Optimal density-functional theory method for zinc-amino acid complexes determined by analyzing structural and Fourier-transform infrared spectroscopy data

Metal-amino acid complexes are important compounds for the human body. Their nutritional value and anticancer, antibacterial, and catalytic properties are the focus of several studies. Density functional theory (DFT) can be used to predict their properties by optimizing their structures and performing electron population analyses. However, conventional computational methods cannot adequately determine the parameters of polymeric metal-amino acid complexes. Therefore, intermolecular interactions of polymers must be considered to correctly predict the properties of metal-amino acid and related metal complexes. In this study, different DFT protocols were used to acquire the infrared spectra and determine interatomic distances of two zinc-amino acid complexes, Zn(Gly)₂ and Zn(Met)₂. The results were compared to spectroscopic and X-ray crystallographic data, revealing that the M06 and M06-L functionals and the 6-311++G(d,p) basis set produced the smallest computational errors. Our results provide a foundation for future theoretical studies on other metal-amino acid and metal-organic complexes.