Hard X-ray spectroscopy: a versatile tool for biophysicists*

Metals play a key role in the variety of processes in nature. Metal activity is usually associated with changes in the oxidation state, spin state, and metal-ligand covalency of corresponding metallocofactors. Thus, the local electronic structure of the transition metal is essential for elucidating mechanisms underlying the dynamic processes involving metal ions. The inner-shell X-ray spectroscopy is the way to directly probe the electronic structure of metal which makes spectroscopic methods crucial for studying these processes. I am going to overview the x-ray spectroscopy techniques used in our laboratory for studying a variety of objects ranging from enzymes such as photosystem II, which catalyzes the fundamental biosphere-sustaining process of light-induced water splitting, to the application of X-ray spectroscopy for situ studies of battery cells. To highlight the sensitivity and powerfulness of the technique I will also present the results of our recent study conducted on the heme-dependent enzyme indoleamine (IDO). IDO is a crucial component to the metabolism of the amino acids suppressing the immune system which makes it a key for disease treatment, in particular cancer. To date the mechanisms of the underlying process are not completely understood, specifically, the spin states of iron and the mechanics of its formation are ambiguous. For determining the spin states of the associated metalloenzyme reaction we collected the Fe Kβ main line emission from crystals of the IDO using a specially designed von Hamos spectrometer.