Density Functional Theory Calculations of Stability and Absorption Spectra of Au₈- Conjugated Complexes with Met and Trp Amino Acids and Trp-Met Dipeptide

Interactions of Au₈ (D_4h) gold cluster with the functional groups of the amino acids; L-Methionine (Met) and L-Tryptophan (Trp), and the dipeptide L-Tryptophyl-L-Methionine (Trp-Met), in their zwitterionic forms and in presence of solvent have been investigated using the density functional theory (DFT). Specifically, the binding energies and Ultraviolet-Visible (UV-Vis) absorption spectra of the bioconjugated complexes of Au₈-Met, Au₈-Trp, and Au₈-Trp-Met have been analyzed. Our calculations show that; the pristine Au₈ (D_4h) cluster has strong absorption spectrum in the near-ultraviolet range, in agreement with the literature. The optical activity of the pristine cluster is the major influencing factor on the absorption spectra of the studied bioconjugated complexes, but the spectra of the complexes carry sufficient remarks that allow them to be distinguished from that of the pristine cluster. The interactions of Au₈ with each specific functional group (either indole or thiol ether) led to very similar absorption spectra whether this group existed in a mono or in a dipeptide amino acid.