The Architecture Of Photosynthesisis Optimized to:

Cover the solar spectrumProtect against photochemical damage

Separate energy and electron transfer

Regulate the efficiency of lightharvesting and repair damage (PSII)

Transmit excitation to thereaction center with near efficiency

An Abstract Question: How much Chl is in Picasso’s tree ?

A Collection of Facts

Pablo Picasso – House and Trees Paris, Winter 1908

A “medium” size tree has ~ 100,000 leaves

An “average” leaf has a surface area of ~ 2.8 x 10-3 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

http://www.hipernet.ufsc.br/wm/paint/auth/picasso/landscapes/picasso.house-garden.jpg

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 How can this be explained ?explained ?

Photosynthetic Reaction Center(RC)

light

LH2

LH1

chemical energy

RC

Light Harvesting Timescales

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:

Photosynthetic organisms experienceexcessive light on a daily basis

excess light

rate

of

lig

ht

ab

so

rpti

on

rate

of

ph

oto

sy

nth

es

is

incident light intensity

time of dayu

mo

l p

ho

ton

s m

-2 s

ec-1

2000

1600

1200

800

400

04 8 12 16 20

Pigments From a Portion of the LH2 Ring

RG1ARG1B

B800B B800A

B850B

B850A

RG2B

RG2A

Photosynthetic organisms experience frequent short-term fluctuations in light intensity.

Külheim et al. (2002) Science 297: 91-93

Photosystem II—3.5 Å

K. N. Ferreira, T. M. Iverson, K. Maghlaoui, J. Barber and S. Iwata. Science. In Press. (2004)

D1 = yellowD2 = orange

Models for Repair of PSII—D1 Protein

E. Baena-Gonzalez and E.-M. Aro. Phil . Trans. R. Soc. Lond. B, 357,1451-1460 (2002).

P. Silva et. al. Phil . Trans. R. Soc. Lond. B, 357, 1461-1468 (2002).

PS II

thylakoid membrane

lumen

stroma

light heat (nonphotochemical quenching)

photochemistryperipheral

LHCinnerLHC

COO-COO- H+

PS IIinnerLHC

short termregulation

long termregulation

Photoprotection involves regulation of light harvesting

LHCprotonation

zeaxanthinsynthesis

and

PQH2

regulation of nuclearLHC gene expression

Photosystem II Supercomplex

What is NPQ?

Nonradiative energy dissipation in PSII Purpose: protects PSII from photochemical damage Main Component: qe - “high energy state quenching”

Chl. Fluorescence vs. Time

Time (min.)

4 6 8 10 12 14 16 18 20

Ch

l flu

ore

sce

nce

co

un

ts

200

300

400

500

Fo

Fm

Fm'

High Light “ON”

High Light (10-20 min.) = Decrease in F (~50%)

Excess hExcess h

Chl

So

S1

T1

Flu

ores

cen

ce (

ns)

ISC (ns)

Flu

ores

cen

ce (

ps)

NPQ

?

S1

S0~

10 p

s

OH

OH

O

OH

OH

O

OH

OH

O

Zeaxanthin

Antheraxanthin

Violaxanthin

excess h

limiting h

limiting h

excess h

a.

b.

Li, X-P, et al., A pigment-binding protein essential for Regulation of photosynthetic light harvesting. Nature 403, 391-395 (2000).

c.

Necessary components for qE

a. ∆pH b. Zeaxanthin c. PsbS

H+

- 2H+

Lumen pH ~ 4-5

Stroma pH ~ 7-8

StromaPSII

Q cycle

PSI

Cyto (b6f)

- 4H+

+ 4H+

+ 4H+

ATPSynth- ase

- 3H++ nH+

pH ~ 3 - 4

wild type

npq4-1

npq4-1 + vector

npq4-1 + psbS

NPQ: low high

Molecular genetic analysis of npq4-1showed that PsbS is necessary for qE.

