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
Photosystem II (PSII) is the enzymefound in plants, algae and cyanobacteria
which uses solar energy to split water into molecular
oxygen and reducing equivalents
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+
From Black Boxto
Atomic Resolution
SPEM
EC X-ray
PSII
1994
1995
1998
2001
2004
Plant PSII
Cyanobacterial PSII
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
PsbE
PsbJ
PsbN
PsbD(D2)
PQ
Diffusion pathway for PQ/PQH2 from the QB pocket to the lipid environment of the membrane
Cubane-like Mn3CaO4 cluster linked to a 4th Mn by a mono-μ-oxo bond
6 + 1 Amino acid ligands5 + 1 D1 protein1 CP43
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.)
H+ exit/H2O entry seems tobe across PsbO
P680
His 190OEC
K317
E312
D61E65
H228
D224 D158
e-H+
D158D222D223D224H228E229
H2O
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)
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
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
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”
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”
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