The structural and bonding evolution in cysteine–gold cluster complexes

Abstract

The bonding characteristics in cysteine–gold cluster complexes represented by thiolate (Au_n·Cys^S (n = 1, 3, 5, 7)) and thiol (Au_n·Cys^SH (n = 2, 4, 6, 8)) is investigated by density functional theory with 6-31G(d,p) and Lanl2DZ hybrid basis sets. The complexes exhibit very different bonding characteristic between these two forms. In the Au_n·Cys^S complexes, the charge transfers from gold clusters to sulfur atoms. The number of S–Au bonds in the Au_n·Cys^S complexes evolves from one to two when n is greater than three. For n equals three, i.e. Au₃·Cys^S, its ground state only has one S–Au bond. While the only S–Au bond in Au₁·Cys^S is mainly covalent, the nature of the S–Au bond in other thiolates is featured with the combination of covalent and donor–acceptor interactions. In particular, one stable isomer of Au₃·Cys^S with two S–Au bonds, which is 2 kcal mol⁻¹ higher in energy than the corresponding ground state, consists of one covalent and one donor–acceptor S–Au bond explicitly. Moreover, the localized three center two electron bonds are formed within the Au clusters, which facilitates the formation of the two S–Au bonds in Au₅·Cys^S and Au₇·Cys^S complexes. In the Au_n·Cys^SH complexes, the donor–acceptor interaction prevails in the Au–SH bond by transferring lone pair electrons from the sulfur atom to the adjacent gold atom. Interestingly, the orbital with much more 6s-component in Au₄·Cys^SH enhances the donor–acceptor bonding character, thus yields the strongest bonding among all the Au_n·Cys^SH complexes studied in this paper. In general, the bonding strength between gold clusters and cysteine is positively correlated with the S–Au overlap-weighted bond order, but negatively correlated with the S–Au bond length. Lastly, the covalent and donor–acceptor S–Au bond strength is computed to be 48 and 18 kcal mol⁻¹, respectively.