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. Author manuscript; available in PMC: 2021 Jan 5.
Published in final edited form as: J Am Chem Soc. 2020 Dec 16;142(52):21679–21690. doi: 10.1021/jacs.0c07914

Figure 7.

Figure 7.

(Left) Free energy landscape for the thermal hydrogen reorganization in the E4 intermediate as obtained from hybrid QM/MM metadynamics simulations. The simulation was performed using the number of hydrogen atoms coordinated to either Fe2 or Fe6, n(Fe2/Fe6), and those coordinated to either Fe3 or Fe7, n(Fe3/Fe7), are used as collective variables to describe to hydrogen location. The coordination numbers are defined with a continuous function (eq S1) that assumes the values of 0, 1, and 2 if no, one, or two hydrogens are located on given side of the Fe2/Fe3/Fe6/Fe7 face of FeMo-co: the Fe2/Fe6 side for n(Fe2/Fe6) and the Fe3/Fe7 side for n(Fe3/Fe7). For further discussion of coordination numbers, see the Supporting Information. (Right) Schematic structure of the limiting forms of E4 isomers (all S = 1/2) identified in previous studies as minima of the potential energy surface of truncated models of the active site (see ref 9). Their location on the free energy surface on the left panel is marked with corresponding letters. Note that the states C and D are not local minima on the free energy landscape, as obtained from the present QM/MM metadynaxnics computations.