Breaching the Fortress: Photochemistry of DNA-Caged Ag106+

A DNA strand can encapsulate a silver molecule to create a nanoscale, aqueous stable chromophore. A protected cluster that strongly fluoresces can also be weakly photolabile, and we describe the laser-driven photochemistry of the green fluorophore C4AC4TC3GT4/Ag106+. The embedded cluster is selectively photoexcited at 490 nm and then bleached, and we describe how the efficiency, products, and route of this photochemical reaction are controlled by the DNA cage. With irradiation at 496.5 nm, the cluster absorption progressively drops to give a photodestruction quantum yield of 1.5 (±0.2) × 10–4, ∼103× less efficient than fluorescence. A new λabs = 335 nm chromophore develops because the precursor with 4 Ag0 is converted into a group of clusters with 2 Ag0 – Ag64+, Ag75+, Ag86+, and Ag97+. The 4–7 Ag+ in this series are chemically distinct from the 2 Ag0 because they are selectively etched by iodide. This halide precipitates silver to favor only the smallest Ag64+ cluster, but the larger clusters re-develop when the precipitated Ag+ ions are replenished. DNA-bound Ag106+ decomposes because it is electronically excited and then reacts with oxygen. This two-step process may be state-specific because O2 quenches the red luminescence from Ag106+. However, the rate constant of 2.3 (±0.2) × 106 M–1 s–1 is relatively small, which suggests that the surrounding DNA matrix hinders O2 diffusion. On the basis of analogous photoproducts with methylene blue, we propose that a reactive oxygen species is produced and then oxidizes Ag106+ to leave behind a loose Ag+-DNA skeleton. These findings underscore the ability of DNA scaffolds to not only tune the spectra but also guide the reactions of their molecular silver adducts.

48,000 NA (molec/mol) 6.02E+23 a t is the irradiation time, P is the power, Al is the absorbance at the laser wavelength (496.5 nm), Nphot is the number of photons absorbed, l is the laser wavelength, h is Planks constant, c is the speed of light, A490 is the cluster absorbance at 490 nm, A490 is the change in the absorbance at 490 nm, Nmolec is the number of photobleached molecules, Vsol is the volume of the solution, b is the pathlength of the cell,  is the extinction coefficient, and NA is Avogadro's number.b Average M/Z differences between observed and predicted values for 3-4 charge states.
Figure S1: Excitation spectra of Ag10 6+ /C4AC4TC3GT4 using em = 534 (black) and 700 (red) nm emission acquired at 77 K in a cryogenic ethylene glycol/buffer matrix.Coincident peaks suggest that the chromophore is excited to its green-emissive S1 state and then relaxes to its lower-energy red luminescent L state (see Figure 1B).The red luminescence is ~100 fold weaker than the green fluorescence.The signal at higher energies is attributed to scattering from the cryogenic matrix.
Figure S2: Time evolution of the green emission and red luminescence using different emission filters.The laser was modulated at 1 kHz with a 50% duty cycle.A 550 ± 88 nm filter (black) shows that the green emission promptly rises and falls, consistent with rapid S1→S0 fluorescence.As the spectral window is red-shifted to 694 ± 44 (blue) and 730 nm longpass (red), the red luminescence is distinguished because it more slowly rises and falls, consistent with disfavored transitions S1→L and L→S0, respectively (see Figure 1B).An exponential fit of the 730 nm LP data gives a lifetime of 66 s.Absorption develops at  ~ 420 nm, supporting precipitation of AgI(s).(B) Fluorescence excitation (dashed) and emission (solid) spectra acquired in an ethylene glycol/buffer matrix at 77K.The spectra were collected before (black), after adding 2 Eq.I -:DNA (blue), and after replenishing the precipitated Ag + by adding 2 Eq.Ag + :DNA (red).The fluorescence drops after adding I -, which suggests that the larger clusters degrade.The fluorescence does not recover after adding Ag + , which suggests that the Ag6 4+ is converted to larger clusters.The emission red shifts ~6 nm after adding 2 equivalents of Ag + , which may be due to a change in the cluster environment.

Figure
Figure S3: (A) An intermediate pair of absorption spectra extracted from Figure 2 before (solid) and after (dashed) waiting 5 minutes without irradiation.Overlapping spectra suggests that photoreaction stops without irradiation.(B/C) Absorption spectra during photoirradiation of Ag10 6+ clusters at  = 496.5 nm in (B) D2O and (C) H2O.The red trace is before irradiation, the blue trace is after 15 minutes of photoirradiation with 2.3 mW, and the black trace is after an additional 10 minutes with 3.5 mW.The clusters degrade similarly, suggesting that the encapsulating DNA host protects its Ag10 6+ cluster.

