Amino-Acid-Conjugated Gold Clusters: Interaction of Alanine and Tryptophan with Au₈ and Au₂₀

Abstract

The stability and electronic properties of gold (Au) clusters interacting with the amino acids alanine (Ala) and tryptophan (Trp) in their canonical and zwitterionic configurations were investigated using first-principles density functional theory (DFT). We found that the geometrical structures of the Au clusters and the polarities of the amino acids determine the nature of the interactions in the gas and solvent phases. In the gas phase, the Au8 (D 4h ) and Au20 (T d ) clusters prefer single-site interactions through the amine group for the canonical amino acids, whereas in the solvent phase, the carboxylic site is preferred for the zwitterionic amino acids. The limited screening of the intermolecular interactions introduced by the solvent medium for the canonical forms of Ala and Trp conjugated with the Au n complexes suggests that the bonding is primarily covalent in nature. The screening is significantly more pronounced for the zwitterionic complexes for which the electrostatic interactions dominate. The cluster sizes and configurations define the extent of the interactions and the overall stability of the complexes. The structures of the Au n clusters govern the charge distribution and electrostatic potential, directing the selectivity toward the preferential binding sites with the Ala and Trp amino acids.

Mechanistic Details and Conformational Behavior of Selective Peptide Linkage Facilitated by Aun Clusters

“…The authors found that canonical amino acids preferred single site interactions through an amine group, while in their zw form, amino acids interact through a carboxylic site (COOH). Our finding about the binding of zw forms of Glycine and Alanine to Au n cluster through their COOH end ( Figure 1) is in congruence with the results of Abdalmoneam et al [59] Positive BE 1 (see Table 2) entails the stability of Glycine/Alanine-Au n complexes. [56,59] There are numerous reports on the anchoring of a single amino acid to gold clusters.…”

“…Our finding about the binding of zw forms of Glycine and Alanine to Au n cluster through their COOH end ( Figure 1) is in congruence with the results of Abdalmoneam et al [59] Positive BE 1 (see Table 2) entails the stability of Glycine/Alanine-Au n complexes. [56,59] There are numerous reports on the anchoring of a single amino acid to gold clusters. However, the interaction of dipeptides with metallic surfaces or clusters has been explored only to a limited extent.…”

“…The interaction of amino acids with Au n clusters has been vastly investigated by considering amino acids in their canonical form. [56][57][58] Recently, Abdalmoneam et al [59] have reported results of their study on the interactions of Au n clusters with amino acids (Tryptophan/Alanine) in both canonical and zwitterionic (zw) forms. The authors found that canonical amino acids preferred single site interactions through an amine group, while in their zw form, amino acids interact through a carboxylic site (COOH).…”

“…The main source of interaction is the charge transfer from the oxygen atom of COOH to the empty σ* and n* orbitals of Au atom in the cluster. [58,59] The charge transfer between amino acid/dipeptides and Au n cluster not only enhances the coulomb interactions, but also overcomes the exclusive repulsion rule that does not permit the two entities to approach each other further than the Van der Waals contact distance. Effect of solvent has been accounted for by re-optimizing the geometries in aqueous phase.…”

“…Previously, it has been suggested that solvent effects cause the binding energies to decrease, indicating that the bonding is electrostatic in nature. [59,60] Consequently, the effective binding energies of amino acids and their dipeptides to the Au n cluster decrease in aqueous medium ( Table 2 and 3). The preferential anchoring of an amino acid to Au n cluster through COOH site (in zw form) and amine site (in canonical form) presents the promising possibility of selective peptide linkage.…”