Structures and Bonding Properties of Gold−Arg-Cys Complexes: DFT Study of Simple Peptide-Coated Metal

We present the structures, bonding characteristics, and infrared spectra of the gold surface (111)−Arg-Cys (Arg-Cys@Au(111)) complex calculated by a periodic plane wave DFT technique. We examine the detailed features of bonding between the gold surface and dipeptide. The dipeptide is revealed to form a covalent bond via the −SH group with 2−3 gold atoms, and also weak noncovalent interactions via the carboxyl and guanidine side chain lying more or less parallel to the gold surface. The S−H bond dissociates as a result of the S−(Au)n bond formation, with the hydrogen atom binding to the guanidine moiety. The acidic proton stays at the carboxyl group in the most stable structure of Arg-Cys@Au(111). The calculated infrared spectra are compared with experimental observations reported by Petoral and Uvdal (Colloids Surf., B 2002, 25, 335). ■ INTRODUCTION Bottom-up self-assembly of hybrid materials produces novel materials for electronics and optical applications. Nanocomposite materials could also be useful as biological probes for diagnosis and treatment of human diseases. Understanding and controlling the metal−biomolecule hybrid systems offers new paradigms in materials science, chemistry, biology, and medicine. Among the various candidates, peptide-coated metal surfaces exhibit appealing properties for biomaterial science and biosensor technology. Protein−surface interactions may also provide a means to control the activity of active sites in enzymes. The gold surface has received a lot of attention as an instrumental template for immobilizing biomolecules such as DNA, peptides, and enzymes for diverse purposes. Adsorption of proteins on gold electrode is of great importance in biosensor research. Because the selectivity and sensitivity of biosensors are highly dependent on the binding characteristic between protein and gold surface, it is very important to understand the nature of their interactions. Protein−surface recognition is also a powerful tool for cellular signal transduction, DNA transcription, and protein antigen/antibody recognition. A considerable amount of computational work has also been reported on the metal−biomolecule complexes at various levels of theory. Self-assembly of sulfide-containing amino acids and related compounds (cysteine, cysteamine, cystamine, etc.) on metal surfaces is a simplistic model system that was investigated by quantum chemical methods in combination with molecular dynamic technique to unravel the structures and bonding properties of metal−protein complexes. Adsorption of large proteins on metal surfaces has been studied largely by classical molecular dynamics simulation based on force fields. Here we study the gold surface−arginine-cysteine complex (Arg-Cys@Au(111)) as a model for a functionalized metal surface. Arginine (Arg) is chosen to examine the binding to gold surface via its very basic side chain guanidine. Cysteine (Cys) is a good model to probe the role of the thiol group. We calculate their structures to examine what part(s) of the dipeptide interacts with the gold surface through what kinds of interactions (covalent or not). We also calculate the infrared spectra to discuss in detail the normal modes in relation to the structure of the complex. The results are compared with experimental observations reported by Petoral and Uvdal, especially focusing on elucidating the similarity and difference in calculated and experimental infrared spectra in terms of the structures of the Arg-Cys moiety and the positions of H atoms resulting from the formation of S−Aun (n = 2−3) bond. ■ COMPUTATIONAL DETAILS A periodic DFT method (GGA-PBE) is employed with a PAW-PBE norm conserving pseudopotential with 400 eV kinetic energy cutoff, a plane-wave basis as implemented in the Vienna ab initio simulation package (VASP) suite of programs. In order to include the weak van der Waals interactions, we used the DFT-D2 Grimme method, which we find to be of better economy than other similar methods. Cut-off radius for pair interactions is taken to be small (7.0 Å) Received: December 19, 2013 Revised: August 12, 2014 Published: August 14, 2014 Article