Based on HERFD-XAS and UV-vis spectroscopy, a histidyl intermediate was claimed to be characterized as an open-shell spin singlet, 41 although it is unclear how the spin state was determined. The histidyl intermediate was only characterized for LsAA9 and we find the spin-state splitting for 3 in LsAA9 to be small, but somewhat functional dependent: the splitting is only 5 kJ/mol with TPPS, the open-shell singlet being most stable. With the B3LYP functional we obtain a splitting of 18 kJ/mol with the triplet being most stable. For TaAA9 the open-shell singlet calculations converged into a closed-shell singlet for 3, while we obtain both triplet and open-shell singlet states for the conformer 3 ′ ; in this case the open-shell singlet and triplet are essentially degenerate (the triplet is only 3 kJ/mol more stable with both TPSS and B3LYP). In light of the results for the tyrosyl radical (see previous subsection), it is likely that small differences in the active site architectures between TaAA9 and LsAA9 lead to small differences in spin-state splittings. Since the splittings are small, this can also lead to differences in which spin states are most stable. However, with the present accuracy of the used functionals, the splittings are generally too small to clearly differentiate the spin states.