Biomedical Engineering Reference
In-Depth Information
by.forming.a.hydrogen-bonding.network.that.involves.several.water.molecules.and.amino.
acid. functional. groups. in. the. extracellular. region. of. the. bR. membrane.
30
. Nevertheless,.
when. the. crystal. structure. is. disrupted. by. solubilization. into. a. monomeric. or. trimeric.
bR. form. or. some. of. the. amino. acid. residues. from. the. bR. structure,. the. protein. loses. its.
robustness.
Because.of.its.long-term.stability.granted.by.its.ability.to.pack.into.2D.crystalline.phases.
on.surfaces,.bR.has.been.the.predominant.focus.of.biohybrid.electronic.applications..The.
range.of.potential.applications.of.bR.includes.the.construction.of.nanoscale.devices.that.
are.capable.of.continuous,.renewable.conversion.of.light.to.chemical.or.electrical.energy..
Interestingly,. Cahen. and. collaborators
31
. have. recently. shown. that. bR. presents. a. rela-
tively.simple.and.stable.biological.system.to.explore.electronic.transport.with.a.biologi-
cal. material. that. can. be. manipulated. with. all. of. the. tools. known. to. modern. chemistry.
and.biology..Futhermore,.recent.simulations
32
.suggest.that.bR.could.have.a.photoinduced.
metal-insulator.transition.(due.to.conformational.changes).that.is.attractive.for.biosolar.
applications.
Recently,.bR.ilms.deposited.on.metals.or.metal.oxides.were.targeted.for.biooptoelectronic.
applications.
33
.The.construction.of.an.excitonic.solar.cell.based.on.bR.triple.mutant.E9Q/
E194Q/E204Q.(3Glu).as.a.photosensitizer.has.recently.been.demonstrated.
34
.Preliminary.
studies. have. examined. the. photoelectrical. response. of. the. constructed. biocell. (bR/TiO
2
.
ilm).in.response.to.light.illumination..Molecular.dynamic.calculations.have.been.carried.
out.to.describe.the.possible.binding.site(s).of.a.genetically.engineered.bR.mutant.onto.the.
surface.of.TiO
2
.
22.3.5.1 bR Proton Pumping Action and Electron Transfer Mechanism on TiO
2
Surfaces
The.process.of.photoreactive.ATP.synthesis.in.bR.occurs.in.several.steps,.where.the.retinal.
chromophore. plays. the. main. role. in. solar. energy. harvesting. and. conversion.. It. isomer-
izes.upon.light.absorption,.causing.several.further.changes.in.the.protein.structure..This.
feature.has.been.used.in.solar.energy.conversion.device.development,.where.the.retinal.
molecule.is.the.photoactive.part.of.the.bR-TiO
2
-based.(or.other.semiconducting.structure).
system.. The. chromophore. acts. as. a. photosensitizer. that,. upon. photoexcitation,. injects.
electrons.to.the.semiconductor.structure..In.such.systems,.bR.undergoes.its.natural.pho-
tocycle.steps,.but.only.the.irst.step.of.the.photocycle.is.believed.to.be.needed.for.TiO
2.
sensitization.to.occur..Further.bR.transformations.following.the.irst.step.are.believed.to.
be.competing.with.the.electron.transfer.from.the.retinal.to.the.electrode.surface,.slowing.
down.this.process.and.at.the.same.time.impairing.the.eficiency.of.the.electron.injection.
into.the.TiO
2
.surface..The.bR.protein,.however,.plays.the.role.of.a.protecting.scaffold,.just.
like. chlorophyll. molecules;. retinal. loses. its. robustness. once. separated. from. its. natural.
environment.. Gai. et. al.
35.
studied. the. role. of. the. protein. and. retinal. on. the. eficiency. of.
the. photocycle,. verifying. the. time. of. retinal. photoisomerization. in. its. natural. environ-
ment.of.the.bR.protein.vs..the.retinal.extracted.from.the.protein.in.a.methanol.solution..
The.results.suggested.that.the.protein.plays.a.catalytic.role.in.the.photoisomerization.of.
retinal..It.was.found.that.the.eficiency.and.selectivity.of.photoisomerization.of.retinal.
in. methanol. solution. are. very. low.. The. protein. surrounding. the. chromophore,. on. the.
contrary,.has.a.positive.impact.on.the.photoreaction,.ensuring.both.high.eficiency.and.
high. selectivity. of. this. process.. When. the. Schiff. base. is. protonated,. the. retinal. present.
in.bR.selectively.photoisomerizes.to.the.13-
cis
form.with.an.eficiency.of.64%,.and.it.is.
highly.decreased.when.the.chromophore.is.extracted.from.the.protein..It.has.also.been.
reported
36,37
.that.the.overall.isomerization.rate.of.retinal.need.not.be.fast.for.the.eficient.
