The Architecture Of Photosynthesis is Optimized to Cover

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)

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 m 2 The “average” Chl content of a C 3 leaf is ~ 5. 6 x 10 -4 mol m-2 The molecular weight of Chl a is 894 g/ mol A Pragmatic Answer: Pablo Picasso – House and Trees Paris, Winter 1908 http: //www. hipernet. ufsc. br/wm/paint/auth/picasso/landscapes/picasso. house-garden. jpg 140 g Chl / tree

How many special pair Chls are in Picasso’s tree? Photosynthetic Reaction Center (RC) 140 g Chl / tree 2 “special pair” Chls initiate primary photochemistry ~ 0. 5 g Chl special Chls / tree How can this be explained ?

Light Harvesting Timescales light LH 2 LH 1 RC chemical energy

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:

2000 excess light incident light intensity umol photons m-2 sec-1 rate of photosynthesis rate of light absorption Photosynthetic organisms experience excessive light on a daily basis 1600 1200 800 400 0 4 8 12 16 time of day 20

Pigments From a Portion of the LH 2 Ring b. B 850 B RG 2 A a. B 850 A B 800 B B 800 A RG 1 B RG 1 A

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

Photosystem II— 3. 5 Å D 1 = yellow D 2 = orange K. N. Ferreira, T. M. Iverson, K. Maghlaoui, J. Barber and S. Iwata. Science. In Press. (2004)

Models for Repair of PSII—D 1 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 heat (nonphotochemical quenching) light short term regulation peripheral LHC inner LHC PS II photochemistry COO- COO H+ zeaxanthin LHC and synthesis protonation long term regulation lumen inner LHC thylakoid membrane PS II PQH 2 stroma regulation of nuclear LHC 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” Excess h S 1 High Light “ON” ~ 10 ps S 1 ISC (ns) Fluorescence (ps) NPQ S 0 T 1 So ? High Light (10 -20 min. ) = Decrease in F (~50%) Chl

Necessary components for q. E a. - 4 H+ a. ∆p. H b. Zeaxanthin c. Psb. S Stroma P S II 4 H+ p. H ~ 4 -5 + 4 H+ - 3 H+ Violaxanthin limiting h excess h Antheraxanthin + n. H+ ATP Synthase Stroma p. H ~ 7 -8 b. P S I Cyto (b 6 f) Q cycle + Lumen - 2 H+ p. H ~ 3 - 4 H+ c. limiting h excess h Zeaxanthin Li, X-P, et al. , A pigment-binding protein essential for Regulation of photosynthetic light harvesting. Nature 403, 391 -395 (2000).

npq 4 -1 wild type Molecular genetic analysis of npq 4 -1 showed that Psb. S is necessary for q. E. npq 4 -1 + vector 4. 4 kb npq 4 -1 + psb. S NPQ: low high genomic DNA hybridization with psb. S

q. E is more than two times greater in the transgenic plants wt+one psb. S gene #5 (4 copies of psb. S) wild type (2 copies of psb. S) wt+one psb. S gene #17 (4 copies of psb. S) npq 4 -1 (no psb. S) 4. 5 4. 0 3. 5 NPQ 3. 0 transgenic plants q. E =2. 9 2. 5 2. 0 wild type q. E =1. 3 1. 5 1. 0 0. 5 0. 0 0 2 4 6 8 10 Time (min) 12 14 16

Transient Absorption Experiment Probe Soret ( 1 Bu) S 2 ( 1 Bu) (1 A g ) S 1 Qx Qy k. ET S 1 (1 A g ) Pump Energy (cm-1) Sn (1 A g ) S o Absorption Car No NPQ So Chl a Car NPQ S o (1 A g )

Transient Absorption Measurements on Arabidopsis Mutants 3 3 wild type + Psb. S wild type more Psb. S and more q. E than wt 2 1 Light OFF Amplitude (a. u. ) Light ON regular q. E 2 1 0 0 Quenched * 1. 9 0 10 20 30 40 50 3 Quenched * 1. 3 0 10 30 40 50 3 npq 4 - E 122 Q E 226 Q npq 4 -1 pump = 664 nm probe = 540 nm 20 no Psb. S no q. E 2 more Psb. S , but a nonfunctional version no q. E 2 Amplitude (a. u. ) 1 1 0 0 0 10 20 30 Time (ps) 40 50

Excited States of Xanthophyll-Chlorophyll Dimers cofacial arrangement Qy S 2 CT Zea-Chl Energy in e. V S 1 CT Anthera-Chl ground state Zea-Chl distance in Angstrom Anthera-Chl distance in Angstrom S 1 Qx Energy in e. V Qx Qy Qy CT Vio-Chl ground state Vio-Chl distance in Angstrom Andreas Dreuw Martin Head-Gordon

Zeaxanthin-Chlorophyll Dimer LUMO HOMO 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 Difference kinetic indicates charge separations quenching during q. E. Near-IR spectra In PS II complexes from Synechocystis PCC 6803 (Tracewell, C. A. et al. (2003) a Biochemistry, 42, 9127).

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+ Psb. S 7. 3 6. 7 210 136 Detect 540 nm WT WT+ Psb. S - ~7 ~7 Car+ • formation is correlated with q. E

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 k. CS ~ 1/(100 -300 fs). k. Rec ~1/(150 ps), corresponds to the recovery dynamics in visible and near-IR regions. g. Ann ~ 1/[(10 ps)*n 0], n 0 – number of initial excitations in the complex. k. Tr, k. Ad. Q ~1/(100 -300 ps). ~1/(10 ps) - corresponds to the net Chl pool decay rate in the vicinity of the charge transfer complex.