Carbohydrate and Trivalent Iron Ion Interactions in the Gas Phase and in Aqueous Solution - Metallic Systems: A Quantum Chemist's Perspective - page 99

symmetrically.located.between.the.O3.and.O4.atoms.with.an.angle.of.54.4.±.12°..In.
the.presence.of.the.second.Fe 3+ .ion,.this.angle.becomes.44.2.±.9.7°.and.the.irst.Fe 3+ .
ion.attached.to.M.is.coordinated.to.three.water.molecules.instead.of.two..The.second.
Fe 3+ .ion.prefers.to.be.arranged.with.six.water.molecules.in.an.octahedral.symmetry.
in.the.second.solvation.shell..The.closest.distance.between.the.irst.and.second.Fe 3+ .
ions.is.10.2.Å..We.also.note.that.the.intermolecular.hydrogen.bond.distance.between.
O2. and. the. coordinated. water. molecule. increases. to. 3.25.±.0.24. Å. in. the. presence.
of.the.second.Fe 3+ .ion..In.agreement.with.our.studies.in.the.gas.phase,.we.ind.that.
the.presence.of.a.second.Fe 3+ .ion.increases.the.distances.between.the.irst.Fe 3+ .ion.
and.O3.and.O4.atoms.of.the.carbohydrate..Coordination.of.the.second.Fe 3+ .ion.to.
the.biomolecule.in.the.gas.phase.is.possible,.whereas.the.inclusion.of.explicit.solvent.
molecules.shows.that.the.presence.of.water.impacts.the.coordination.chemistry.since.
the.second.Fe 3+ .ion.coordinates.to.water.molecules.rather.than.to.the.carbohydrate.
(see.above).
Our.NPA.analysis.(including.all.solute.and.solvent.molecules).indicates.that.the.
partial.positive.charge.on.the.irst.Fe 3+ .ion.increases.from.0.7.to.1.4.in.the.presence.
of.the.second.Fe 3+ .ion.(Table.3.2)..This.inding.is.consistent.with.our.simulations.pre-
sented.above.since.the.distances.of.the.coordinated.hydroxyl.group.oxygen.atoms.of.
M.to.the.irst.Fe 3+ .ion.are.larger.when.a.second.Fe 3+ .ion.is.present.in.the.system,.indi-
cating.less.electron.donation.capability.for.the.carbohydrate-coordinated.Fe 3+ .ion.
The.present.irst.principles.results.are.in.agreement.with.the.experiments,.which.
show. only. one. Fe 3+ . ion. coordinated. to. the. carbohydrate. in. solution.. In. contrast. to.
the.experiment.and.ab.initio.simulations,.our.CMD.simulations.show.that.both.Fe 3+ .
ions. form. complexes. only. with. water. molecules. and. do. not. coordinate. to. the. car-
bohydrate.. These. discrepancies. can. be. attributed. to. the. quality. of. the. force. ield.
parameters.for.the.transition.metal.ion,.usage.of.ixed.partial.charge.parameters.for.
the.different.hydroxyl.oxygen.atoms.in.the.carbohydrate.(neglecting.charge.polariza-
tion),.failure.to.distinguish.between.equatorial.and.axial.orientations.of.the.carbohy-
drate.hydroxyl.groups,.and.ignoring.the.conformation.dependence.of.steric.effects.
The.relative.torsional.energies.of.the.glycosidic.linkage.(φ).of.M.(see.above).were.
studied.using.CPMD.and.CMD.simulations.in.water..Changes.arising.in.PMF.as.a.
function.of.torsional.angle.derived.from.these.calculations.are.presented.in.Figure.
3.6..Figure.3.6A.shows.the.calculated.PMF.values.of.the.glycosidic.linkage.for.M,.
M-Fe 3+ ,.and.based.on.CPMD.simulations.(see.Section.3.2)..Different.trends.for.the.
conformational. preference. of. the. glycosidic. linkage. of. M. are. observed. in. CPMD.
simulations:. the. g+ > t > g −. trend. observed.in. the. free. M. becomes. g+ > g− > t .
and.the.minimum.shifts.from.60°.to.70°.upon.coordination.of.one.Fe 3+ .ion.to.M.in.
water..The.presence.of.the.second.Fe 3+ .ion.does.not.inluence.the.calculated.thermo-
dynamic.trend.for.the.glycosidic.linkage.conformation.in.the.vicinity.of.a.single.Fe 3+ .
ion,.consistent.with.the.fact.that.a.second.Fe 3+ .ion.does.not.coordinate.to.the.M-Fe 3+ .
complex.in.water.
According.to.our.CMD.simulations,.the.trend.obtained.for.the.glycosidic.linkage.
conformation. g+ > g− > t .for.the.free.carbohydrate.is.retained.in.aqueous.solution.
in.the.presence.of.the.Fe 3+ .ions,.which.are.coordinated.to.water.molecules.rather.than.
to.the.carbohydrate..Furthermore,.in.stark.contrast.to.the.CPMD.results,.our.CMD.
simulations.do.not.indicate.a.signiicant.shift.of.the.minimum.energy.torsional.angle.