Bimolecular processes
Electron transfer
*A + B
A+ + B-
*A + B
A- + B+
EA
IP
*EA
*IP
LUM O
An excited state is a better
oxidant and a better reductant
than the ground state
HOM O
X
X*
Kinetic of electron transfer: Marcus Model
kel = nNkexp(-DG#/RT)
A+A-
A.A
autoexchange reaction
E
Energy profile
of reactants
l
DG#
The meaning of the various terms can be
conveniently discussed considering the energy
profiles.
For simplicity we consider an auto-exchange
electron transfer reaction.
These curves represent potential energies of
reactants and products as a function of a
reaction coordinate. This is the combination of
internal coordinates (bonds and angles) and of
arrangement of solvent molecules surrounding
reactants and reagents.
At the equilibrium geometry of reactants,
products have a very higher energy (l) called
“reorganization energy”
Fluctuations around equilibrium geometry can
lead reactants to the geometry of the crossing
between the two curves. At this geometry
reactants and products have the same energy
Energy profile and the electron can be easily transferred: then
of products
products relaxed to their equilibrium geometry
Nuclear configuration
kel = nNkexp(-DG#/RT)
k is the trasmission coefficient of the
reaction, that is the probability that the
reactants convert into products once
they have reached the geometry of the
crossing point. Its values vary from 0 to
1.
A+A-
A.A
nN is the nuclear frequency factor of the
reaction. It is the weighted mean of the
frequencies of the nuclear vibration
modes involved in the reaction
coordinate.
autoexchange reaction
E
l is the reorganization energy (energy of
products at the equilibrium geometry of
reactants).
Energy profile
of reactants
l
DG#
Energy profile
of products
Nuclear configuration
DG# is the activation energy, that is the
energy difference between the crossing
point and the reactant minimum.
When the electron transfer process involves
two different reactants A and B, a free energy
variation (DG0) has to be considered
kel = nNkexp(-DG#/RT)
E
A.B
In this case DG# depends on DG0:
A+B-
l
DG#
DG0
Nuclear configuration
DG# =
(DG0 + l)2
4l
DG#
(DG0 + l)2
=
kel = nNkexp(-DG#/RT)
4l
DG0 = 0; DG# = l/4
Log ket
-l < DG0 < 0, logket increases when DG0 decreases:
“normal”
DG0 = -l; DG# =0 and logket is maximum: “activationless”
-l
0
DG0
DG0 < -l; DG#>0 and logket decreases: “inverted”
Photosynthesis
photosynthesis
H2O + CO2
1/n (CH2O)n + 3O2
DH = + 470 kJ/mole
respiration
Photosynthesis is an example of uphill process and it is energy
consuming.
For this reason it has a high degree of complexity and a very high
organization.
In plants, the primary photosynthetical events take place in the
membrane of special vesicles inside chloroplasts.
Efficiency in light absorption is fulfilled through the presence of different
organic pigments
Tetrapyrrolic ring containing Mg2+ is the light
absorbing component (molar extinction
coefficient is about 105 M-1 cm-1).
Long hydrophobic chain binds chlorophyll to
the membrane
Other pigments allow to cover a broader
spectral absorption range
Light-harvesting
The absorption of photons by the pigments is quick (femtoseconds) and yields to singlet excited state
that are very short lived.
The major part of chlorophylls (> 98%) act as antenna devices and collect available photons.
The absorbed energy is then transferred through consecutive energy transfer reactions to the actual
photoreaction center which contain less than 2% of the total chlorophyll content.
Energy transfer does not require
any movement: many chlorophylls
are arranged in spatial proximity
with a specific orientation: they are
able to funnel the light energy to the
reaction centers with 95% of
efficiency within 10-100 ps.
Energy-transfer cascade for antenna pigments
Energy transfer proceeds via spectral overlap of emission bands of the donor
excited species with the absorption bands of the acceptor. So the light
harvesting complexes of the photosynthetic membrane feature a spatially as
well as spectrally optimized cross-section for photon capture.
S1
12 ps
22 ps
52 ps
530 nm
578 nm
640 nm
660 nm
685 nm
S0
phycoerythrin
phycocyanin
allo-phycocyanin
chlorophyll a
Energy-transfer cascade for antenna pigments in light-harvesting
complexes of the algae Porphyridium cruetum
Role of magnesium in chlorophyll
• It contributes to the three dimensional organization of chlorophylls.
They are fixed and correctly oriented not only by the bond of their long
chain to the membrane, but also through the two axial coordination
sites of magnesium.
• it has proper size, sufficient natural abundance and strong tendency
for hexacoordination
• it is a light atom with a small spin orbit coupling constant. So inter
system crossing is inhibited so favouring the energy transfer from the
excited singlet states
• it is not a redox metal and does not interfere in the charge separation
steps.
Charge-separation step
*P680 Pheo QA QB etc…
P680+. Pheo-. QA QB
P680+. Pheo QA.- QB
P680+. Pheo QA QB .- and so on
Summarizing, the key aspects for successful charge-separation are:
-The arrangement of the components is the basis for the strong
preference for charge separation steps instead of charge
recombination
-The arrangement and the confinement of the components reduces
the activation energies for the forward electron transfer and
accelerates the electron transfer reaction.
- Back electron transfer process falls in the Marcus inverted region,
where reaction rate decreases in spite of an increase of DG0, i.e. in
spite of a more favorable equilibrium.
Photophosphorilation process
Ferredoxin-NADP reductase (FNR)
2Fd2+red + 2H+ + NADP+ (in stroma) 2 Fd3+ox + NADPH + H+
Dark reaction: Calvin cycle
Water photolysis
2 H2O
O2 + 4 H+ + 4 e-
Photoexcitation of PSII leaves
the chlorophyll a in the
reaction center with a deficit
of electron. This electron must
be obtained from some other
reducing agent. The external
source of electrons is water
OEC is a redox active structure
containing manganese as redox species.
Oxygen evolving complex
Oxygen evolving complex or water splitting complex
P680+. accepts an eletron from Tyr.
Oxygen evolving complex
can exist in 5 states: S0 to
S4. photons trapped by
PSII move the complex
from one state to the next
one. S4 is unstable and
reacts with water to
produce O2
Tyr+. Accepts an electron from
manganese ion that changes its
oxidation state in OEC.
After 4 of these steps OEC takes 4
electrons from the oxidation of two
water molecules, releasing O2.
4 H+ are also released in the lumen so
contributing to the proton gradient.
No intermediates between H2O and O2
are released.
O2 + 4 H+ + 4 e2 H2O
S0
S1
S2
S3
S4
4+
3+
Mn
Mn
4+
3+
Mn
Mn
4+
3+
Mn
Mn
4+
4+
Mn
Mn
4+
4+
Mn
Mn
3+
3+
Mn
Mn
4+
3+
Mn
Mn
4+
4+
Mn
Mn
4+
4+
Mn
Mn
3+ 5+
4+
Mn
Mn
Role of manganese in the OEC
Mn(III) and Mn(IV) oxides or hydroxides were certainly
available in sea water under the conditions of developing
photosynthesis (3x109 years ago)
Manganese has a large variety of stable oxidation states (+II,
+III, +IV, +VI, +VII)
The importance of manganese for the O2 metabolism is not
restricted to photosynthesis.