Understanding the physical mechanisms of thiolated molecules adsorption on metal surfaces has required copious research, particularly on Au–cysteine systems due to the affinity of sulfur molecules to gold surfaces, as well as the interesting structural modifications that this strong interaction induces and the peculiar optical, chiroptical, and electronic properties of Au(SR) systems. Here, we present vibrational experimental data on the adsorption of L‐ and D‐cysteine on small gold nanoparticles (<2 nm) by means of Raman spectroscopy. L‐ and D‐cysteine molecules adopt the same strained conformation upon adsorption on colloidal gold nanoparticles, regaining structure due to the stabilization that the gold nanoparticle induces on the cysteine, reflected in the recuperation of vibrational bands from their polymorphically distinctive crystalline forms. Through the analysis of Raman vibrational modifications after adsorption, we found experimental evidence that confirms a stabilized cysteine conformation locating the carboxyl group in the antiposition (PC isomeric rotamer) for both molecules. This result is supported by extensive density functional theory (DFT) calculations and simulated Raman spectra, considering zwitterionic cysteine adsorbed on a Au34 cluster, emulating experimental nanoparticle sizes. Our Raman spectroscopy experimental and DFT results determine one of the oxygen atoms of the carboxyl group as a second adsorption site after the sulfur atom, confirming that independent of its polymorphism and enantiomerism, zwitterionic cysteine interacts with gold nanoparticles through the thiol group and the carboxyl group as adsorption sites.

Gold, silver, and copper small nanoparticles (NPs), with average size ≈2 nm, are synthesized and afterward protected with l‐ and d‐cysteine, demonstrating emergence of chiroptical activity in the wavelength range of 250–400 nm for all three metals with respect to the bare nanoparticles and ligands alone. Silver‐cysteine (Ag‐Cys) NPs display the higher anisotropy factor, whereas gold‐cysteine (Au‐Cys) NPs show optical and chiroptical signatures slightly more displaced to the visible range. A larger number of circular dichroism (CD) bands with smaller intensity, as compared to gold and silver, is observed for the first time for copper‐cysteine (Cu‐Cys) NPs. The manifestation of optical and chiroptical responses upon cysteine adsorption and the differences between the spectra corresponding to each metal are mainly dictated by the metal–ligand interface, as supported by a comparison with calculations of the oscillatory and rotatory strengths based on time‐dependent density functional theory, using a metal–ligand interface motif model, which closely resembles the experimental absorption and CD spectra. These results are useful to demonstrate the relevance of the interface between chiral ligands and the metal surfaces of Au, Ag, and Cu NPs, and provide evidence and further insights into the origin of the transfer mechanisms and induction of extrinsic chirality.

Cysteine-protected metal nanoparticles (NPs) have shown interesting physicochemical properties of potential utility in biomedical applications and in the understanding of protein folding. Herein, cysteine interaction with gold, silver, and copper NPs is characterized by Raman spectroscopy and density functional theory calculations to elucidate the molecular conformation and adsorption sites for each metal. The experimental analysis of Raman spectra upon adsorption with respect to free cysteine indicates that while the C−S bond and carboxyl group are similarly affected by adsorption on the three metal NPs, the amino group is sterically influenced by the electronegativity of each metal, causing a greater modification in the case of gold NPs. A theoretical approach that takes into consideration intermolecular interactions using two cysteine molecules is proposed using a S−metal−S interface motif anchored to the metal surface. These interactions generate the stabilization of an organo−metallic complex that combines gauche (P H ) and anti (P C ) rotameric conformers of cysteine on the surface of all three metals. Similarities between the calculated Raman spectra and experimental data confirm the thiol and carboxyl as adsorption groups for gold, silver, and copper NPs and suggest the formation of monomeric "staple motifs" that have been found in the protecting monolayer of atomic-precise thiolatecapped metal nanoclusters.