X-ray Crystal Structure of a Metalled Double-Helix Generated by Infinite and Consecutive C*-AgI -C* (C*:N1 -Hexylcytosine) Base Pairs through Argentophilic and Hydrogen Bond Interactions

Abstract

The synthesis of a metalled double-helix containing exclusively silver-mediated C*-C* base pairs is reported herein (C*=N¹ hexylcytosine). Remarkably, it is the first crystal structure containing infinite and consecutive C*-Ag^I -C* base pairs that form a double helix. The Ag^I ion occupies the center between two C* residues with N(3)-Ag bond lengths of 2.1 Å and short Ag^I -Ag^I distances (3.1 Å) suggesting an interesting argentophilic attraction as a stabilization source of the helical disposition. The solid-state structure is further stabilized by metal-mediated base-pairs, hydrogen bonding and π-stacking interactions. Moreover, the angle N(3)-Ag-N(3) is almost linear in the [Ag(N¹ hexylcytosine)₂ ]⁺ motif and the bases are not coplanar, thus generating a double-strand helical aggregate in the solid state. The noncovalent and argentophilic interactions have been rationalized based on DFT calculations.

Keywords: X-ray crystallography; density functional calculations; helical structures; silver; solid-state structures.