Virus-Based Nanobiotechnology - The Nanobiotechnology - page 162

particle.(Anderson.et.al..2006)..At.clinically.relevant.magnetic.ield.strengths,.a.threefold.
increase. in. relaxivity. per. gadolinium. ion. when. conjugated. to. MS2. capsids. was. noted..
Additional.research.compared.the.relaxivities.of.Gd 3+ .ions.when.conjugated.to.either.the.
exterior. or. interior. surface. of. the. capsid,. where. interior. surface. modiication. had. better.
performance.(Hooker.et.al..2007)..The.observed.increase.in.signal.was.later.shown.to.be.
due.to.conjugation.of.the.gadolinium.ions.to.rigid.tyrosine.residues.on.the.capsid.interior.
versus.more.lexible.lysine.residues.on.the.capsid.exterior.(Datta.et.al..2008)..Hyperpolarized.
xenon.chemical.exchange.saturation.transfer.(Hyper.CEST).is.a.technique.with.sensitivities.
of.up.to.10,000-fold.greater.than.traditional.MRI..Modiied.MS2.capsids.have.been.used.
as. carriers. of. Xenon-containing. cryptophane. cages. for. application. in. Hyper. CEST. MRI.
(Meldrum.et.al..2010)..In.this.study,.cysteine.(N87C).residues.were.genetically.introduced.
on.the.capsid.interior.for.attachment.of.up.to.125.cryptophane.molecules.per.MS2.virion..
The.detection.limit.of.xenon-modiied.capsids.was.0.7.pM,.demonstrating.the.advantages.
of. packaging. a. xenon-based. molecular. sensor. inside. a. virus. protein. cage.. Furthermore,.
modiied.MS2.capsids.have.been.investigated.into.their.use.as.carriers.of.radionuclides.for.
use.in.positron.emission.tomography.(PET)..In.one.study,.over.100.copies.of.the.positron.
emitter,. [ 18 F]luorobenzaldehyde. were. conjugated. to. tyrosine. residues. on. the. interior. of.
empty.MS2.capsids.(Hooker.et.al..2008)..Biodistribution.of.positron-emitting.MS2.capsids.
was. monitored. after. intravenous. administration. to. Sprague. Dawley. rats,. and. increased.
blood.circulation.times.compared.to.free.radionuclides.were.observed..Additionally,.the.
interior. of. the. MS2. capsids. could. be. dually. modiied. with. both. 18 F. and. the. luorophore.
coumarin.without.any.deleterious.effect.to.the.positron.emitter.
As.mentioned.earlier,.porphyrins.have.been.conjugated.to.the.interior.of.MS2.capsids.
for. the. purpose. of. site-speciic. PDT.. However,. porphyrins. have. also. been. conjugated.
to. the. exterior. of. MS2. to. capitalize. on. the. photocatalytic. properties. of. these. aromatic.
molecules.. In. one. study,. zinc. porphyrins. were. conjugated. to. an. unnatural. amino.
acid,. p-aminophenylalanine. (pAF),. on. the. exterior. of. the. capsid. (Stephanopoulos. et. al..
2009).. The. pAF. residue. was. genetically. introduced. through. the. use. of. the. amber. codon.
suppression.technique.and.offer.precise.control.by.attaching.only.one.porphyrin.molecule.
per.capsid.monomer.(Xie.and.Schultz.2006)..Moreover,.luorophores.were.also.covalently.
linked.to.cysteine.residues.engineered.into.the.capsid.interior..The.dual.labeling.allowed.
luorescence. resonance. energy. transfer. (FRET). through. the. 2.nm. thick. capsid. wall,.
demonstrated. by. exciting. the. luorophores. conjugated. to. the. interior. and. observing. the.
light. emitted. from. the. zinc. porphyrin. on. the. exterior.. These. results. hold. potential. for.
opening.up.new.techniques.for.diagnostic.imaging.in.addition.to.possible.applications.for.
nanoengineered.MS2.capsids.in.photocatalysis.
7.4.4 Nanoscale Assembly
The.same.methods.used.to.modify.the.MS2.phage.for.use.in.drug.delivery.and.diagnostic.
applications.have.been.retooled.for.other.purposes..Controlling.the.precise.placement.of.
molecules.and.structures.at.the.nanometer.level.is.extremely.challenging,.and.researchers.
continually.rely.on.nature.for.inspiration..The.properties.of.virus.capsids.make.them.model.
contenders.for.structuring/templating.materials.at.the.nanoscale..To.this.end,.MS2.has.been.
engineered.to.form.one-dimensional.nanoarrays.with.good.precision.using.DNA.origami.
tiles. (Stephanopoulos. et. al.. 2010a).. MS2. capsids. were. modiied. on. the. interior. with. the.
luorophore.Oregon.Green,.maleimide-linked.to.genetically.engineered.cysteines.(N87C)..
Short. DNA. sequences. were. conjugated. to. the. MS2. exterior. that. were. complimentary. to.
probe.sequences.extending.from.the.DNA.origami.tiles..Through.this.approach,.the.authors.