A combined experimental and modelling investigations on mixed bioligand complexes of divalent cobalt and copper in β-cyclodextrin

The biologically active divalent metal complexes [Co(II)(GluArg)(H2O)(2)] (1) and [Cu(II)(GluArg)] (2) (where Co = cobalt and Cu = copper) of mixed glutamic (Glu) and l-arginine (Arg) amino acids form a stable 1:1 inclusion complex with beta-cyclodextrin (beta-CD) in water. The preferred orientation of guest molecules into the host is simulated by quantum chemical computations. Geometry-optimized results using ONIOM technique identified the structure and show that the glutamic acid chelate moiety of both metal ions complexes is encapsulated within the beta-CD cavity while the l-arginine chelate is partially located outside the cavity. The calculated inclusion binding energy (Delta E-B, kJ mol(-1)) and other thermodynamic parameters reveal the noticeable thermal stability of 2-(beta-CD) over the 1-(beta-CD) capsulate. A reverse trend is experimentally found from the determined association constant values (K, m(-1)). The chemical reactivity and reactive site selectivity of these complexes were elucidated via conceptual density functional theory and an electrostatic potential surface map. Global electronic molecular descriptors [such as chemical potential (mu), hardness (eta) and electrophilicity (omega)] validated the experimental findings. The reactivity of the encapsulated complexes is altered vis-a-vis the free complexes. The obtained results and contradictions are discussed and rationalized.