Interaction of gold nanoclusters of different size with adenine: A density functional theory study of neutral, anionic and cationic forms of [adenine + (Au)n=3,6,9,12] complexes

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In the present work, we have studied the interaction of adenine and gold nanoclusters of different size at B3LYP level of theory using 6-31G (d) basis set except for gold atoms. For the gold atoms, LANL2DZ effective core potential (ECP) was used. In order to understand the binding interaction, neutral, anionic and cationic forms of [adenine+(Au)n=3,6,9,12] complexes have been selected. We found that the adenine has high binding affinity at their N1, N3 and N7 atomic sites for each size of gold nanoclusters. The distance between gold clusters (Au, Au2, Au3 and Au4) and adenine (N1, N3 and N7) was found to be larger than 2Å. Thus, it favors the formation of non-covalent interaction between adenine and gold clusters. The distance between adenine to gold nanoclusters is found to decrease by increasing the cluster size. Mulliken charge analysis was performed for all three forms of [adenine+(Au)12] complexes for determining the nature of charge transfer between gold nanoclusters to adenine and vice versa. It was observed that the gold nanoclusters acquire a large amount of negative charge in anionic form of [adenine+(Au)12] complex. However, in case of cationic form of the complex, adenine gains a substantial amount of positive charge. The adiabatic electron affinity (AEA) of [adenine+(Au)n=3,6,9,12] complexes was also calculated. The AEA values of [adenine+(Au)n=3,6,9,12] complexes were found to be less than that of the isolated adenine.