The first X-ray structure of a protein adduct of a half-sandwich ruthenium-arene complex could pave the way to a new class of hybrid catalyst. Peter Sadler and his team at the University of Edinburgh built adducts starting from various ruthenium complexes and the single chain protein lysozyme and carried out a crystallographic analysis that shows the resulting structure in fine detail. They explain that ruthenium arene binds selectively to the imidazole ring of the only histidine amino acid in the protein chain and results in the formation of a water-repelling, hydrophobic, binding pocket around the ruthenium. Such a site could provide the starting point for a new class of catalyst, explain the researchers. They add that ruthenium arene complexes also have anticancer activity so obtaining a clearer picture of their binding sites to proteins could provide new clues to how they work and perhaps lead to more effective derivatives.