1. The Architecture Of Photosynthesis
is Optimized to:
Cover the solar spectrum
Protect against photochemical damage
Separate energy and electron
transfer
Transmit excitation to the
reaction center with near
efficiency
Regulate the efficiency of light
harvesting and repair
damage (PSII)

2. A Pragmatic Answer: 140 g Chl / tree An Abstract Question:
How much Chl is in Picasso’s tree ?
A Collection of Facts
A “medium” size tree has
~ 100,000 leaves
An “average” leaf has a surface
area of
~ 2.8 x m2
The “average” Chl content of a C3
leaf is
~ 5.6 x 10-4 mol m-2
The molecular weight of Chl a is
894 g/ mol
A Pragmatic Answer:
140 g Chl / tree
Pablo Picasso – House and Trees
Paris, Winter 1908

3. How many special pair Chls are in Picasso’s tree?
140 g Chl / tree
2 “special pair” Chls
initiate primary
photochemistry
~ 0.5 g Chl special Chls /
tree
How can this be explained ?
Photosynthetic
Reaction Center
(RC)

4. Light Harvesting Timescales
LH2
LH1
chemical energy RC

5. Current Model of the PSU
Net Reaction of PSI and PSII:
ATP synthase: Uses electrochemical potential to synthesize ATP from ADP
Net Reaction of the
Calvin Cycle:

6. Photosynthetic organisms experience excessive light on a daily basis
time of day
umol photons m-2 sec-1
2000
1600
1200
800
400 4 8 12 16 20
excess light
rate of photosynthesis
rate of light absorption
incident light intensity

7. Pigments From a Portion of the LH2 Ring
RG2B
bB850B
RG2A
aB850A
B800B
B800A
RG1B
RG1A

8. Photosynthetic organisms experience frequent short-term fluctuations in light intensity.
Külheim et al. (2002) Science 297: 91-93

9. Photosystem II—3.5 Å D1 = yellow D2 = orange
K. N. Ferreira, T. M. Iverson,
K. Maghlaoui, J. Barber and
S. Iwata. Science. In Press.
(2004)

10. Models for Repair of PSII—D1 Protein
E. Baena-Gonzalez and E.-M. Aro.
Phil . Trans. R. Soc. Lond. B, 357,
(2002).
P. Silva et. al. Phil . Trans. R. Soc.
Lond. B, 357, (2002).

11. Photoprotection involves regulation
of light harvesting
light
heat (nonphotochemical quenching)
short term
regulation
peripheral
LHC
inner
LHC
PS II
photochemistry
COO-
COO- H+
zeaxanthin
synthesis
LHC
protonation
and
lumen
long term
regulation
inner
LHC
PS II
thylakoid membrane
PQH2
stroma
regulation of nuclear
LHC gene expression

12. Photosystem II Supercomplex

13. What is NPQ? ?  Nonradiative energy dissipation in PSII Excess h NPQ
Purpose: protects PSII from photochemical damage
Main Component: qe - “high energy state quenching”
High Light “ON”
Excess h
Chl So S1 T1
Fluorescence (ns)
ISC (ns)
Fluorescence (ps)
NPQ ? S0
~ 10 ps
High Light (10-20 min.) = Decrease in F (~50%)

14. Necessary components for qE
a. ∆pH
b. Zeaxanthin
c. PsbS a. H+
- 2H+
Lumen pH ~ 4-5
Stroma pH ~ 7-8
Stroma P S II
Q cycle I
Cyto
(b6f)
- 4H+
+ 4H+
ATP
Synth-
ase
- 3H+
+ nH+
pH ~ 3 - 4
Li, X-P, et al., A pigment-binding protein essential for
Regulation of photosynthetic light harvesting. Nature 403, (2000). c. b.
Zeaxanthin
Antheraxanthin
Violaxanthin
excess h
limiting

15. Molecular genetic analysis of npq4-1
showed that PsbS is necessary for qE.
wild type
wild type
npq4-1
+ vector +
psbS
NPQ: low high
npq4-1
4.4 kb
genomic DNA
hybridization with psbS

16. qE is more than two times greater in the transgenic plants
wild type (2 copies of psbS)
wt+one psbS gene #5 (4 copies of psbS)
npq4-1 (no psbS)
wt+one psbS gene #17 (4 copies of psbS)
Time (min)
NPQ
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5 2 4 6 8 10 12 14 16
transgenic
plants
qE =2.9
wild type
qE =1.3

17. Transient Absorption ExperimentSn
Probe
Soret
( 1Bu) S2
Energy (cm-1)
S2 ( 1Bu)
(1Ag) S1 Qx
kET Qy
S1 (1Ag)
Pump
(1Ag) So
So (1Ag) So
Car
Chl a
Car
Absorption
No NPQ
NPQ

18. Transient Absorption Measurements on Arabidopsis Mutants3 3
wild type + PsbS
wild type
more PsbS and
regular qE 2
more qE than wt 2 1 1
Quenched * 1.9
Quenched * 1.3
Light ON
Amplitude (a.u.) 10 20 30 40 50 10 20 30 40 50
Light OFF 3 3
npq4-1
npq4- E122Q E226Q
more PsbS 2
no PsbS 2 ,
but a
pump = 664 nm
probe = 540 nm
no qE
nonfunctional
version no qE 1 1
Amplitude (a.u.) 10 20 30 40 50 10 20 30 40 50
Time (ps)
Time (ps)

19. Excited States of Xanthophyll-Chlorophyll Dimers
cofacial arrangement Qy Qx S2 Qy S1 S1 CT CT
Energy in eV
Energy in eV
Zea-Chl
Anthera-Chl
ground state
ground state
Zea-Chl distance in Angstrom
Anthera-Chl distance in Angstrom S1 Qx Qy CT
Energy in eV
Vio-Chl
Andreas Dreuw
Martin Head-Gordon
ground state
Vio-Chl distance in Angstrom

20. Zeaxanthin-Chlorophyll Dimer
HOMO
LUMO
Andreas Dreuw
Martin Head-Gordon

21. TA studies in the near-IR: Formation of the carotenoid radical cation.
Spinach thylakoids
λPump = 664 nm; λProbe = 1000 nm
Near-IR spectra
a In PS II complexes from Synechocystis PCC 6803 (Tracewell, C. A. et al. (2003) Biochemistry, 42, 9127).
Difference kinetic indicates charge separations quenching during qE.

22. Near-IR Arabidopsis thaliana Studies
(λpump = 664 nm; λprobe = 1000 nm)
Time (ps)
Difference Kinetic Fits
τrise(ps) τdecay (ps)
Detect 1000 nm WT
WT+ PsbS
Detect 540 nm
WT ~ 7
WT+ PsbS ~ 7
Car+• formation is correlated with qE

23. One proposed quenching mechanism
Corresponds to the negative (bleaching) signal in the 540 nm region.
Gives rise to the positive signal at 1000 nm.
Assigned time constants
kCS ~ 1/( fs).
kRec ~1/(150 ps), corresponds to the recovery dynamics in visible and near-IR regions.
gAnn ~ 1/[(10 ps)*n0], n0 – number of initial excitations in the complex.
kTr, kAdQ ~1/( ps).
~1/(10 ps) - corresponds to the net Chl pool decay rate in the vicinity of the charge transfer complex.