Figure S4 :
Figure S4: Time evolution of A490 based on data in Figure 2 (see inset).The data was fit using an exponential model, and the linear trend using a logarithmic scale supports a firstorder or pseudo first-order reaction.

Figure S5 :
Figure S5: Isotopologue distributions for -4 charged DNA-Ag10 6+ anions based on the molecular formula C169H212N53O110P17Ag10. Alternative models with ± 1 H + with respective formulas C169H211N53O110P17Ag10 (open circles) and C169H213N53O110P17Ag10 6+ (open squares) have distributions that are statistically different based on the standard deviations for the intensities.These standard deviations are 4.5%, 12.1%, and 9,7% for the respective formulas with H212, H211, and H213.Based on the smaller deviation for H212, the cluster is assigned to be Ag10 6+ with a +6 oxidation state.

Figure S6 :
Figure S6: Mass spectra of Ag + -DNA complexes with initial stoichiometries of 2 Ag + :DNA, 4 Ag + :DNA, and 8 Ag + : DNA (top to bottom).These were diluted with 100x volumes of 5 mM ammonium acetate to dissociate weaker adducts.Similar distributions for the three spectra suggest that 3-4 Ag + :DNA strongly bind with this DNA.

Figure
Figure S7: (A) Absorption spectra before (solid) and after (dotted) adding 3 Eq I -:DNA.Absorption develops at  ~ 420 nm, supporting precipitation of AgI(s).(B) Fluorescence excitation (dashed) and emission (solid) spectra acquired in an ethylene glycol/buffer matrix at 77K.The spectra were collected before (black), after adding 2 Eq.I -:DNA (blue), and after replenishing the precipitated Ag + by adding 2 Eq.Ag + :DNA (red).The fluorescence drops after adding I -, which suggests that the larger clusters degrade.The fluorescence does not recover after adding Ag + , which suggests that the Ag6 4+ is converted to larger clusters.The emission red shifts ~6 nm after adding 2 equivalents of Ag + , which may be due to a change in the cluster environment.1

1
Figure S7: (A) Absorption spectra before (solid) and after (dotted) adding 3 Eq I -:DNA.Absorption develops at  ~ 420 nm, supporting precipitation of AgI(s).(B) Fluorescence excitation (dashed) and emission (solid) spectra acquired in an ethylene glycol/buffer matrix at 77K.The spectra were collected before (black), after adding 2 Eq.I -:DNA (blue), and after replenishing the precipitated Ag + by adding 2 Eq.Ag + :DNA (red).The fluorescence drops after adding I -, which suggests that the larger clusters degrade.The fluorescence does not recover after adding Ag + , which suggests that the Ag6 4+ is converted to larger clusters.The emission red shifts ~6 nm after adding 2 equivalents of Ag + , which may be due to a change in the cluster environment.1

Figure S8 :
Figure S8: Mass spectra of C4AC4TC3GT4/Ag10 6+ following photolysis and then dialysis with 100x (top) and 10000x (bottom) volumes of buffer.No significant change in the distribution of adducts that all silvers are strongly bound.

Figure S9 :
Figure S9: Absorption spectra with no halide (solid), with I -(dashed), with Br -(dotted), and Cl -(long dashes).The amounts are 0.5 equivalents halide:DNA.Similar results without and with Cl -suggest the Ag + /Cl -affinity is too low to precipitate AgCl(s).

Figure S10 :
FigureS10: Absorption spectra collected before (black) and then in 1 min intervals using 1 mW at 490 nm (blue, green, aqua, magenta, and red, respectively).The arrows emphasize a more efficient reaction with O2 vs N2.

Table S1 :
Example Calculations for Photodestruction Quantum Yield (d) a

Table S4 :
Isotopologue M/Z and Intensity Analysis for (C4AC4TC3GT4/Ag8 6+ ) -4 a a See footnotes for TableS2.Peak positions based on the formula C169H212N53O110P17Ag8.

Table S5 :
Isotopologue M/Z and Intensity Analysis for (C4AC4TC3GT4/Ag7 5+ ) -4 a a See footnotes for TableS2.Peak positions based on the formula C169H213N53O110P17Ag7.
a See footnotes for TableS2.Peak positions based on the formula C169H214N53O110P17Ag6.b c Absolute M/Z differences between observed and predicted values.These differences are reported in ppm.