Exploring bonding interactions of biochemical relevance in silicon, platinum(II) and iron(III) positively charged species

Elements playing a biological role that are present in nature or in synthetic drugs, such as silicon, platinum(II) and iron(III) usually appear coordinated to ligands in more or less composite molecular architectures. This notion is particularly true when a metal ion is placed in the active center of an enzyme or otherwise integrated into simple biomolecules and proteins. Whereas multifaceted factors affect a charged (metal) centre in a biological environment, the gas-phase provides an interesting medium for elucidating intrinsic interactions between metal ions and biological targets. The idea underlying this doctoral thesis is to highlight how state of the art techniques combining mass spectrometry, IR spectroscopy and computational chemistry can be applied to the study of ionic complexes in an isolated state. In a first section the reactivity behavior of gaseous complexes from the (CH3)3Si+ addition to acetylene has been fully explored by FT-ICR mass spectrometry and ab initio calculations. In this way the C5H11Si+ potential energy surface has been elucidated and the computational results nicely account for the experimental evidence showing an isomerization process from a primarily formed complex (a β-silyl-substituted vinyl cation acquiring an asymmetric cyclic geometry) to CH2=C(CH3)-Si(CH3)2+ silyl cation. The computational methods tested in dealing with the C5H11Si+ ion problem have been further applied to more challenging systems. In a second and third section a comprehensive investigation of the structural features of the key intermediates which are formed from cisplatin by replacement of chloro ligands by water or methionine is described. Here the experimental approach has involved vibrational spectroscopy carried out with a recently designed and assembled apparatus. The NH/OH stretching region has been found highly structurally diagnostic in the aqua complexes where hydrogen bonding interactions are crucial in determining relative conformer stability. The infrared characterization of the monofunctional adducts of platinum(II) drugs with methionine has led to identify distinct modes of interaction with cisplatin and transplatin derived species. In fact, the preferred ligand atom (S or N) seems to be depending on the specific isomer (cis- or trans-) that is reacting with the metal. Cisplatin and transplatin derived species have been sampled both experimentally and computationally, taking into account relativistic effects in the heavy metal. In a fourth task the binding properties of azole ligands toward ferric heme have been examined. Starting from simple ligands such as pyridine, 1-methylimidazole and 1H-1,2,4-triazole, the focus was then directed to imidazole- and triazole-based antifungal drugs. These drugs are known to act as inhibitors of CYP51enzyme, through binding to the heme prosthetic group. Relative binding energies were determined experimentally by energy variable collision induced dissociation experiments performed on the selected ionic complexes and evaluated theoretically by Car-Parrinello molecular dynamics calculations. To this end, theoretical investigations were carried out during a training period spent at the “Parc Cientific de Barcelona”, under the supervision of Research Professor Carme Rovira. Imidazole-based drugs consistently display higher dissociation energies when compared to triazole-based drugs.