Biomedical Engineering Reference
In-Depth Information
The.complexes.and.pigments.of.the.PS.I.peripheral.antenna.have.developed.a.speciic.
structure.that.allows.it.to.play.two.signiicant.functions,.light.harvesting,.prompt.and.efi-
cient.delivery.of.the.excited.energy.to.the.RC,.and.photoprotection.of.the.photosynthetic.
unit.
19
PS.I.sensitization.of.a.semiconducting.solid.surface.has.been.recently.studied.by.Frolov's.
and. Sepunaru's. research. groups.. Frolov. et. al.
47.
built. a. junction. between. specially. engi-
neered. unique. cysteine. mutants. of. PS. I. and. GaAs,. a. semiconductor. of. current. interest..
This. PS. I-GaAs. system. sustained. its. stability. for. a. few. months.. Moreover,. an. electron.
transfer.from.PS.I.to.the.semiconductor.was.observed.when.0.3.and.-0.47.V.(vs..normal.
hydrogen.electrode.(NHE)).of.photovoltage.was.obtained.with.PS.I/p-GaAs.and.PS.I/n-
GaAs,.respectively..In.a.later.study.of.the.PS.I/GaAs.system,.Sepunaru.et.al.
48
.observed.
that. the. negative. photovoltage. in. the. PS. I/p-GaAs. junction. was. a. result. of. the. electron.
transfer,.similar.to.the.one.in.the.original.PS.I—from.the.primary.electron.donor,.chloro-
phyll.chromophore.(Chl),.to.the.inal.acceptor,.FeS..This.was.found.to.be.opposite.with.the.
n-GaAs.structure,.where.positive.photovoltage.was.observed..This.was.a.result.of.fast.elec-
tron.transfer.between.the.photoexcited.reaction.center.of.the.PS.I.and.its.primary.electron.
acceptor.(Chl).to.the.n-GaAs.structure..Sepunaru.et.al..reported.that.this.occurrence.was.
a.consequence.of.a.0.9.V.energy.gap.generation,.resulting.from.the.light-induced.charge.
separation..These.researchers.reported.that.the.generated.energy.gap.forced.the.electrons.
from.the.exited.PS.I.reaction.center.to.the.n-GaAs.conducting.band,.causing.a.decrease.in.
band.bending.
22.3.4 Photosystem II (PS II)
Contrary.to.its.numeric.designation,.photosystem.II.is.actually.where.the.irst.steps.of.pho-
tosynthetic.pathways.in.modern.cyanobacteria,.algae,.and.higher.plants.occur.
20
.Working.
in.conjunction.with.photosystem.I,.several.light-activated.reactions.take.place.to.produce.
factors.necessary.to.motivate.the.“dark”.portions.of.photosynthesis..Such.factors.include.
excitation.of.electrons,.establishment.of.a.proton.gradient,.and.the.reduction.of.NADP+..
Being. the. irst. step. in. photosynthesis,. photosystem. II. is. responsible. for. the. light-driven.
splitting. of. water. as. a. source. of. electrons. and. protons;.as. a. result,. much.work. has. gone.
toward.elucidating.the.natural.mechanisms.responsible.for.eficient.use.of.light.to.decon-
struct.water's.stable.bonds..Similar.to.PS.I,.PS.II.is.a.transmembrane.protein.complex.that.
consists.of.multiple.polypeptide.subunits,.each.responsible.for.a.particular.function..Some.
structures.of.interest.include.the.antenna.assemblies.designated.CP47.and.CP43.and.reac-
tions.centers.D1.and.D2.(see.Figure 22.5).
21
.CP47.and.CP43.are.thought.to.be.involved.in.
the. resonance. energy. adsorption. and. transfer;. as. such,. it. is. these. subunits. that. are. the.
subject.for.possible.application.in.BSSC.devices..X-ray.diffraction.and.electron.microscopic.
characterization.illustrate.that.the.interior.of.CP47.houses.14.chlorophyll.molecules.held.
in. very. speciic. conformations. within. 17.Å. of. each. other,. theoretically. optimizing. them.
for.photon.adsorption.and.transfer..From.these.antennae.subunits,.the.absorbed.energy.
is.transferred.into.the.reaction.center.through.a.noncovalently.bonded.Chl.a.
22
.Subunits.
D1.and.D2.house.these.reaction.centers,.where.the.primary.electron.donor,.a.special.pair.
of. chlorophylls. called. P680,. and. water-cleaving. components. reside.. Here,. charge. sepa-
ration. occurs. when. the. P680. is. excited. by. either. direct. absorption. of. a. photon. or,. more.
likely,.a.transfer.from.antenna.chlorophyll.molecules..The.strongly.oxidizing.P680
+
.has.its.
electrons.replaced.by.a.secondary.electron.donor.identiied.as.a.speciic.Tyr.amino.acid,.
located. on. D1. and. D2. structures,
23
. which. in. turn. replenishes. its. electrons. from. a. 4. Mn.
atom.cluster,.the.ultimate.electron.source.and.site.of.water.oxidation.
