  Hydrogen bonding between nucleobases produces diverse DNA structural motifs,
including canonical duplexes, guanine (G) quadruplexes and cytosine (C)
i-motifs. Incorporating metal-mediated base pairs into nucleic acid structures
can introduce new functionalities and enhanced stabilities. Here we
demonstrate, using mass spectrometry (MS), ion mobility spectrometry (IMS) and
fluorescence resonance energy transfer (FRET), that parallel-stranded
structures consisting of up to 20 G-Ag(I)-G contiguous base pairs are formed
when natural DNA sequences are mixed with silver cations in aqueous solution.
FRET indicates that duplexes formed by poly(cytosine) strands with 20
contiguous C-Ag(I)-C base pairs are also parallel. Silver-mediated G-duplexes
form preferentially over G-quadruplexes, and the ability of Ag+ to convert
G-quadruplexes into silver-paired duplexes may provide a new route to
manipulating these biologically relevant structures. IMS indicates that
G-duplexes are linear and more rigid than B-DNA. DFT calculations were used to
propose structures compatible with the IMS experiments. Such inexpensive,
defect-free and soluble DNA-based nanowires open new directions in the design
of novel metal-mediated DNA nanotechnology.
