Investigations of the electronic-molecular structure of bio-inorganic systems using modern methods of quantum chemistry

A few bio-inorganic systems have been studied by means of modern quantum chemistry (QM) methods, such as the Density Functional Theory (DFT) in connection with a large set of basis functions. At this level, DFT has proven to give reliable (semi-quantitative) description of several properties of transition metals complexes which are either relevant part of a natural enzyme or ad hoc built to mimic the behaviour of enzymes. The first example considered is the N,N'-ethylenebis(acetylacetonatiminato)Co(II) (Coacacen), a very famous synthetic oxygen carrier. We report results of a DFT investigation of Coacacen in its free form as well as in penta-coordinated form with a suitable axial ligand and, finally, in hexa-coordinated form bound to dioxygen. The problems related to the electronic structure of Coacacen derivatives are very old, and have been investigated (mainly experimentally) for several years. Coacacen has been the first bio-inorganic system investigated in the group of Renato Ugo in the 1970s. In addition, a short review will be presented concerning the “modern” problems of the bio-inorganics, as they are presently investigated in our research group based at the University of Milano-Bicocca (Italy): the catalytic cycle characteristic of Fe-Fe hydrogenases and the model compounds describing the interaction between copper and amyloid β-peptides.