QM study of interaction between arginine amino acid and Au clusters and the effects on arginine acidity

Understanding the various structures of gold clusters and the interaction modes between Au clusters and biomolecules is an important issue in material science such as biosensors and catalysts. The binding of small gold clusters (Au_n n?=?2–5) with neutral and anionic forms of arginine (Arg) amino acid is investigated in this study using density functional theory (DFT) and B3LYP level. The relative stability among different forms of Au clusters including linear, zigzag, planar, and three-dimensional Au clusters was estimated, initially. The calculated findings show that the zigzag structure for Au₃ and the planar structure for Au₄ and Au₅ are the best form. Furthermore, the different modes of interaction were taken into account from thermodynamic view point between the most stable conformers of Arg and Arg^? with gold clusters. Finally, the arginine is considered as a weak organic acid to investigate the impact of Au clusters on the gas phase acidity. The acidity of isolated arginine and the acidity of [Au_n/Arg] complexes were also compared. Based on the obtained results, upon the complexation with Au clusters at 298?K, for the interaction of Au, Au₂, Au₃, Au₄, and Au₅ clusters with arginine, the gas phase acidity (GPA) of arginine alters from 342.12 to 314.17, 303.04, 299.42, 303.41, and 331.66?kcal/mol respectively. These calculated values predict that when a weak organic acid is complexed with Au clusters, it will be altered to super acid. Furthermore, for isolated and complexed species of Arg, pK_a values were evaluated in water solvent.