Comparative assessment of theoretical methods for the determination of geometrical properties in biological zinc complexes

In the present study, we have compared the performance of the density functional theory (DFT) functionals B1B95, B3LYP, B97-2, BP86, and BPW91 with MP2 for geometry determination in biological mononuclear Zn complexes. A total of 15 different basis sets, of rather diverse complexity, were tested, several which included also three different types of common effective-core potentials: Los Alamos, Steven-Basch-Krauss, and Stuttgart-Dresden. In addition, the ability to describe mononuclear Zn biological systems using relatively simple models of the metal coordination sphere, comprising only the metal atom and a simplified representation of the ligands at the first coordination sphere, starting from a set of high-resolution X-ray crystallographic structures, is evaluated for 90 combinations of method/basis set. The results show that the use of such models allows for a relatively accurate description of the Zn-ligand bond lengths, although failing to correctly represent the topology of the metal coordination sphere (namely, the angles involving the metal atom) if constraints at the C alpha atoms are not considered. Globally, B3LYP had the best average performance in the test, closely followed by MP2, whereas B1B95 was the least accurate method. The study also points out B3LYP/CEP-121G and B3LYP/SDD, which use, respectively, the Steven-Basch-Krauss and the Stuttgart-Dresden effective-core potentials, as the best compromise between accuracy and CPU time for the geometrical characterization of metal-ligand bond lengths in Zn biological systems.