Solvation of Al-Guanine Complexes with NH 3 : A Theoretical Study Marco-Vinicio Va ´ zquez, Anastassiia Moussatova, and Ana Martı ´nez* Instituto de InVestigaciones en Materiales, UNAM. Circuito Exterior s/n, Ciudad UniVersitaria, 04510, Coyoaca ´ n, Me ´ xico D.F., Me ´ xico O. Dolgounitcheva, V. G. Zakrzewski, and J. V. Ortiz Department of Chemistry, Kansas State UniVersity, Manhattan, Kansas 66506-3701 ReceiVed: March 18, 2004; In Final Form: May 4, 2004 Geometry optimizations on complexes composed of an Al atom, a guanine molecule, and an ammonia molecule have been performed with density functional methods. In the most stable structures, the ammonia molecule forms hydrogen bonds with previously studied Al-guanine complexes. The two lowest structures correspond to an unusual tautomer of guanine in which both N atoms of the five-membered ring, N7 and N9, are protonated. Within 3 kcal/mol in energy lie two additional structures in which a proton is shifted from N9 to N3. Ionization energies calculated with ab initio, electron-propagator methods for the two latter structures are in close agreement with the experimentally observed ionization threshold. Higher ionization energies are obtained for the two lowest structures. Dyson orbitals for the lowest ionization energies are guanine π* functions. The order of isomers in the cationic species is different from that of the neutrals. Energies of ammonia dissociation are approximately equal for all forms of the Al-guanine-NH 3 complex, except for a structure with an Al- NH 3 dative bond. Introduction Interactions between metal ions and DNA have far-reaching biological consequences. 1-4 Metal-base complexes may have tautomerization energies and structures that differ from those of the Watson-Crick bases. Metal complexation therefore has the potential to disrupt the replication of genetic material. Information on the energetics of metal-ion coordination to base N atoms and on the effects of these interactions on acid-base and keto-enol equilibria may be useful in understanding the origins of a variety of reproductive pathologies. Electronic properties of base pairs and stacks also may be affected by metal complexation. These effects may influence the structural and electronic properties of molecular electronic devices constructed with DNA fragments. 5,6 Complexed metal atoms may function as electron acceptors or donors and have been successfully used to study charge transport through strands of DNA. Solvents also have important structural and energetic effects on the bonding capabilities of DNA bases. With the advent of improved techniques for the synthesis and isolation of inter- molecular complexes with specified numbers of solvent mol- ecules, spectroscopic studies may now provide detailed infor- mation on individual binding energies. Competition for solvent molecule coordination between bases and metal ions may be examined through synthesis and characterization of metal- base-solvent assemblies. These developments invite computational investigation of corresponding structures, for tautomerism, metal-ligand coor- dination isomerism, and hydrogen bonding imply an abundance of alternative geometries. Predictions on spectroscopic properties may provide corroboration of calculated structures. After testing computational methodologies against experimental data to ensure reliability, well-calibrated methods may be applied to species that lie beyond the capabilities of current experimental techniques. A study of gas-phase Al-guanine-(NH 3 ) n (n ) 0, 1, and 2) complexes prepared with laser ablation and characterized by photoionization spectroscopy and mass spectrometry has ap- peared recently. 7 Photoionization efficiency spectra were col- lected and used to determine ionization energies of the gas- phase Al-guanine complex. Introduction of ammonia into the He carrier gas resulted in a shift of the onset of ion signal from 5.6 ( 0.1 eV to 4.65 ( 0.08 eV for the Al-guanine (n ) 0) complex. Two explanations were proposed for this effect. First, association of ammonia molecules might produce a species in an electronically excited or geometrically distorted form. Loss of NH 3 during a photoexcitation-ionization process then would produce a species with a significantly lower value for the ionization energy. In an alternative view, an ammonia molecule might facilitate the formation of an isomer of the Al-guanine complex with a shifted ionization potential. Preliminary calculations 7 and a subsequent, more complete study 8 employing density functional and correlated ab initio methods were compatible with the second explanation. After optimization of several structures and prediction of their vertical ionization energies, good agreement with the higher threshold value was obtained for a complex where an Al atom is coordinated to an unusual guanine tautomer in which both nitrogens of the five-membered ring (N7 and N9) are protonated. (In contrast, the Watson-Crick tautomer is not protonated at N7; see Figure 1.) This unprecedented tautomer is related to another form by a proton transfer from N9 to N3. Coordination of an Al atom to the tautomer with protonated N3 produces a complex whose predicted ionization energy is close to the lower threshold. Calculations also indicated that the species with the higher ionization energy is slightly more stable. In addition, a 5845 J. Phys. Chem. A 2004, 108, 5845-5850 10.1021/jp048778k CCC: $27.50 © 2004 American Chemical Society Published on Web 06/15/2004