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Macroscopic Reaction Vessel
2H2O O2 + 4H+ + 4eSunlight
Organic moleculeof life
+CO2
2H2O 4H+ + 4e CH2OOrganicmolecules
CO2
CO2
O2
O2
Solar Energy
photosynthesis
respiration biomassfood
fossil fuelscombustion
light reactions dark reactions
Biology adopted the perfect solution to the energy problem
Energy
5 Billion Year Clock
Big Bang ofEvolution
H2O splitting
Oxygenic atmosphere&
Ozone layer
Photosynthetic water splitting is“The Engine of Life”
Photosystem II (PSII) is the enzymefound in plants, algae and cyanobacteria
which uses solar energy to split water into molecular
oxygen and reducing equivalents
Plants and Green Algae
0
-1
+1
P680+
P680*Pheo-
PQA
PQB
RedoxscaleeV
P680+ is highly oxidisingEm ~ 1.3eV at pH 7
Tyrz (YZ)
H2O
Red.
Oxid.
Electron transfer in Photosystem II
H+
Water splitting reactionis a four photon process
2H2O + 2PQ O2 + 2PQH2Light (4hv)
PSII
Oxygen emission induced by flashesJoliot & Kok ~1969
+
+
+
2
S-state cycle – Kok & Joliot
dark stable state
Photosystem II (PSII)
Water splitting rapid turnover of D1“a repair process”
From Black Boxto
Atomic Resolution
SPEM
EC X-ray
PSII
1994
1995
1998
2001
2004
Plant PSII
Cyanobacterial PSII
PSII Dimeric Core of T. elongatusDuncan, Nield, Barber (2001) unpublished
46,630 atoms5138 amino acids
X-ray CollaboratorsKristina FerreiraTina IversonKarim MaghlaouiSo Iwata
Joanna KargulJames Murray
Also
19 protein subunits 16 intrinsic + 3 extrinsics35 transmembrane helices57 cofactors
Ferreira et al Science 303(2004)1831-1838Pdf coordinates ref. 1S5L.
Mn4-Ca cluster Mn4-Ca cluster
Per monomer
Comparison of the purple photosynthetic bacterial reaction centre with photosystem II
Bacterial RC D1/D2 PSII monomer
L
M
D1
D2
H
19 subunits4 subunits
Cyt
R. sphaeroides D1 PS IIR. viridis D2
Comparison of bacterial RC and D1/D2 of PSII
Extinsics important for producing 3D crystals
PsbO
P212121
Lumenal view of PSII monomer within the dimer
Side view of cofactors, chlorophylls and carotenoids
14Chls 6Chls 16Chls7 Carotenoidsb
CP43 CP47
14 Chls 16 Chls
13 conserved
CP43 Chls + CP47 Chls
Electron transport cofactors
Active Branch Protective Branch
Non-haem iron and quinonesbicarbonate ligand
PsbE
PsbJ
PsbN
PsbD(D2)
PQ
Diffusion pathway for PQ/PQH2 from the QB pocket to the lipid environment of the membrane
Lumenal view of PSII monomer within the dimer
PQ/PQH2
Electron transport cofactors
Active Branch Protective Branch
“P680” Chlorophylls
Oxygen Evolving CentreAnomalous diffration for Mn (1.89A) and Ca (2.25A)
bicarbonate?
Cubane-like Mn3CaO4 cluster linked to a 4th Mn by a mono-μ-oxo bond
6 + 1 Amino acid ligands5 + 1 D1 protein1 CP43
E
A
BC
D E
Ade
ab
cd
D1 protein
CP43 CP47
Large loops of CP43 and CP47
GGETMRFWD
FFESFPVIL
Water Splitting-Oxygen Evolving Catalytic Site
A344
Is the Ca2+ in the approx right position?Yes it is based on replacement by Sr2+
(Kargul et al 2007 BBA)
Where is Chloride in the OEC?(Murray et al 2008 Energy and Environ.)
