The interactions between a size-expanded Guanine analogue x-Guanine (xG) and gold nanoclusters, Au n ( n = 2, 4, 6, and 8), were studied theoretically using density functional theory. Geometries of neutral complexes were optimized using the B3LYP functional with the 6-31+G(d,p) basis set for xG and the LANL2DZ basis set for gold clusters. The binding modes, interaction strength, and the charge-transfer properties of different Au n-xG complexes were investigated. Natural population analysis was performed for natural bond order charges. It was found that gold nanoclusters form stable complexes with xG and these binding results in a substantial amount of electronic charge being transferred from xG to the gold clusters. The vertical first ionization potential, electron affinity, Fermi Level, and the HOMO-LUMO gap of xG and its complexes with gold nanoclusters were also analyzed. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]