4.4 kb

genomic DNAhybridization

with psbS

wil

d t

ype

np

q4-

1

qE is more than two times greaterin the transgenic plants

Time (min)

NP

Q

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0 2 4 6 8 10 12 14 16

wild type (2 copies of psbS)

npq4-1 (no psbS) wt+one psbS gene #17 (4 copies of psbS)

wt+one psbS gene #5 (4 copies of psbS)

wild typeqE =1.3

transgenicplantsqE =2.9

0.00.20.40.60.81.01.21.40

4000

8000

12000

16000

20000

24000

28000

32000

Chl a

En

erg

y (

cm

-1)

Car Car NPQNo NPQ

Qy

Qx

Absorption

So (1Ag)

S2 ( 1Bu)

So

Sn

Soret

Pum

p

Prob

e

kET

S1 (1Ag)

(1Ag) So

(1Ag) S1

( 1Bu) S2

Transient Absorption Experiment

Transient Absorption Measurements on Arabidopsis Mutants

Light ON

Light OFF

0 10 20 30 40 50

0

1

2

3

wild type regular qE

Time (ps)

0 10 20 30 40 50

0

1

2

3

npq4- E122Q E226Q more PsbS , but a nonfunctional version no qE

Time (ps)

0 10 20 30 40 50

Am

plitude (a.u.)

0

1

2

3

0 10 20 30 40 50

Am

plitude (a.u.)

0

1

2

3

wild type + PsbS more PsbS and more qE than wt

npq4-1 no PsbS no qE

Quenched * 1.9 Quenched * 1.3

pump = 664 nmprobe = 540 nm

Excited States of Xanthophyll-Chlorophyll Dimers

cofacial arrangement

Zea-Chl Anthera-Chl

Vio-Chl Andreas DreuwMartin Head-Gordon

En

erg

y in

eV

En

erg

y in

eV

Zea-Chl distance in Angstrom Anthera-Chl distance in Angstrom

En

erg

y in

eV

Vio-Chl distance in Angstrom

S1

CT

S2

ground state

Qy

CT

ground state

S1

Qy

Qx

ground state

CT

S1

Qy

Qx

Zeaxanthin-Chlorophyll Dimer

LUMO HOMO

Andreas DreuwMartin Head-Gordon

TA studies in the near-IR: Formation of the carotenoid radical cation.

a In PS II complexes from Synechocystis PCC 6803 (Tracewell, C. A. et al. (2003) Biochemistry, 42, 9127).

Spinach thylakoidsλPump = 664 nm; λProbe = 1000 nm Near-IR spectra

Difference kinetic indicates charge separations quenching during qE.

0

1x10-5

2x10-5

3x10-5

4x10-5

5x10-5

0 100 200 300 400 500

0

1x10-5

2x10-5

0 20 40 60

0

1x10-5

2x10-5

3x10-5

4x10-5

5x10-5

Quenched

A (

OD

)

Unquenched

150 ps10 ps

Difference

Time (ps)

A (

OD

)

Time (ps)

900 950 1000 1050 1100

0.0

0.2

0.4

0.6

0.8

1.0

Transient spectrum at 20 ps delay

-carotene cation spectruma

Diff

ere

nce

A

(a

.u.)

(nm)

npq4-1

Time (ps)0 100 200 300 400

Nor

mal

ized

O

D (

a.u

.)

0.0

0.2

0.4

0.6

0.8

1.0Difference Kinetic Fits

τrise(ps) τdecay (ps)

Detect 1000 nmWT 7.3 210WT+ PsbS 6.7 136

Detect 540 nmWT - ~ 7 WT+ PsbS - ~ 7

Wild Type + PsbS

0 100 200 300 400

Nor

mal

ized

O

D (

a.u

.)

0.0

0.2

0.4

0.6

0.8

1.0Wild Type

0 100 200 300 400

Nor

mal

ized

O

D (

a.u

.)

0.0

0.2

0.4

0.6

0.8

1.0

Near-IR Arabidopsis thaliana StudiesNear-IR Arabidopsis thaliana Studies ((λpump = 664 nm; λprobe = 1000 nm))

Time (ps)

Car+• formation is correlated with qE

One proposed quenching mechanism

Gives rise to the positive signal at 1000 nm.

Corresponds to the negative (bleaching) signal in the 540 nm region.

Assigned time constants

kTr, kAdQ ~1/(100-300 ps).

~1/(10 ps) - corresponds to the net Chl pool decay rate in the vicinity of the charge transfer complex.

kCS ~ 1/(100-300 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.