Download Metal Ions Involved in Photosynthesis

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)
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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
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The structures of
chlorophylls
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Absorption spectra of pigments
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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
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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
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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
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Flow of energy through a photosynthetic antenna complex
by EXCITON transfer
Light green circles: antenna pigments
Dark green circles: Photosynthetic Reaction Centre (RC)
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Energy transfer cascade for antenna pigments in the lightharvesting complexes of Porphyridium cruetum
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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
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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)
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Structure of Light-harvesting complex LH2
from Rhodospirillum molischianum
cytoplasm
White: Mg(II)
88 structure (solved by
Michel)
Yellow: lycopenes
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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
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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
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Structure of a subunit of trimeric
LHC-II from pea chloroplasts
Thylakoid
membrane
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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
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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
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Structure of the RC from Rhodobacter sphaeroides
Cyan:
M subunit
Orange:
L subunit
H subunit
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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)
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Special pair: two closely
associated BChl b molecules
Note the two-fold symmetry of
the ring systems!
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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
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RCs from Rhodopseudomonas viridis
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Photosynthetic electron transport system in purple photosynthetic
bacteria (R. viridis)
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Standard reduction potentials
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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)
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