A DNA-Encapsulated Silver Cluster and the Roles of Its Nucleobase Ligands

摘要： Silver clusters consisting of ~10 atoms are readily bound by and encapsulated within DNA strands to yield strong absorption and emission. The coordination environments, however, are poorly understood, so cluster adducts can only be empirically tuned. This work describes the C4AC4TC3G strand that templates a particular cluster adduct. Its sequence has three types of nucleobases with distinct roles – tracts of cytosines that collectively coordinate the cluster, thymine acting as a junction in the overall strand, and the adenine/guanine pair that exclusively forms the cluster. In relation to the native oligonucleotide, the DNA-silver cluster complex diffuses faster and is more compact, thus suggesting that the strands folds because of the cluster. The Ag10 6+ adduct emits with λex/λem = 490/540 nm, a 19% quantum yield, and a biexponential 1.1/2.1 ns lifetime. The electronic environment for the cluster is controlled by the heteroatoms in the adenine and guanine. Most significantly, the N7 and the N2 in the guanine change the fluorescence quantum yield by 60-fold and shift the fluorescence lifetime by ~3.8 ns. Thus, our studies discern distinct spectroscopic and structural roles for the nucleobase ligands in C4AC4TC3G, and these finding may help develop new DNA templates for other silver cluster adducts.