Carbohydrate and Trivalent Iron Ion Interactions in the Gas Phase and in Aqueous Solution - Metallic Systems: A Quantum Chemist's Perspective - page 102

stronger.due.to.the.coordination.of.Fe.to.O3.and.O4..In.addition,.we.ind.that.two.
water.molecules.are.coordinated.to.the.carbohydrate-coordinated.Fe 3+ .ion.with.aver-
age.Fe-water.oxygen.distances.of.2.1.±.0.2.and.2.2.±.0.1.Å..In.contrast,.CMD.simula-
tions.predict.intramolecular.hydrogen-bonds.between.O2.and.O3.and.between.O3.
and.O4.in.the.presence.of.Fe.(Table.3.4)..The.hydroxyl.oxygen.atoms.are.also.coor-
dinated. to. water. molecules. with. average. intermolecular. hydrogen. bond. distances.
varying.between.2.7.and.3.5.Å.
According.to.the.CPMD.simulations,.the.presence.of.the.second.Fe 3+ .ion.does.not.
impact.the.number.of.intra.and.intermolecular.hydrogen.bonds.of.the.carbohydrate..
However,.both.intra.and.intermolecular.hydrogen.bond.distances.are.slightly.larger.
in.comparison.to.those.calculated.for.the.hydrogen.bonds.in.the.vicinity.of.a.single.
Fe 3+ .ion.(Table.3.4)..Instead.of.only.two.water.molecules,.three.water.molecules.are.
coordinated. to. the. carbohydrate-coordinated. Fe 3+ . ion. (Figure. 3.5).. This. arrange-
ment.might.be.due.to.the.larger.distances.between.Fe.and.the.O3.and.O4.atoms.of.
the.carbohydrate.(see.above).since.it.may.allow.electron.donation.from.more.than.
two.water.ligands.as.shown.by.our.NPA.results..The.average.distance.between.the.
second.Fe.and.its.six.coordinated.water.oxygen.atoms.is.2.2.±.0.3.Å.in.accord.with.
our.recent.studies.and.with.diffraction.spectroscopy.measurements.for.hydrated.Fe 3+ .
ion.[47]..The.irst.shell.coordination.number.was.found.to.be.six.in.all.cases.for.the.
second. Fe 3+ .ion,.indicating. that.the.interaction. between. Fe 3+ .and. the.carbohydrate.
does. impact. the. coordination. number. of. water. molecules. reported. for. a. hydrated.
Fe.ion..In.contrast,.CMD.simulations.suggest.that.both.Fe 3+ .ions.are.coordinated.to.
water.molecules.and.not.to.the.carbohydrate.
The. presence. of. water. molecules. affects. the. chemical. reactivity,. coordination.
chemistry,.and.structural.and.thermodynamic.properties.of.biometallic.complexes.
and.calls.for.care.when.one.attempts.to.predict.the.chemical.properties.of.biometal-
lic.complexes.in.solution.from.the.results.of.gas-phase.calculations.
3.5 CONCLUSION
In.this.chapter,.we.have.presented.a.means.of.identifying.sites.of.carbohydrates.reac-
tive. toward. metal. ions. in. the. gas. phase. and. in. aqueous. solution. via. study. of. the.
coordination.chemistry.of.methyl-α-mannopyranoside.and.Fe 3+ .ion..Our.approach.
is.based.on.chemical.reactivity.calculations,.NPA,.and.CPMD.simulations..The.irst.
principles.calculations.and.simulations.show.that.three.hydroxyl.groups.are.coordi-
nated.to.Fe.in.the.gas.phase,.whereas.two.hydroxyl.groups.and.two.water.molecules.
are.coordinated.to.Fe.in.a.square-planar.arrangement.in.aqueous.solution..The.sec-
ond.Fe 3+ .ion.is.coordinated.to.six.water.molecules.in.an.octahedral.arrangement.in.
the.second.solvation.shell.and.does.not.coordinate.to.the.carbohydrate..We.compared.
the. results. obtained. from. CPMD. simulations. to. those. obtained. from. CMD. simu-
lations,. and. we. found. that. the. CMD. simulations. cannot. capture. the. coordination.
chemistry.of.biomolecules.and.transition.metal.ions.in.the.gas.phase.and.in.aqueous.
solution;.both.Fe 3+ .ions.coordinate.to.water.molecules.instead.of.the.carbohydrate.in.
the.CMD.simulations.
Our. theoretical. studies. using. irst. principles. methods. show. that. more. than. one.
hydroxyl.oxygen.atom.of.methyl-α-mannopyranoside.coordinates.to.the.Fe 3+ .ion.in.