Biomedical Engineering Reference
In-Depth Information
22.3 PhotoactiveProteinSystems
Considering.the.wide.variety.of.living.systems.in.existence,.different.proteins.responsible.
for.photoactivity.are.surprisingly.uniform.from.species.to.species..In.this.section.we.will.
give.a.brief.overview.of.common.antenna.proteins.photosystems.I.and.II,.major.motivators.
in.photosynthesis.of.almost.all.plant.life..We.will.also.describe.the.structure.and.use.of.
bacteriorhodopsin,.a.photoactive.protein.found.in.bacteria.that.exhibits.remarkable.struc-
tural.integrity.
22.3.1 Antenna Proteins
Many. photoactive. protein. systems. consist. of. several. proteins. bound. together,. each. per-
forming. speciic. tasks. associated. with. light. absorption,. charge. transport,. stabilization,.
and.chemical.reactions..Antenna.proteins.are.typically.associated.with.the.initial.stages.
of.photosynthesis.and.are.assigned.the.task.of.collecting.solar.radiation;.as.such,.they.are.
of.particular.interest.for.BSSC.applications.for.which.they.exhibit.several.desirable.traits..
First,. they. can. harvest. photons. at. low. intensities. of. illumination. by. eficient. funneling.
of the.delocalized.states..Second,.during.high.light.intensities.and.excited.state.formation,.
the. antenna. proteins. may. dissipate. the. superluous. energy,. protecting. the. system. from.
thermal.deterioration.
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.Light-harvesting.antenna.systems.are.based.on.chromophore-pro-
tein.complexes,.where.the.protein.acts.as.a.scaffold.responsible.for.the.orientation.of.the.
chromophore.molecule..The.chromophore.orientation.is.responsible.for.both.light.adsorp-
tion.and.excitonic.transfer,.providing.an.eficient.transport.(~100%.of.illuminating.photon.
energy. is. trapped). of. the. antenna's. excitation. energy. toward. the. lower-energy. reaction.
center.(RC),
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.where.the.electron.potential.is.used.to.motivate.biochemical.reactions.
The. sophisticated. antenna's. structure. provides. absorption. among. broad-spectrum.
regions.and.increases.the.effective.molecular.absorptivity..This.is.one.advantage.that.these.
complex.systems.have.over.conventional.silicon-based.solar.cells,.which.are.most.active.
under. a. narrow. range. of. photon. energies.. The. antenna's. chromophores. are. carotenoids.
and. chlorophylls.. The. main. function. of. the. former. is. photoprotection. against. harmful.
excessive. energy,. and. to. provide. structural. stabilization. of. the. antenna.. Second,. carote-
noids.also.act.as.light.harvesters.
A.classical.example.of.naturally.occurring. light-harvesting.antenna.systems.is.purple.
bacteria,. whose. pigments. (bacteriochlorophylls. and. carotenoids). can. harvest. the. light.
energy.in.the.near-infrared.(NIR).and.green.regions.of.the.solar.spectrum.
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.In.most.cases,.
it.contains.two.types.of.light-harvesting.complexes,.LH1.and.LH2..The.irst.is.a.part.of.the.
RC-LH1.core.complex.situated.in.the.inner.side.of.the.photosynthetic.units..In.comparison,.
the.LH2.complexes.are.situated.around.the.LH1-RC.core.complex.(see.Figure 22.6).
Each. type. is. associated. with. speciic. bacteriochlorophyll. molecules. absorbing. differ-
ent. light. intensities.. The. bacterial. chlorophyll. associated. with. LH2. absorbs. light. with. a.
shorter.wavelength.vs..the.chromophores.associated.with.the.LH1..This.structure.allows.
the.absorbed.energy.by.LH2.to.be.funneled.to.the.LH1,.and.inally.to.the.RC..LH2.con-
sists.of.two.rings.of.bacterial.chlorophyll.molecules:.B800.with.an.absorption.maximum.
at.800.nm,.and.B850.absorbing.light.at.850.nm.
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.These.complex.energy.transfer.schemes.
are.commonly.seen.in.photoactive.structures.and.are.responsible.for.the.eficient.energy.
transfers.required.to.move.excitons.from.antenna.structures.to.reaction.centers..The.main.
