Figure 2.

The His62 and His294 MD densities agree with the crystallographic density when the residues are protonated at the ɛ atom, but disagree when they are doubly protonated. Coordinates of S are shown as sticks with 2F _o − F _c density in blue (1σ isosurface) and the total density from a 90–100 ns segment of a 200 kJ mol⁻¹ nm⁻² MD simulation in pink (1σ isosurface). (a) His62: MD density from a doubly protonated (HIP) simulation. (b) His62: MD density from simulating with histidine singly protonated (on the ɛ N atom; HIE). (c) His294: MD density from a doubly protonated (HIP) simulation. (d) His294: MD density from simulating with histidine singly protonated (on the ɛ N atom; HIE).