Biology
Porphyrins
Metalloporphyrins play a key role in a wide range of biological processes, such as photosynthesis or the functioning of haem proteins. Here they are responsible e.g. for the uptake, transport and release of small ligands, which are essential to life (e.g. oxygen) or for the protection of organisms against oxidants. Due to their remarkable optical, electronic and magnetic properties, they are promising for applications e.g. in the field of spintronics and photovoltaics, in the form of porphyrin-based molecular devices. Surprisingly, very little is known about their electronic structure under realistic conditions, i.e. in solutions at room temperature and in the presence of counter-ions. With the help of L-edge spectroscopy on their metal active centre we aim to understand the functionality of porphyrins.
Catalase
Catalase and methaemoglobin have very similar haem groups, which are both ferric, yet catalase decomposes hydrogen peroxide to water and oxygen very efficiently, while methaemoglobin does not. Structural studies have attributed this behaviour to their different
distal environments. Here we present Fe L2,3-edge X-ray absorption spectra of these proteins in physiological solutions, which reveal clear differences in their electronic structures, in that p back-donation of the Fe atom occurs in catalase, which confers on it a partial ferryl (Fe4+) character, while this is not the case in methaemoglobin. The origin of the Fe4+ character stems from the proximal tyrosine residue. We also find that both systems are in a high spin state. Temperature effects influence the spectra of catalase only weakly, in agreement with previous studies of its chemical activity. We conclude that the high activity of catalase is not only determined by its distal environment but also by its partial ferryl character.
