Complexation of Al(III) by aromatic amino acids in the gas phase.

The coordination properties of three natural aromatic amino acids (AAAs)-phenylalanine (Phe), tyrosine (Tyr), and tryptophan (Trp)-to AlIII are studied in this work, devoting special attention to the role of the aromatic side chain. A comparison with aluminum(III)-alanine complexes is also presented. The polarizability arising from the ring has been seen to be a key factor in the stability of the complexes, with the order being Trp-AlIII > Tyr-AlIII > Phe-AlIII, starting from the most stable one. Cation-pi interactions between the metal and the aromatic ring are present in the lowest energy conformers, especially for Trp, which seems to be very well suited for these kinds of interactions, occurring with both the six- and five-membered rings of the indole side chain. The most stable coordination mode for the three AAAs is found to be tricoordinated with the N and O of the backbone chain and the aromatic ring, as was found theoretically and experimentally for other metals.

Assessment of Approximate Density Functional Methods for the Study of the Interactions of Al(III) with Aromatic Amino Acids.

Four approximate Density Functional Theory methods, the standard hybrid B3LYP functional, the hybrid mPW1PW91 functional designed to account for van der Waals forces, the one-parameter meta hybrid TPSSh functional, the general-purpose meta hybrid MPWB1K functional and one Molecular Orbital Theory method, the standard Moller-Plesset perturbation theory up to second-order MP2, have been assessed for studying the complexation modes of the highly acidic Al(III) cation with the three aromatic amino acids, phenylalanine (Phe), tyrosine (Tyr), and tryptophan (Trp). Based on their performance toward the prediction of the geometrical structure of a number of lowest energy isomers and their relative binding energies, it is concluded that the B3LYP approximate functional renders the desired accuracy at the minimum computational cost.

The ferrocene--lithium cation complex in the gas phase.

The stable isomers of the ferrocene--lithium cation gas-phase ion complex have been studied with the hybrid density functional theory. The method of calculation chosen has been tested checking its performance for the more studied protonated ferrocene species. Our calculations demonstrate that the procedure used is reliable. We have found two isomers of the ferrocene--lithium cation complex separated by a barrier of 25.6 kcal/mol. The most stable isomer of this complex has Li(+) on-top of one of the cyclopentadienyls, while in the least stable isomer Li(+) binds the central iron metal. The latter isomer has been characterized as a planetary system in the sense that Li(+) has one thermally accessible planar orbit around the central ferrocene moiety. Our calculations lead to a value of ferrocene's gas-phase lithium cation basicity of 37.4 kcal/mol for the on-top complex and 29.4 kcal/mol for the metal-bound complex.