Resolution in the 3.0 t0 3.5 Å region
ASP 170
ALA 344
ASP 342
GLU 189
HIS 332
GLU 333CP43
GLU 354
Mn4
Srr
Mn
Kargul et al (2007) BBA Anomalous diffraction at 0.7Å
Is the Ca2+ in the approx right position?Yes it is based on replacement by Sr2+
(Kargul et al 2007 BBA)
Where is Chloride in the OEC?(Murray et al 2008 Energy and Environ.)
Anomalous difference map forbromide at 0.92 ÅMurray et al 2008
What about channelsat the water oxidation site?
2H2O O2 + 4H+ + 4eLight (4hv)PSII
In Out Out P680+
H+ exit/H2O entry seems tobe across PsbO
P680
His 190OEC
K317
E312
D61E65
H228
D224 D158
e-H+
D158D222D223D224H228E229
H2O
H+
Oxygen channel ?
H+Murray & BarberJ. Struct. Biol.
Possible oxygen channel
Murray & BarberJ. Struct. Biol.
2H2O O2 + 4H+ + 4e
Does the structure help understand the mechanism ofwater splitting?
Diagrammatic representationof the water splitting
centre
With two substrate water molecules bond
Mn4
Ca2+
CaO
O O
MnIV MnIV
O
MnIVMnV
O
OHighly electrophilic oxo (or oxyl radical)
S4
Nucleophilic attackH H Cl
Cu+Fe2+
e-
H+
2e-
O2
Fe4+=O HO-Cu2+
3H+
2H2O 2H2O
Mn5+=O HO-Ca2+
O2e-
3H+
e-
H+
Mn2+ Ca2+
3e-
Cyt oxidase PSII
Y/W
DFT-QM/MM S-State Cycle
H+ , O2
H+ , e-
e-
H+ , e-
H+ , e-
2 H2O
Victor Batista, Gary Brudvigand coworkers (Yale)
Chemically feasible?
Fe1
Fe4Fe2
Fe3
Fe4Sx Ni - cluster
Ni
CO + H2O CO2 + 2H+ + 2e
Carbon monoxide dehydrogenase
CO
OH H
Mishra, Christou et al Chem.Commun 2005
*
*
Imperial Yale
Model based on QM/MM calculations by Basita, Brudvig and colleagues in Yale
Sproviero, Gascon, McEvoy, Brudvig, Batista, (2008) J. Am. Chem. Soc. 130, 3428.
This structure fitted the polarisedEXAFS spectra of Yano et al (2006) Science 314 , 821.
Mn3CaO4-cubane with the 4th Mn linked to the cubane by one of its bridging oxygens
remains a feasible working model
Leonardo’s Dream
Our dream
2H2O
Solar energy
2H2
O2
From Natural
A Q Q B
P 680
O
Mn
Mn
Tyr
Acceptor
N
EtO 2 C
NH
O
NN ON N
N NO OO
Me Me
Me
MnMn
Ru
NN
N
NN
to
Photosynthesis
Artificial
Hammerstrom & StyringU. of Uppsala
Fe
O
BOH
Design of an artificial water splitting system
H
MnIVMnIVMnIVMnV
O
OCa
O
OO
Cl
O
Daniel NoceraMIT
Kanan/Nocera Colbalt/phosphate based electrochemical catalyst
2H2O 2H2
O2
O2
Solar Energy (100,000TW)
EnergyTotal global (14TW)
“If a leaf can do it we can do it”
“Its only chemistry”
One hour of solar =annual global energy consumption
“If a leaf can do it we can do it…even better!It’s just chemistry”
EnergyTotal global
(14TW)
Solar Energy (100,000TW)O2
O2
2H2O 2H2
“Nature is miserably inefficient in converting light to useable energy and surely science properlyappliedcould do better”
Lord George PorterOM FRS
Starch Photographof
Jules Verne’s Dream (1875)
“I believe that water will one day be used as a fuel, because the hydrogen and oxygen which constitute it, used separately or together, will furnish an inexhaustible source of heat and light. I therefore believe that, when coal deposites are oxidised, we will heat ourselves by means of water. Water is the fuel of the future”
