Spin cascade and doming in ferric hemes

SwissFEL — Another iron dome

In biology, structure and function are closely interwoven. A case in point is oxygen transport in the lungs, which relies on ferrous heme proteins adopting dome-like shapes. Previous work has suggested, however, that no such doming takes place in their ferric counterparts, where the coordination centre is Fe(III) rather than Fe(II). A study combining femtosecond X-ray emission spectroscopy (XES) and X-ray absorption near-edge structure (XANES) now challenges that assumption. Experiments performed at SwissFEL and the European XFEL provide evidence that in the ferric form of the heme protein cytochrome c, a cascade among excited spin states of the iron ion occurs upon photoexcitation, causing in turn doming — and raising fresh questions about the structure–function relationship in these complexes.