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During my PhD studies at the ETH Zurich and the Paul Scherrer Institute I worked in the field of time-resolved serial femtosecond X-ray crystallography (TR-SFX), an emerging structural biology method that is performed at X-ray free-electron lasers (XFELs). The first advantage of XFELs is their provision of ultrafast 10-100 fs pulses, which allow radiation damage effects to be avoided, and obtaining structures from data collected at room temperature. The second advantage of XFELs comes upon installing a pump laser that induces protein conformational changes and allows obtaining crystallographic datasets at different times after protein activation. The protein that was subject to the study, Krokinobacter eikastus rhodopsin 2 (KR2), is the first discovered light-driven sodium pump. It was intriguing to elucidate the protein’s new sodium-pumping mechanism. It had been long debated if such a non-H+ cation pump could exist, because Na+ was supposed to be electrostatically repulsed from a positively charged retinal chromophore Schiff base. Due to the fact that outward pumping of Na+ silences neuronal signalling, KR2 is considered a next-generation tool for optogenetics.