photo: Jeff Wilson

In 1874 the science fiction writer Jules Verne wrote, "Water will one day be employed as fuel - the hydrogen and oxygen that constitute it, used singly or together, will furnish an inexhaustible source of heat and light".

In the early 21st century, Professor Tom Wydrzynski and Dr Warwick Hillier, along with their colleagues in the School of Biology Photobioenergetics group, are hoping to achieve just this by mimicking the energy-transducing enzymes of photosynthetic organisms.

Green plants, algae and cyanobacteria use their chlorophyll to trap photons from sunlight and siphon off energy to split water molecules. The enzyme that catalyses this reaction is unique in nature and its operation is a thermodynamic wonder because, with only a weak driving force (energy from absorbed photons), it's being used to split water which is a remarkably stable molecule. In arguably the most unlikely chemical event to occur in our biosphere, the water-splitting process proceeds ‘uphill' through a series of increasing energetic steps, thanks to the operation of the ‘oxygen clock' - a kind of manganese (Mn) -based ‘capacitor' buried within the enzyme which stores energy. Collectively, an endless series of water molecules yield atmospheric oxygen, hydrogen ions (H⁺), and electrons.

Remarkably, a few organisms retain a capacity to reunite the H⁺ into molecular hydrogen (H₂). However, molecular oxygen (O₂) inactivates the hydrogenase activity, so that photosynthetic water splitting and hydrogenase catalysis have to be kept separate if H₂ production from water is to proceed. Mutant organisms can be configured in this way, and over the past seven years, the Photobioenergetics group has begun to exploit nature's blueprint in developing a solar-driven biocatalyst (based on the use of abundant metals such as manganese, iron and possibly nickel) that will ultimately split water into H₂ and O₂ all within the same molecular assembly. In effect, they are reverse-engineering the natural water-splitting and hydrogenase reactions and are using a robust protein nanostructure (a hollow sphere, about eight nanometres in diameter) to compartmentalise these reactions.

While sustained operation of an artificial protein device remains a challenge, if solved, the implications of such a device for a future H₂-based energy economy are profound. Meanwhile, the artificial protein device is proving invaluable as a model system for further study of natural photosynthetic reactions, with their intriguing Mn-based oxygen clock.

Further Info Online:

• http://www.rsbs.anu.edu.au/ResearchGroups/PBE/profiles/Tom_Wydrzynski