Amino acids conjugated gold clusters: Interaction of Au₈ and Au₂₀ clusters with Tryptophan and Alanine in the gas and aqueous phases

The site-specific interactions in the amino acids conjugated gold clusters taking account of the solvation effect and the zwitterionic nature of amino acids were investigated. Specifically, bioconjugated complexes of Au₈ (D_4h) and Au₂₀ (T_d) with tryptophan (Trp) and alanine (Ala) amino acids were considered. The calculated results based on density functional theory find the site-specificity of interaction depends strongly on the polarity of amino acids. For example, Au₈ and Au₂₀ interacting with the neutral Trp and Ala prefer the amine group, whereas the zwitterionic Trp and Ala in the aqueous phase prefer the carboxylic site. The solvation induced effects appear to slightly decrease the strength of the interaction in the complexes. Additionally, analysis of the frontier molecular orbitals show that the fluorescence properties of the bioconjugated complex will be controlled by the Au clusters.