Interrupted DNA and Slow Silver Cluster Luminescence

Abstract

A DNA–silver cluster conjugate is a hierarchical chromophore with a partly reduced silver core embedded within the DNA nucleobases that are covalently linked by the phosphodiester backbone. Specific sites within a polymeric DNA can be targeted to spectrally tune the silver cluster. Here, the repeated (C2A)6 strand is interrupted with a thymine, and the resulting (C2A)2-T-(C2A)4 forms only Ag10 6+, a chromophore with both prompt (∼1 ns) green and sustained (∼102 μs) red luminescence. Thymine is an inert placeholder that can be removed, and the two fragments (C2A)2 and (C2A)4 also produce the same Ag10 6+ adduct. In relation to (C2A)2T(C2A)4, the (C2A)2 + (C2A)4 pair is distinguished because the red Ag10 6+ luminescence is ∼6× lower, relaxes ∼30% faster, and is quenched ∼2× faster with O2. These differences suggest that a specific break in the phosphodiester backbone can regulate how a contiguous vs broken scaffold wraps and better protects its cluster adduct.

Breaching the Fortress: Photochemistry of DNA-Caged Ag106+

“…Second, it luminesces at λ ∼ 700 nm, which we attribute to a metastable L state situated between its ground S 0 and green-emissive S 1 states ( Figure 1 A(inset),B). 40 , 44 , 55 Three observations support this assignment: (1) The red luminescence and green fluorescence have the same λ ex = 490 nm, suggesting that S 0 → S 1 excitation precedes and feeds the L state ( Figure S1 ). (2) The red luminescence is relatively weak, even in a cryogenic matrix, implying that the radiative L → S 0 transition is disfavored.…”

“… 81 Additionally, a DNA-bound silver cluster is quenched more efficiently by O 2 when the phosphodiester backbone is split. 40 This protection is also observed for rotoxanes and cyclodrexins that encapsulate and protect organic dyes from photooxidation. 82 , 83 A DNA is analogous to these synthetic scaffolds because its sequence and structure can be tuned and controlled.…”

“…The low ϕ c ∼ 0.05 for S 1 → L aligns with other strongly fluorescent DNA-bound silver clusters that favor radiative S 1 → S 0 relaxation. 26 , 40 , 73 Weak S 1 → L transitions may be intersystem crossing because frontier orbitals of the silvers and their ligated nucleobases can mix. 71 Second, the L state has an ∼80 μs lifetime, so an energized, long-lived form of Ag 10 6+ could react with O 2 .…”

“…An oligonucleotide with 10–30 nucleobases strongly binds and locally concentrates silvers to form clusters with 6–30 silvers. , Second, a DNA sequence prescribes the set of nucleobase ligands that collectively chelate a cluster. − The natural nucleobases and their derivatives bind silvers with varying affinities; therefore, a pattern of nucleobases can be chosen to form a specific cluster. Third, DNA tracts assemble and provide a three-dimensional framework for the cluster. ,− These segments of a longer DNA strand can be manipulated to control the cluster spectra, brightness, and quenching. These binding sites not only program the spectra but also guide the chemistry of their silver cluster adducts.…”

“…Third, DNA tracts assemble and provide a three-dimensional framework for the cluster. 7 , 36 − 40 These segments of a longer DNA strand can be manipulated to control the cluster spectra, brightness, and quenching. These binding sites not only program the spectra but also guide the chemistry of their silver cluster adducts.…”