The Light Reactions of Photosynthesis: Physical Mechanisms and Chemical Patterns

1
The Light Reactions of PhotosynthesisPhysical
MechanismsandChemical Patterns

• Julio de Paula
• Department of Chemistry
• Haverford College
• jdepaula_at_haverford.edu
• http//www.haverford.edu/chem/depaula/

2
Outline

• Overview of photosynthesis
• Energy transfer
• Forster theory
• Light-harvesting complexes
• Electron transfer
• Thermodynamics
• Kinetics Marcus theory
• Design of electron transfer systems in biology

3
Overview of oxygenic photosynthesis
4
Overview of oxygenic photosynthesis
5
The chloroplast
6
The action spectrum of plant photosynthesis
7
The light reactions of plant photosynthesis
8
Absorption spectra of photosynthetic pigments
9
The pigments
10
The pigments
11
The pigments
12
The pigments
13
The absorption and emission of light
heat
excited state
Energy
Fluorescence
Absorption
ground state
14
Energy capture and transfer
15
Exciton transfer mechanisms Förster theory
excited states
Energy
Absorption
ground states
D
A
16
Exciton transfer mechanisms Förster theory
excited states
Energy
Emission
ground states
D
A
17
Exciton transfer mechanisms Förster theory
excited states
Energy
Absorption
ground states
D
A
18
Exciton transfer mechanisms Förster theory
excited states
Energy
Fluorescence
ground states
D
A
19
Exciton transfer mechanisms Förster Theory
Energy transfer is a down-hill process, but
energy emitted by D must be able to excite A.
20
Exciton transfer mechanisms Förster Theory
The efficiency decreases with increasing distance
between D and A.
21
The LH2 complex of purple bacteria
2 nm
2 nm
22
Energy transfer in purple bacteria

• Equilibration within a LH2 unit 1 ps.
• Transfer among LH2 units, then to LH1 10-50 ps.
• Transfer from LH1 to the RC 50-100 ps.

23
Phycobilisomes of cyanobacteria
24
Exciton transfer in the phycobilisome
12 ps
24 ps
excited states
50 ps
576 nm
Energy
642 nm
660 nm
ground states
RC
PE
PC
AP
25
The LHCII complex of photosystem II
26
Chlorosomes of green bacteria
100 nm
27
Chlorosomes of green bacteria
aggregates of Bchl c
bacteriochlorophyll c
28
Chlorosomes of green bacteria
aggregates of Bchl c
baseplate Bchl a
antenna proteins
reaction center
29
Chlorosomes of green bacteria
light
aggregates of Bchl c
baseplate Bchl a
antenna proteins
reaction center
30
Chlorosomes of green bacteria
aggregates of Bchl c
baseplate Bchl a
antenna proteins
reaction center
31
Chlorosomes of green bacteria
aggregates of Bchl c
baseplate Bchl a
antenna proteins
reaction center
32
Chlorosomes of green bacteria
aggregates of Bchl c
baseplate Bchl a
antenna proteins
reaction center
33
Thermodynamics of electron transfer

• Diversity of electron donors in photosynthesis
• plants, algae, and cyanobacteria
• CO2 2 H2O ? CH2O H2O O2
• green sulfur bacteria
• CO2 2 H2S ? CH2O H2O 2 S
• purple sulfur bacteria
• CO2 H2O 2 HSO3- ? CH2O 2 HSO4-
• nonsulfur photosynthetic bacteria
• CO2 2 H2 ? CH2O H2O
• CO2 2 Lactate- ? CH2O H2O 2 Pyruvate-
• Water is the most plentiful electron donor but
  the most difficult to oxidize
• O2 4H 4 e- ? 2 H2O E? 0.82 V

34
Thermodynamics of electron transfer

• The light reactions of oxygenic photosynthesis
  must be driven by absorption of energy
• 2 NADP 2 H2O ? O2 2 NADPH 2 H
• E? -1.135 V and ?rG? 438.0 kJ mol-1
• In general, an organic molecule is a better
  reducing agent in an excited state than in a
  ground state.
• Transfer of energy from light-harvesting
  complexes to a special dimer of chlorophylls in
  the reaction center generates a strong reducing
  agent that initiates an electron transfer cascade.

35
Electron transfer in purple bacteria
36
Electron transfer in purple bacteria
37
Electron transfer in purple and green bacteria
E? (V)
38
Electron transfer in purple and green bacteria
exciton
chlorophyll dimer
-

resistor electron trap
heat
generator cyt bc1 complex
chemical work ATP
39
Electron transfer in plant photosynthesis
40
The Z scheme
E? (V)
41
Photosystem II
42
Photosystem II
43
Photosynthetic water oxidation
44
Photosynthetic water oxidation
45
Photosynthetic water oxidation
46
Photosystem I
47
The cytochrome b6f complex
48
Kinetics of electron transfer Marcus Theory
49
Kinetics of electron transfer Marcus Theory
50
Kinetics of electron transfer Marcus theory

• The rate of electron transfer decreases with
  increasing distance between donor and acceptor.
• Duttons ruler (empirical) for
• Exergonic reactions (?rG lt 0)
• with distances in Ångstroms and energies in eV.
• Endergonic reactions (?rG gt 0)

51
Electron transfer in purple bacteria
E? (V)
52
Design of electron transfer systems
53
Design of electron transfer systems
E? (V)
54
Design of electron transfer systems
E? (V)
55
Design of electron transfer systems
E? (V)