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Metal Ions Involved in Photosynthesis energy-consuming process leading to the production of reduced carbon and dioxygen certain bacteria, algae and green plants are photosynthetically active model complexes show only moderate success in mimicking aspects of photosynthesis (because of the demanding requirements for “uphill catalysis” -> this also explains the high complexity of photosynthesis in biological systems) 1 Photosynthesis in eukaryotes (algae and higher plants) takes place in chloroplasts Chloroplasts are membranous subcellular organelles (resembling mitochondria in many ways) Stroma: concentrated solution of enzymes, DNA, RNA, ribosomes involved in synthesis of chloroplast proteins Thylakoid (from Greek: sac or pouch) is a highly folded vesicle, its membrane houses the photosynthetic apparatus 2 1 The structures of chlorophylls 3 Absorption spectra of pigments 4 2 Modes of Decay Absorption: < 10-15 s Internal Conversion common mode of decay energy converted to kinetic energy of molecular motion, i.e. heat completed in < 10-11 s Fluorescence decay involves emission of one photon (with lower energy than the incident photon) completed in ca. 10-8 s in plants: fluorescence only accounts for 3-6 % of the total dissipation of energy 5 Modes of Decay (cont.) Exciton Transfer also known as Förster Resonance Energy Transfer (FRET) excited molecule transfers excitation energy directly to a neighboring unexcited molecule (thereby returning to its ground state) achieved by appropriate overlap of molecular orbitals of both molecules (donor and acceptor) if many donor/acceptor molecules are in close proximity and in the proper spatial orientation, then exciton transfer can happen over long distances 6 3 Modes of Decay (cont.) Photooxidation excited state molecule loses an electron by transferring it to an acceptor (which is thereby reduced) the electron is lost relatively easily because of the less tight binding of the electron to the donor in the excited state (as compared to the ground state) In photosynthesis: Chlorophyll can absorb a photon and be photooxidized. Oxidized Chl(+) is a cation radical which can return to its ground state by oxidizing some other molecule 7 Flow of energy through a photosynthetic antenna complex by EXCITON transfer Light green circles: antenna pigments Dark green circles: Photosynthetic Reaction Centre (RC) 8 4 9 Energy transfer cascade for antenna pigments in the lightharvesting complexes of Porphyridium cruetum 10 5 Structure of a 1-dim aggregate in crystals of ethyl chlorophyllide dihydrate conjugation highlighted in bold, H-bonds with dashed lines such close spatial arrangement is not found in light-harvesting proteins however: the arrangement reveals a possible pathway for exciton transfer here: the double coordinatively unsaturated Lewis acid Mg(II) interacts (via H-bonding water molecules) with the carbonyl group of the cyclopentanone ring (Lewis base) of adjacent chlorophyll molecules 11 Why Magnesium? Properties of Mg(II): hard Lewis acid, prefers hexacoordination The well-defined spatial orientation of pigments cannot be solely guaranteed by anchoring the chlorophyll molecules in the membrane (via phytyl side chains) Mg(II) contributes to the particular arrangement of pigments resulting in virtually loss-free exciton transfer to the photosynthetic reaction centre Mg(II) does so by interacting with polypeptide side chain ligands, which fill the axial positions (“three point fixing” for spatial orientation) Mg(II) is the only main group metal ion which has the proper size and charge for chlorophyll, sufficient natural abundance, non-catalytic function and preference for hexacoordination! Mg(II) is a light element and has therefore a small spin-orbit coupling constant (this avoids detrimental inter-system crossings from excited singlet to long-lived triplet states, which would lead to undesired light and heat-producing processes and chemical reactions) 12 6 Structure of Light-harvesting complex LH2 from Rhodospirillum molischianum cytoplasm White: Mg(II) 88 structure (solved by Michel) Yellow: lycopenes 13 LH2 is a transmembrane 16-mer protein complex It absorbs light at short wavelength. The energy is then funneled to LH1, and then further to the reaction centre LH2 binds 24 bacteriochlorophyll a (BChl a) and 8 lycopene (carotenoid) molecules 16 BChl a molecules form a 16-bladed “turbine”, in which individual BChl a molecules are in VdW contact each of the 16 BChl a molecules has an axial His ligand 8 remaining BChl a molecules contain Asp as an axial ligand both sets of BChl a molecules are in contact with lycopene molecules Accessory pigments (such as lycopene or -carotene) absorb in spectral regions where chlorophylls do not 14 7 LHC-II in plants most abundant membrane protein in chloroplasts of green plants 232-residue transmembrane protein contains ca. half of the chlorophylls in the biosphere (each LHC-II contains at least 7 Chl a’s, 5 Chl b’s and two carotinoids through electronic interactions, the carotenoids prevent the reaction of chlorophylls with O2 15 Structure of a subunit of trimeric LHC-II from pea chloroplasts Thylakoid membrane 16 8 aquatic photosynthetic organisms contain additional types of accessory pigments because light in the range from 450-550 nm is almost completely absorbed by water (10 m) in red algae and cyanobacteria: Chl a is replaced by phycoerythrobilin and phycocyanobilin 17 The photosynthetic reaction centre (of photosynthetic bacteria) Nobel Prize for Chemistry (1988) was awarded to Deisenhofer, Huber, Michel for the structural elucidation of a bacterial photosynthetic reaction centre 18 9 Structure of the RC from Rhodobacter sphaeroides Cyan: M subunit Orange: L subunit H subunit 19 Cytoplasmic side Cofactors in the RC of purple photosynthetic bacteria 4 BChl b (960 nm max) 2 Bacteriopheophytin b (BPheo b) -> Mg(II) replaced by 2 protons 1 nonheme/non-FeS Fe(II) ion 1 molecule of ubiquinone 1 molecule of menaquinone (vitamin K2) 20 10 Special pair: two closely associated BChl b molecules Note the two-fold symmetry of the ring systems! 21 The Fe(II) is hexacoordinated by 4 His and 2 Glu residues and lies almost mid-way in between the ubiquinone and menaquinone Rapid removal of electron from excited state special pair is virtually 100 % efficient 22 11 RCs from Rhodopseudomonas viridis 23 Photosynthetic electron transport system in purple photosynthetic bacteria (R. viridis) 24 12 Standard reduction potentials 25 Some final comments on photosynthesis in photosynthetic bacteria Photosynthetic bacteria use photophosphorylation to drive endergonic processes they do not produce reducing equivalents (unlike cyanobacteria and plants through water oxidation); instead, they need these red. eq. from the environment (as H2S, S, S2O32-, H2, etc.) PS of photosynthetic bacteria probably resemble the “original PS” (early earth: reducing eq. were abundant) evolution: rise of cyanobacteria (which developed a photosystem “strong enough” to abstract electrons from water) 26 13