Credit: © 2010 Wiley

The Watson–Crick base pairing underlying DNA's helical structure is very well known, but nucleobases can form a variety of other hydrogen-bonded motifs. Guanines, in particular, can assemble into dimers, ribbons or macrocyclic 'G-quartets' that are relevant to fields as diverse as organic electronics or drug design. Now, a team from the universities of Strasbourg and Bologna, led by Paolo Samorì and Gian Piero Spada, has directly observed the reversible conversion between guanine-based ribbons and G-quartets at a liquid–solid interface by scanning tunnelling microscopy (STM)1.

Guanine-based assemblies on surfaces have been widely investigated by STM, but under ultra-high vacuum. To study liquid–solid interfaces, Samorì and colleagues used solutions of guanines bearing a long alkyl side chain that were so dilute they can all be adsorbed on the surface. The molecules first assembled into extended ribbons on the highly oriented pyrolitic graphite surface. The addition of potassium ions to the solution then triggered the formation of macrocyclic G-quartets centred around the cations. When the potassium ions were subsequently captured by addition of a cryptand molecule, the ribbons re-assembled.

On acidification of the solution, the protonated cryptands released the potassium ions, inducing the re-formation of the G-quartets. Finally, the formation of ribbons could be triggered again by further addition of cryptands. The surface areas covered by either the ribbons or the quartets were found to be very similar, suggesting that the conversions occur on the surface without involving desorption and re-adsorption steps.