Biomedical Engineering Reference
In-Depth Information
(a)
(b)
FIGURE 22.8
(a). Structure. of. PS. II. as. viewed. from. along. the. membrane. plane.. Antenna. units. CP43. and. CP47. are. situated.
on.either.side.of.the.D1.and.D2.reaction.centers..(b).PS.II.as.seen.perpendicular.to.the.membrane.plane..P680.
reaction.center.chromophores.can.be.viewed.situated.within.the.D1.and.D2.structures..(From.Shinkarev,.V..P.,.
Biophys. J.
,.85,.435,.2003.)
This. water-splitting. mechanism,. more. commonly. referred. to. as. the. oxygen. evolution.
cycle,.has.been.the.subject.of.intense.study..While.the.precise.structure.of.the.Mn.cluster.
is.still.not.known,.several.theories.describing.the.overall.pathways.have.been.proposed.
to.explain.how.the.structure.creates.enough.potential.to.overcome.the.chemical.bonds.of.
water..In.1970,.Kok.and.colleagues
25,26
.hypothesized.that.with.every.photonic.excitation,.
the.Mn.clusters.donate.electrons.toward.the.P680+.cluster,.and.there.is.a.succession.of.oxi-
dizing.equivalents.that.accumulate.on.the.metal.atoms..Only.when.the.Mn.ions.reach.a.+4.
oxidative.state.do.they.extract.four.electrons.from.two.water.molecules,.producing.O
2
,.4H
+
,.
and.returning.to.their.original.state.to.start.the.cycle.again..This.makes.PS.II.of.interest.
in.biohybrid.devices.for.various.reasons..Like.the.PS.I.system,.it.houses.the.intricate.array.
of. antennae. choromophores. capable. of. capturing. a. wide. range. of. the. visible. spectrum..
Moreover,.its.ability.to.directly.cleave.water.is.an.attractive.feature:.hydrogen.production.
directly.from.solar.energy.
22.3.5 Bacteriorhodopsin (bR)
Chlorophyll.and.its.derivatives,.such.as.photosystems.I.and.II,.are.sensitive.to.extreme.ther-
mal.conditions.once.extracted.from.their.source,.an.in.vitro.environment..Alternatively,.bacte-
riorhodopsin.(bR),.a.natural.light-activated.protein,.found.in.
Halobacterium salinarum,
is.more.
robust,.holding.much.promise.for.solar.energy.conversion
27.
with.high.quantum.eficiency.
Upon.absorption.of.visible.light,.the.bR.protein.undergoes.a.photocycle.during.which.
protons.are.pumped.from.the.cytoplasmic.side.of.the.membrane.to.the.extracellular.side,.
producing. a. proton. gradient.. This. proton. gradient. leads. to. the. formation. of. adenosine.
triphosphate.(ATP).molecules..Due.to.its.natural.harsh.environment.in.salt.marshes,.bR.
has.became.a.robust.protein.with.a.photosensitivity.to.illumination.that.is.higher.than.that.
of.synthetic.photochromic.materials..In.the.dry.stage,.bR.occurs.in.a.2D.crystal.structure,.
providing.exceptionally.high.stability.toward.chemical.and.thermal.degradation.
28
.bR.is.
very. stable. at. high. salinities. (up. to. 5. M. NaCl). and. high. temperature,. even. up. to. 140°C..
Furthermore,. bR. sustains. its. functionality. in. a. broad. range. of. pH. values. (5-11).
29
. These.
highly.stable.properties.of.bR.are.developed.by.acquiring.its.speciic.structural.features.
