Tug-of-War between DNA Chelation and Silver Agglomeration in DNA-Silver Cluster Chromophores.

ABSTRACT

Supramolecular chromophores form when a DNA traps silvers that then coalesce into clusters with discrete, molecular electronic states. However, DNA strands are polymeric ligands that disperse silvers and thus curb agglomeration. We study this competition using two chromophores that share three common components a dimeric DNA scaffold, Ag+-nucleobase base pairs, and Ag0 chromophores. The DNA host C4-A2-iC4T mimics structural elements in a DNA-cluster crystal structure using a phosphodiester backbone with combined 5' → 3' and 3' → 5' (indicated by "i") directions. The backbone directions must alternate to form the two silver clusters, and this interdependence supports a silver-linked structure. This template creates two chromophores with distinct sizes, charges, and hence spectra (C4-A2-iC4T)2/Ag117+ with λabs/λem = 430/520 nm and (C4-A2-iC4T)2/Ag148+ with λabs/λem = 510/630 nm. The Ag+ and Ag0 constituents in these partially oxidized clusters are linked with structural elements in C4-A2-iC4T. Ag+ alone binds sparsely but strongly to form C4-A2-iC4T/3-4 Ag+ and (C4-A2-iC4T)2/7-8 Ag+ complexes, and these stoichiometries suggest that Ag+ cross-links pairs of cytosines to form a hairpin with a metallo-C4/iC4 duplex and an adenine loop. The Ag0 are chemically orthogonal because they can be oxidatively etched without disrupting the underlying Ag+-DNA matrix, and their reactivity is attributed to their valence electrons and weaker chelation by the adenines. These studies suggest that Ag+ disperses with the cytosines to create an adenine binding pocket for the Ag0 cluster chromophores.