Carbohydrate and Trivalent Iron Ion Interactions in the Gas Phase and in Aqueous Solution - Metallic Systems: A Quantum Chemist's Perspective - page 101

of. the. O2. atom. to. a. water. molecule. (as. compared. to. the. O3-water. and. O4-water.
hydrogen.bonds).via.the.shorter.O2-water.distance.(see.Table.3.4)..This.inding.is.
in.agreement.with.our.chemical.reactivity.calculations,.which.identify.the.O2.atom.
as.the.most.reactive.carbohydrate.hydroxyl.oxygen.toward.ligands..CPMD.simula-
tions.show.that.the.water.ligand.tends.to.coordinate.to.O2.rather.than.to.O3.or.O4..
CMD.simulations.do.not.show.a.signiicant.difference.in.intermolecular.hydrogen.
bond.distances.for.these.hydroxyl.oxygen.atoms.in.water..In.addition,.a.comparison.
with. the. results. obtained. for. the. gas. phase. indicates. that. intermolecular. hydrogen.
bonds.between.the.carbohydrate.and.solvent.molecules.increases.the.intramolecular.
hydrogen.bond.distances.O2-O3,.O3-O4,.and.O4-O6.by.values.varying.between.
0.4.and.1.7.Å.
Figure.3.7.shows.the.average.number.of.hydrogen.bonds.between.the.carbohydrate.
and. surrounding. water. molecules. obtained. from. three. distinct. CPMD. simulations.
for.the. g +,. g −,.and. t .orientations.of.the.glycosidic.linkage.(each.simulation.was.per-
formed.for.60.ps)..For.these.analyses,.following.the.work.of.Molteni.and.Parrinello.
[48].and.our.previous.studies,.the.criteria.for.the.existence.of.a.hydrogen.bond.were.
deined.as.O D -H.≤.1.5.Å.and.H-O A .≤.2.4.Å,.where.O D .and.O A .represent.donor.and.
acceptor. oxygen. atoms,. respectively.. The. angular. criterion. was. values. larger. than.
120°..According.to.this.analysis,.the.total.number.of.intermolecular.hydrogen.bonds.
is.highest.for.the. g +.conformation,.while.the. t .conformation.exhibits.a.slightly.larger.
number.than.the. g −.orientation.
CPMD.simulations.and.a.comparison.with.our.results.obtained.from.the.simu-
lations.in.the.gas.phase.indicate.that.the.presence.of.a.single.Fe 3+ .ion.weakens.the.
intramolecular.hydrogen.bonds.between.the.O3.and.O4.atoms.since.these.hydroxyl.
oxygens.are.coordinated.to.the.metal.ion.(see.Table.3.4)..The.average.O4-O6.intra-
molecular.hydrogen.bond.distance.becomes.shorter.in.comparison.to.those.obtained.
for.the.aqueous.carbohydrate.without.a.metal.ion.(Table.3.4)..This.result.indicates.
that. the. intramolecular. hydrogen. bonds. between. these. hydroxyl. groups. become.

3

2.5

2

1.5

1

0.5

0

01 02 03 04 05 06

FIGURE 3.7 Average. number. of. hydrogen. bonds. between. speciic. methyl-α-
mannopyranoside.oxygens.and.water.molecules;.for.the.(—). g +,.(---). t ,.and.(…). g −.orienta-
tions.of.the.glycosidic.linkage.of.methyl-α-mannopyranoside.via.CPMD.simulations.