Spin cascade and doming in ferric hemes: Femtosecond X-ray absorption and X-ray emission studies

The structure-function relationship in central to biology, while the structural dynamics are driven by electronic changes. Doming of ferrous heme proteins, which is central to the respiratory function of hemoglobin, ensues from populating high-spin states. However, for ferric heme proteins, doming was excluded. Here, we show that high-spin states are populated in photoexcited ferric cytochrome c, and we present evidence for doming. We also conclude that photo- or thermally activated doming occurs in a wide variety of ferric heme proteins, calling for a deeper understanding of its role in their respective functions.