Atomic Structure of a Fluorescent Ag₈ Cluster Templated by a Multistranded DNA Scaffold

Abstract

Multinuclear silver clusters encapsulated by DNA exhibit size-tunable emission spectra and rich photophysics, but their atomic organization is poorly understood. Herein, we describe the structure of one such hybrid chromophore, a green-emitting Ag 8 cluster arranged in a Big Dipper shape bound to the oligonucleotide A 2 C 4. Three 3' cytosine metallo-base pairs stabilize a parallel Aform-like duplex with a 5' adenine-rich pocket, which binds a metallic, trapezoidal-shaped Ag 5 moiety via Ag-N bonds to endo-and exo-cyclic nitrogens of cytosine and adenine. The unique DNA configuration, constrained coordination environment, and templated Ag 8 cluster arrangement highlight the reciprocity between the silvers and DNA in adopting this structure. These first atomic details of a DNA-encapsulated Ag cluster fluorophore illuminate many aspects of biological assembly, nanoscience, and metal cluster photophysics.

Crystal structure of a NIR‐Emitting DNA‐Stabilized Ag16 Nanocluster

“…Furthermore, each Ag atom is in close contact with neighboring Ag atoms ( d =2.7–2.8 Å). Similar Ag–Ag atom interactions were also observed in the green‐emitting silver nanocluster by Huard et al . The structure is further stabilized by hydrogen bonds ( d =2.8–3.1 Å) between the carbonyl group O2 and amino group N4 of the neighboring cytosine bases.…”

“…Since their introduction in 2004, attempts have been made to characterize the exact composition and structure of these DNA–AgNCs using techniques such as mass‐spectroscopy, extended X‐ray absorption fine structure (EXAFS), X‐ray photoelectron spectroscopy (XPS), and more recently the first atomic structure information appeared using X‐ray diffraction . In the study of Huard et al., the cationic silver–DNA precursor was first crystallized and then chemically reduced. This crystal structure contains a silver cluster with novel Ag–Ag interactions and a segment with cationic silver‐mediated base‐pair interactions, similar to a previously reported silver–DNA hybrid nanowire .…”

“…The close distance between Ag and N1(G) suggests that guanine residues must be deprotonated at the N1 position. Binding of Ag to deprotonated adenine and cytosine residues was also suggested in the crystal structure of the green‐emitting silver nanocluster by Huard et al . The Ag−N3(C)/N1(G) distances are in the range of 2.3–2.4 Å.…”

“…DNA oligomers allow the creation of atomically monodisperse silver nanoclusters (DNA–AgNCs), usually below 30 Ag atoms that, depending on the DNA sequence, can result in bright emitters . Since their introduction in 2004, attempts have been made to characterize the exact composition and structure of these DNA–AgNCs using techniques such as mass‐spectroscopy, extended X‐ray absorption fine structure (EXAFS), X‐ray photoelectron spectroscopy (XPS), and more recently the first atomic structure information appeared using X‐ray diffraction . In the study of Huard et al., the cationic silver–DNA precursor was first crystallized and then chemically reduced.…”

C3‐Symmetric Assemblies from Trigonal Polycarbene Ligands and MI Ions for the Synthesis of Three‐Dimensional Polyimidazolium Cations

“…[17a] While the encapsulations studies have not yet provided remarkable results,t he flexible polyimidazolium cations are also promising candidates for the encapsulation of metallic nanoparticles of varying size.DNA templated silver nanoparticles have been obtained from Ag I mediated base pairs upon Ag I reduction. [18] Much more facile would be the deprotonation and metalation of the polyimidazolium salts presented here followed by metal reduction and encapsulation of the nanoparticles via NHC-M interactions. [13b] In summary,wepresent the preparation of trigonal poly-NHC precursors,w hich were employed for the synthesis of unique metallosupramolecular architectures containing six (Ag I or Au I )o rn ine (Ag I )m etal ions sandwiched in between two trigonal hexakis-or nonakis-NHC ligands.F unctionalization of the poly-NHC ligands with terminal cinnamic ester groups allowed for apostsynthetic [2+ +2] cycloaddition reaction connecting two coordinated trigonal ligands by three cyclobutane linkers.T ot he best of our knowledge,t his led to the largest known threedimensional polyimidazolium derivatives featuring 12 or 18 imidazolium groups.C ontrary to many supramolecular hosts composed of polydentate ligands and metal ions, [1][2][3][4][5][6] we have developed purely organic polyimidazolium derivatives featuring only stable C À Ca nd C À Nb onds.I nvestigations into the use of the novel polyimidazolium salts as polydentate Lewis acids and as hosts for nanoparticles are in progress.…”