A vast array of different structures can be formed by exploiting reversible, multiple weak intermolecular interactions. The structures that are formed are, for the most part, stable at equilibrium. Inspiration for research into such self-assembled structures often comes from natural systems, but in reality these often exist far from equilibrium and require the constant input of chemical energy to remain assembled. Now, Jan van Esch and co-workers from Delft University of Technology have developed1 a gel-forming system that consumes a 'chemical fuel' to mimic this behaviour.

The system is based on the known gelator dibenzoylcysteine, a disulfide formed from the N-protected amino acid. The molecule shows pH-responsive gelation: a gel is formed at low pH but breaks down at high pH owing to electrostatic repulsion between the negatively charged carboxylates. If the diacid is esterified, however, then the gel is stable at high pH. Such an esterification can be achieved by the addition of an alkylating agent — methyl iodide — to the system. The diester forms a gel but the ester is slowly hydrolysed by the presence of hydroxide (base is slowly added to the system to maintain the pH).

Because the rate of hydrolysis is slower than the rate of esterification, a steady state where gel is present but is constantly being formed and degraded can be reached as long as enough fuel — in the form of methyl iodide — is available. As the fuel is used up, the rate of ester hydrolysis overtakes that of ester formation and the system can return to its initial liquid state.