Biomedical Engineering Reference
In-Depth Information
Among. bacterial. photosynthetic. reaction. centers,. the. RC. of. the. purple. bacterium.
Rhodobacter sphaeroides
.has.been.known.to.be.the.simplest.of.structures..It.exists.in.anaero-
bic.environments.in.deeper.layers.of.lakes,.and.it.consists.of.three.protein.subunits:
•. L,.through.which.electron.transport.takes.place.
•. M.and.H.coordinating.six.pigment.molecules,.a.bacteriochlorophyll.dimer.known.
as.the.special.pair.
•. P,.two.monomer.bacteriochlorophylls.(BL.and.BM),.two.bacteriopheophytins.(HL.
and.HM),.and.two.quinines.(Qa.and.Qb)..The.quinone.Qb.acts.as.the.main.elec-
tron.acceptor.in.the.complex.
12
Due. to. its. much. simpler. structural. arrangement,. compared. to. plant. photosystems,.
the. bacterial. RC. can. be. easily. isolated. from. its. natural. culture,. sustaining. its. stability.
against.denaturation..This.feature.makes.bacterial.reaction.centers.an.attractive.compo-
nent. for. BSSC. applications.. However,. manipulation. of. orientation. and. conformation. of.
the. photosynthetic. complexes. at. the. surface. of. electrodes. has. been. a. signiicant. chal-
lenge. for. obtaining. an. optimum. speed. of. electron. transport. across. the. device.
13
. It. has.
been. observed. that. immobilization. of. the. light-harvesting. complex. of. purple. bacteria.
on.a.solid.semiconducting.surface.(i.e.,.TiO
2
).results.in.an.impaired.effectiveness.of.the.
protein.as.a.photosensitizer..One.proposed.explanation.is.that.the.biomolecules.undergo.
interfacial.interaction-induced.structural.deformation.of.their.light-harvesting.complex..
The. possible. forces. responsible. for. this. deformation. are. discussed. in. Section. 22.5.. The.
photosynthetic. reaction. center. of. purple. bacteria. has. been. studied. as. a. light. harvester.
by.Lu's.research.group.
13
.The.RC.immobilization.effectiveness.was.measured.for.various.
morphologies.of.the.TiO
2
-WO
3
.surface..It.was.found.that.the.most.eficient.RC.immobi-
lization. was. achieved. when. a. three-dimensional. (3D). worm-like. pore. structure. of. the.
semiconductor.was.used.
22.3.3 Photosystem I (PS I)
Photosystem.I.is.an.eficient.natural.light-harvesting.device..It.consists.of.various.protein.
subunits,.including.antenna.and.reaction.proteins.to.absorb.photons.of.a.wide-spectrum.
range.and.transfer.that.energy.to.reaction.centers,.ultimately.to.reduce.NADP+.(nicoti-
namide.adenine.dinucleotide.phosphate).into.NADPH,.which.is.used.in.the.“dark.reac-
tions”.of.phothosynthesis..PS.I.absorbs.light.in.the.spectral.range.between.700.and.645.
nm.
14,15
. It. consists. of. an. absorption. band. at. the. red. spectrum. (~715. nm),. which. is. the.
unique.feature.of.PS.I..Its.reaction.center.itself.is.complex,.comprising.up.to.15.subunits,.
among.which.3.subunits.are.found.only.in.higher.plants.(PS.I-G,.PS.I-H,.and.PS.I-N).
11
.
This.complexity.yields.some.unique.challenges.to.utilizing.multiunit.structures.as.bio-
sensitizers.in.BSSC.applications,.namely,.how.to.isolate.only.the.subunits.necessary.for.
photon. absorption. and. charge. carrier. generation,. as. well. as. those. subunits. responsible.
for.overall.stabilization.
PS.I.consists.of.light-harvesting.antennae.whose.diverse.structure.ensures.very.eficient.
use. of. illuminating. light. under. various. environmental. conditions. with. sustained. light-
harvesting. functionality. over. a. long. period. of. time.
16,17
. The. PS. I. core. antenna. complex.
consists.of.2.protein.subunits.binding.a.number.of.cofactors,.including.96.chlorophyll.a
molecules,.22.B-carotenes,.4.lipids,.3.iron-sulfur.clusters,.and.2.phylloquinones.
17,18
.Unlike.
other. photosynthetic. systems,. whose. structure. involves. distinct. complexes. of. antennae.
