial photosynthesis system must be able to use sun energy to drive thermodynamically uphill reaction of abundant materials to produce a fuel.
Lubitz et al, Energy & Environmental Science 1(2008)15
32
Processes at electrode – enzyme interface
Electroactive orientation of enzyme at the surface Stability: no mechanism for repairing enzymes Stability towards O2
A: depicts how a film of protein is formed on a pyrolytic graphite “edge” electrode by spotting dilute protein onto the surface. B: a scanning electron micrograph of the “edge” surface of pyrolytic graphite polished with 1 μm R-alumina, rinsed with water, and then sonicated for 10 s in water. Particles of alumina remain on the surface but are removed upon further sonication.
Enzyme adsorption on the PG electrode surface
Armstrong et al, Chemical Reviews, 107(2007)4366
38
Hydrogen evolution by PS I and hydrogenase
The hydrogen evolution in biophotolysis process is possible due to activity of hydrogenase enzyme in green algae and cyanobacteria and nitrogenase enzyme in cyanobacteria.
H2O PSII PSI Ferrodoxin Hydrogenaze H2 O2
2H+ + 2e H2
H2 H+ + H- 2H+ + 2e
Problems: sensitivity to O2: closed reactors, gen. modification
39
The artificial approach to hydrogen evolution
1% H2 in N2 (dotted) and 1% H2 in air (bold), blank graphite electrode under 1% H2 in air (dashed)
1. Use of enzyme’s hydrogenase deposited on electrode (graphite)
2. Synthesis of artificial metal complexes (Fe, Ni, Ru, Ir)
Armstrong et al, JACS 130(2008)424
electrode
H2ase H2ase H2ase
J. Am. Chem. Soc. 132(2010)9672
H2 production by a 1:1 mixture of nc-CdTe-H2aseA (0.25 μM).
Rate of H2 production under illumination and in the dark gray in
0.1 M ascorbic acid (pH 4.75).
Electrochemical tests
DEMS Differential electrochemical mass spectroscopy (with help of Dr.P. Bogdanoff)
0.1M phosphate buffer pH=6, saturated with N2 Light 40 mWcm-2
44
ALGAE FARM TO RECYCLE CO2 FOR BIO-HYDROGEN AIRSHIP
Belgian architect Vincent Callebaut has designed a conceptual transport system that would involve airships powered by seaweed (green algae).
45
Ca
Mn Mn Mn
Mn
Phil.Trans.R.Soc. B 363(2008)1237
The oxygen-evolving complexes manganese clusters: (Mn ions), (Ca ions).
PSII- oxygen-evolving complexes
Nature Reviews Molecular Cell Biology 5, 971-982 (2004)
46
The artificial complexes for water decomposition
ruthenium “blue dimmer effective can lose its catalytic efficiency after a few cycles
(bpy)2(H2O)RuORu(H2O)(bpy)24+
4 Ce(IV) + 2 H2O O2 + 4H+
Ru, Mn, Ir, Co
48
How to involve light in operation of biocatalytic system?
Antennas system Semiconductor electrolyte
Nature 414, 589-590 (2001)
49
Inorg. Chem. 2005, 44, 6802-6827
use light absorption and excited-state electron transfer to create oxidative and reductive equivalents for driving relevant fuel-forming half-reactions such as the oxidation of water to O2 and its reduction to H2
1. Light absorption, either at a single “reaction center” chromophore (C) or by excitation of an antenna array 2. Electron-transfer quenching, of a donor-chromophoreacceptor (D-C-A) array either oxidatively, D-C*-A →D-C+-A-, or reductively, D-C*-A →D+-C--A. 3. Redox separation by electron transfer, D-C+-A- → D+-C-A- or D+-C--A → D+-C-A-
51
Artificial light harvesting systems
adsorption maxima at 380 nm and 518 nm
The main optical transition have a metal-to-ligand charge transfer character: exited electron is transferred from the metal center to the π* system of the carboxylate ligand
The best photovoltaic performances have been achieved with polypyridyl complexes of ruthenium or osmium
general formula is cis-X2 bis(2,2’-bipyridyl-4,4’dicarboxylate)-ruthenium(II), where X = Cl-, Br-, I- and SCN-
53
Molecular wires: carbon nanotubes
Redox polymer wiring enzyme on anode
enzyme
Nano Lett. 7(2007)3528
55 J. Am. Chem. Soc., 2008, 130 (6), pp 2015–2022
NADH electron donor at the photoanode
Overpotential of over 1V
56 Heller et al JACS 124(2002)12962
The anode electrocatalyst film comprises glucose oxidase (Gox), while the cathode electrocatalyst consists of bilirubin oxidase (BOD)