Carbohydrate and Trivalent Iron Ion Interactions in the Gas Phase and in Aqueous Solution - Metallic Systems: A Quantum Chemist's Perspective - page 95

preference.of.the.glycosidic.linkage.of.M.were.obtained.via.PMF.calculations.using.
both.simulation.techniques.(see.Figure.3.4A):. g+ > t > g −..Furthermore,.the.differ-
ence.of.the.Gibbs.free.energy.values.calculated.for.the.prediction.of.the.lexibility.of.
the.glycosidic.linkage.with.the.PMF.and.perturbation.methods.varies.between.2%.
and.9%.(Figure.3.4A).
Based.on.the.thermodynamic.calculations,.the.coordination.of.the.Fe 3+ .ion.to.
M. impacts. the. predicted. preference. of. the. glycosidic. linkage. in. the. gas. phase..
Namely,. the. minimum. energy. torsional. angle. shifts. from. 60°. to. 70°,. and. the.
trend.for.the.glycosidic.linkage.lexibility.becomes. g+ > t ≈ g −.(see.Figure.3.4B)..
Interestingly,.the.CMD.simulation.results.are.insensitive.to.the.coordination.of.the.
metal.ion.and.do.not.present.a.change.in.the.predicted.glycosidic.linkage.lexibility.
in. the. vicinity. of. the. metal. ion. (see. Figure. 3.4B).. Overall,. these. calculations. do.
not.present.only.the.structural.properties.regarding.the.coordination.chemistry.of.
M.and.the.Fe 3+ .ion.but.also.the.thermodynamics.obtained.via.CPMD.and.CMD.
simulations.
The. observed. trends. in. the. glycosidic. linkage. lexibility. can. be. described. via.
detailed.analysis.of.the.structural.properties..According.to.the.CPMD.simulations,.
the. average. intramolecular. hydrogen. bond. distances. between. O2. and. O3,. O3. and.
O4,.and.O4.and.O6.atoms.(see.Figure.3.1).vary.between.1.7.and.2.8.Å,.whereas.these.
distances. vary. between. 1.8. and. 3.3. Å. with. the. CMD. simulations. (see. Table. 3.3)..
In.addition,.our.studies.show.intramolecular.hydrogen.bonds.between.O2.and.O3,.
and.between.O4.and.O6.atoms.for.each.trajectory.(see.Table.3.3)..These.structural.
parameters.could.not.be.demonstrated.with.the.CMD.simulations.due.to.the.reasons.
explained.above.(neglecting.quantum.effects,.ixed.partial.charges.on.the.hydroxyl.
groups,.and.not.treating.the.d.orbital.interactions.of.the.metal.ion).
For. the. most. preferred. glycosidic. linkage. conformation. of. M. (+60°),. we. ind.
short-lived. intramolecular. hydrogen. bonds. between. O2. and. O3,. O3. and. O4,. and.
O4.and.O6.atoms.via.CPMD.simulations..On.the.other.hand,.for.the. g −.and. t .ori-
entations.of.the.glycosidic.linkage.of.M,.we.ind.no.intramolecular.hydrogen.bonds.
between.the.O2.and.O3.atoms..This.inding.might.indicate.that.the.intramolecular.
hydrogen.bonds.observed.between.O2.and.O3.for.the. g +.orientation.of.φ.stabilizes.
this.conformation.over.its. t .and. g −.orientations.in.the.gas.phase..For.the. t .orientation,.
the.intramolecular.hydrogen.bond.between.O3.and.O4.is.23%.shorter-lived.in.com-
parison.to.that.computed.for.the. g −.orientation..This.result.might.be.indicative.of.the.
role.of.intramolecular.hydrogen.bonding.between.the.O3.and.O4.atoms.in.stabilizing.
the. g −.orientation.of.the.glycosidic.linkage.over.its. t .orientation.
In. the. vicinity. of. a. single. Fe 3+ . ion,. the. intramolecular. hydrogen. bond. between.
O2.and.O3.atoms.reported.for.M.(see.above).diminished.according.to.our.CPMD.
simulations..Short-lived.intramolecular.hydrogen.bonds.were.observed.between.O4.
and.O6.atoms..The.average.number.of.intramolecular.hydrogen.bonds.remained.low..
For. the. g +. orientation. of. φ,. which. is. coordinated. to. a. single. Fe 3+ . ion,. we. observe.
intramolecular. hydrogen. bonds. between. O2. and. O3,. and. O4. and. O6. atoms.. The.
intramolecular.hydrogen.bond.observed.for.O2.and.O3.atoms.diminished.for.the. t .
and. g −.orientations.of.φ..This.inding.indicates.that.the.hydrogen.bond.between.O2.
and.O3.atoms.stabilizes.the. g +.orientation.of.the.glycosidic.linkage.over.its. g −.and.
t .orientations.