We have studied the interactions between the aromatic ring containing aminoacid chains with the aluminum (III) and magnesium (II) cations. We have characterized several types of complex stable minima on their corresponding potential energy surfaces. Namely, complexes in which the metal cation interacts with the aromatic ring, complexes in which the magnesium interacts with the oxygen of the alcohol group of Tyr, also complexes where the magnesium interacts with the nitrogen of the Trp, and finally complexes where the cations interact with three carbons of Trp (X-c3).
The strongest binding occurs between the aluminum (III) cation and the indole, with a binding
energy of 447 kcal/mol Al-c3. For the magnesium (II) the strongest binding among these complexes was
in the methylated Mg-benzene complex, with a binding energy of 151 kcal/mol. Phe and Tyr
have binding energies of 404 and 418 kcal/mol with aluminum and 133 and 136 kcal/mol with
magnesium. These binding energies
are significantly smaller than the binding energies between these cations and the negatively charged
ligands like COO
or the SH with binding energies around 740 and 380 kcal/mol for aluminum
and magnesium cations respectively. Aluminum (III) and magnesium (II) interactions with neutral
ligands, however, are around 40 kcal/mol for aluminum and around 10 kcal/mol weaker than the interactions with
the Trp chain.
An important difference between the aluminum (III) and magnesium (II) complexes is that the aromatic ligands loose the aromaticity after bind to the aluminum while the magnesium complexes retain it.
The charge transfer between the cations and the ligand has a maximum in the
X-indole complex, where the aluminum cation has a charge of 1.976 e and the magnesium 1.761 e.
This transfer is not as larger as that observed in the sulfur containing aminoacid complexes, but is
significantly larger than the charge transfer found in the acid and acid derivative aminoacids.
The Bader analysis reports covalent bonds between the aluminum and the carbons, and ionic for the magnesium, while the NBO describe the X-C bonds being due to second order interactions. NBO reports similar interactions between all the carbons and the metal cation, while the Bader analysis only reports bonds between certain carbons of the aromatic ring. However, the high ellipticity of these bonds together with the similar value of the bond and ring critical point densities indicate that these bonds may collapse easily into different X-C bonds, in agreement with the NBO picture. The Bader analysis also shows in a very elegant way the loss of aromaticity of the benzene rings upon complexation with the aluminum (III) cation.