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)

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 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 Pablo Picasso – House and Trees Paris, Winter 1908 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 explained ? Photosynthetic Reaction Center (RC)

Light Harvesting Timescales LH2 LH1 chemical energy RC

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 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

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

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

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)

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).

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

Photosystem II Supercomplex

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%)

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, 391-395 (2000). c. b. Zeaxanthin Antheraxanthin Violaxanthin excess h limiting

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

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

Transient Absorption Experiment Sn 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

Transient Absorption Measurements on Arabidopsis Mutants 3 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)

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

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

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.

Near-IR Arabidopsis thaliana Studies (λpump = 664 nm; λprobe = 1000 nm) Time (ps) Difference Kinetic Fits τrise(ps) τdecay (ps) Detect 1000 nm WT 7.3 210 WT+ PsbS 6.7 136 Detect 540 nm WT - ~ 7 WT+ PsbS - ~ 7 Car+• formation is correlated with qE

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/(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. 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.