178
L. Mammino
polarity increases, as generally noted for ACPLs [26]. The free energy of solvation
(G solv ) is negative in water, with small magnitudes, and positive in the other two
solvents, with greater values in acetonitrile with respect to chloroform.
The relative energies of the complexes show similar trends: the energy of a given
complex decreases as the medium polarity increases, with greater decreases for water
solution. The energy decreases are not regular, leading to different relative-energy
sequences for some complexes, above all in water solution. The relative energy ranges
(fig. S11) show clear tendency to lower values in water.
The natural charge on the ion increases slightly as the medium polarity increases,
but it always remains less than 1 a.u., except for the high energy d*-β-δ-e-Cu-O18O10-π1 complex; the ranges of its values depend significantly on the ion binding site.
The spin density on Cu may increase or decrease slightly with the medium polarity;
the values corresponding to the O25˛O30˛O12 binding site are considerably greater
than the others. The dipole moment of the complexes increases as the medium polarity
increases, and the ranges of its values depend on the conformer type and on the ion
binding site.
Differently from the uncomplexed conformers, G solv is negative in all the solvents, with large magnitudes. The magnitude is smaller in chloroform, greater in
acetonitrile and greatest in water; it dominantly depends on the solvent, whereas
the dependence on the ion binding site is comparatively minor. The difference with
respect to the uncomplexed conformers is due to the fact that the entire molecule is an
ion with a positive charge (≈+1), and the Cu ion attached to it also has a ≈+1 charge;
the charge greatly increases the strength of the interactions between the solute and
the solvent.
4 Discussion and Conclusions
The study of 58 complexes of FNGB, with different geometries of the molecule and
different binding sites of the ion, shows that the ion is effectively reduced in all the
complexes and in all the media considered. This is consistent with the experimentallyproven antioxidant activity of FNGB. The obtained detailed information about the
energetics and properties of the complexes highlights considerable dependence on
the binding sites of the ion.
FNGB is the fourth antioxidant ACPLs whose complexes with a Cu
2+ ion have
been studied extensively within the ongoing research to which the current work
pertains—the other ones being hyperjovinol A (HPJA [6]), arzanol (ARZ [7]) and
hyperguinones A and B [8] (HPGB being taken as representative of the two hyperguinones for the comparisons in this section). The structures of these molecules differ
greatly. R
is a geranyl-type chain with an additional OH in HPJA, a substituted pyranoid ring linked to the phloroglucinol moiety through a methylene bridge in ARZ, a
substituted pyranoid ring fused to the benzene ring at the C3–C4 bond in HPGB, and
a substituted furanoid ring fused to the benzene ring at the C3–C4 bond in FNGB.
ARZ, HPGB and FNGB also have a prenyl chain at C5. In terms of the OH groups
L. Mammino
polarity increases, as generally noted for ACPLs [26]. The free energy of solvation
(G solv ) is negative in water, with small magnitudes, and positive in the other two
solvents, with greater values in acetonitrile with respect to chloroform.
The relative energies of the complexes show similar trends: the energy of a given
complex decreases as the medium polarity increases, with greater decreases for water
solution. The energy decreases are not regular, leading to different relative-energy
sequences for some complexes, above all in water solution. The relative energy ranges
(fig. S11) show clear tendency to lower values in water.
The natural charge on the ion increases slightly as the medium polarity increases,
but it always remains less than 1 a.u., except for the high energy d*-β-δ-e-Cu-O18O10-π1 complex; the ranges of its values depend significantly on the ion binding site.
The spin density on Cu may increase or decrease slightly with the medium polarity;
the values corresponding to the O25˛O30˛O12 binding site are considerably greater
than the others. The dipole moment of the complexes increases as the medium polarity
increases, and the ranges of its values depend on the conformer type and on the ion
binding site.
Differently from the uncomplexed conformers, G solv is negative in all the solvents, with large magnitudes. The magnitude is smaller in chloroform, greater in
acetonitrile and greatest in water; it dominantly depends on the solvent, whereas
the dependence on the ion binding site is comparatively minor. The difference with
respect to the uncomplexed conformers is due to the fact that the entire molecule is an
ion with a positive charge (≈+1), and the Cu ion attached to it also has a ≈+1 charge;
the charge greatly increases the strength of the interactions between the solute and
the solvent.
4 Discussion and Conclusions
The study of 58 complexes of FNGB, with different geometries of the molecule and
different binding sites of the ion, shows that the ion is effectively reduced in all the
complexes and in all the media considered. This is consistent with the experimentallyproven antioxidant activity of FNGB. The obtained detailed information about the
energetics and properties of the complexes highlights considerable dependence on
the binding sites of the ion.
FNGB is the fourth antioxidant ACPLs whose complexes with a Cu
2+ ion have
been studied extensively within the ongoing research to which the current work
pertains—the other ones being hyperjovinol A (HPJA [6]), arzanol (ARZ [7]) and
hyperguinones A and B [8] (HPGB being taken as representative of the two hyperguinones for the comparisons in this section). The structures of these molecules differ
greatly. R
is a geranyl-type chain with an additional OH in HPJA, a substituted pyranoid ring linked to the phloroglucinol moiety through a methylene bridge in ARZ, a
substituted pyranoid ring fused to the benzene ring at the C3–C4 bond in HPGB, and
a substituted furanoid ring fused to the benzene ring at the C3–C4 bond in FNGB.
ARZ, HPGB and FNGB also have a prenyl chain at C5. In terms of the OH groups