Atomic structure of a peptide coated gold nanocluster identified using theoretical and experimental studies

Abstract

Peptide coated gold nanoclusters (AuNCs) have a precise molecular formula and atomic structure, which are critical for their unique applications in targeting specific proteins either for protein analysis or drug design. To date, a study of the crystal structure of peptide coated AuNCs is absent primarily due to the difficulty of obtaining their crystalline phases in an experiment. Here we study a typical peptide coated AuNC (Au₂₄Peptide₈, Peptide = H₂N-CCYKKKKQAGDV-COOH, Anal. Chem., 2015, 87, 2546) to figure out its atomic structure and electronic structure using a theoretical method for the first time. In this work, we identify the explicit configuration of the essential structure of Au₂₄Peptide₈, Au₂₄(Cys–Cys)₈, using density functional theory (DFT) computations and optical spectroscopic experiments, where Cys denotes cysteine without H bonded to S. As the first multidentate ligand binding AuNC, Au₂₄(Cys–Cys)₈ is characterized as a distorted Au₁₃ core with O_h symmetry covered by two Au(Cys–Cys) and three Au₃(Cys–Cys)₂ staple motifs in its atomic structure. The most stable configuration of Au₂₄(Cys–Cys)₈ is confirmed by comparing its UV-vis absorption spectrum from time-dependent density-functional theory (TDDFT) calculations with optical absorption measurements, and these results are consistent with each other. Furthermore, we carry out frontier molecular orbital (FMO) calculations to elucidate that the electronic structure of Au₂₄(Cys–Cys)₈ is different from that of Au₂₄(SR)₂₀ as they have a different Au/S ratio, where SR represents alkylthiolate. Importantly, the different ligand coatings, Cys–Cys and SR, in Au₂₄(Cys–Cys)₈ and Au₂₄(SR)₂₀ cause the different Au/S ratios in the coated Au₂₄. The reason is that the Au/S ratio is crucial in determining the size of the Au core of the ligand protected AuNC, and the size of the Au core corresponds to a specific electronic structure. By the adjustment of ligand coatings from alkylthiolate to peptide, the Au/S ratio could be controlled to generate different AuNCs with versatile electronic structures, optical properties and reaction stabilities. Therefore, we propose a universal approach to obtain a specific Au/S ratio of ligand coated AuNCs by adjusting the ligand composition, thus controlling the chemicophysical properties of AuNCs with ultimately the same number of Au atoms.