Base-Directed Formation of Fluorescent Silver Clusters

Small silver clusters that form with short oligonucleotides are distinguished by their strong fluorescence. Previous work showed that red and blue/green emitting species form with the cytosine oligonucleotide dC₁₂. To understand how the bases and base sequence influence cluster formation, the blue/green emitting clusters that form with the thymine-containing oligonucleotides dT12, dT₄C₄T₄, and dC₄T₄C₄ are discussed. With dT₁₂ and dT₄C₄T₄, variations in the solution pH establish that the clusters associate with the N3 of thymine. The small clusters are bound to the larger DNA template, as demonstrated by fluorescence anisotropy, circular dichroism, and fluorescence correlation spectroscopy (FCS) studies. For dT₄C₄T₄, FCS studies showed that approximately 50% of the strands are labeled with the fluorescent clusters. Absorption spectra and the gas dependence of the fluorescence show that nonfluorescent clusters also form following the reduction of the silver cation - oligonucleotide conjugates. Fluorescent cluster formation is favored by oxygen, thus indicating that the DNA-bound clusters are partially oxidized. To elaborate the sequence dependence of cluster formation, dC₄T₄C₄ was studied. Cluster formation depends on the oligonucleotide concentration, and higher concentrations favor a red emitting species. A blue/green emissive species dominates at lower concentrations of dC₄T₄C₄, and it has spectroscopic, physical, and chemical properties that are similar to those of the clusters that form with dT₁₂ and dT₄C₄T₄. These results suggest that cytosine- and thymine-containing oligonucleotides stabilize a preferred emissive silver cluster.