Computational Spectroscopy Studies of Model Ternary Complexes of Copper(II), C-Peptide, and Serum Albumin Using Excited State-Targeted Geometry Optimization.

C-Peptide has been identified as an important biomarker and is involved in insulin production and used to treat various pancreatic diseases. It is also reported to be able to interact with metal ions in the human body such as Cu(II) as well as metal-binding sites in transport proteins such as serum albumin. Recent experimental studies described a ternary complex where Cu(II) is simultaneously coordinated to the C-peptide and bovine serum albumin (BSA) and provided ultraviolet-visible (UV-vis) absorption spectra in which the absorption intensity in the 400-500 nm region depended on the order in which the species were added to the mixture. Our main goal in this study is to obtain structural insights into this ternary complex. We computed UV-vis absorption spectra for structural models of the ternary Cu(II)/C-peptide/BSA complex using time-dependent density functional theory, which can accurately reproduce experimental spectra for binary complexes of Cu(II) with the C-peptide and the metal-binding sites of BSA. A new energy penalty geometry optimization method tailored to searching for structures with high absorption at specific wavelengths is proposed. By applying this method, a model structure of a ternary complex characterized by absorption frequencies in close agreement with experiment (<0.05 eV) was found. Analysis of the electronic character of the excitations highlights the strong effect of the distance between Cu(II) and backbone ligands in this complex in modulating the absorption wavelength.