Credit: © 2010 Wiley

The materials that come to mind when 'porous solids' are mentioned are usually zeolites, silicates and carbon-based frameworks. Dipeptides, however, have recently been shown to form hydrogen-bonded microporous crystals, and have attracted interest for their gas sorption properties. Now, a team of Portuguese researchers led by Luís Gales at the Institute of Molecular and Cell Biology in Porto have observed1 that dipeptide single-crystals can act as permeable membranes able to distinguish between argon, nitrogen and oxygen — a process of interest for air separation, but difficult to carry out because of the similarity in size between the species.

The permeability of three dipeptides — L-leucyl-L-serine (LS), L-valyl-L-isoleucine (VI), and L-alanyl-L-alanine (AA), which crystallize into different lattices with different porosities — was found to be size-dependent. The crystals that displayed narrower pores combined lower gas absorption abilities and guest diffusivities, resulting in poorer permeabilities. Thus, LS, with nanochannels that are larger than argon, nitrogen and oxygen, is permeable to all three. VI, whose channel size is close to that of the gas molecules, was only permeable to oxygen and nitrogen. And although AA's porosity consists of discrete pockets rather than channels, it was found to be permeable to oxygen.

The AA dipeptide retained its crystallinity after oxygen permeation, suggesting that its guest-induced flexibility is reversible. This dynamic response was also guest-dependent: AA was more permeable to oxygen than to helium — even though helium is a smaller species. This excellent selectivity suggests that dipeptide crystals hold promise for a variety of separation processes.