Quantitative prediction of electronic absorption spectra of copper(II)-bioligand systems: Validation and applications

The visible region of the electronic absorption spectra of Cu(II) complexes was studied by time-dependent density functional theory (TD-DFT). The performance of twelve functionals in the prediction of absorption maxima (λ_max) was tested on eleven compounds with different geometry, donors and charge. The ranking of the functionals for λ_max was determined in terms of mean absolute percent deviation (MAPD) and standard deviation (SD) and it is as follows: BHandHLYP > M06 ≫ CAM-B3LYP ≫ MPW1PW91 ~ B1LYP ~ BLYP > HSE06 ~ B3LYP > B3P86 ~ ω-B97x-D ≫ TPSSh ≫ M06-2X (MAPD) and BHandHLYP > M06 ~ HSE06 > ω-B97x-D ~ CAM-B3LYP ~ MPW1PW91 > B1LYP ~ B3LYP > B3P86 > BLYP ≫ TPSSh ≫ M06-2X (SD). With BHandHLYP functional the MAPD is 3.1% and SD is 2.3%, while with M06 the MAPD is 3.7% and SD is 3.7%. The protocol validated in the first step of the study was applied to: i) calculate the number of transitions in the spectra and relate them to the geometry of Cu(II) species; ii) determine the coordination of axial water(s); iii) predict the electronic spectra of the systems where Cu(II) is bound to human serum albumin (HSA) and to the regions 94-97 and 108-112 of prion protein (PrP). The results indicate that the proposed computational protocol allows a successful prediction of the electronic spectra of Cu(II) species and to relate an experimental spectrum to a specific structure.