Histidine rotation

Classical simulations of MbCO using the CHARMM force field were performed for different tautomerization states of the distal histidine residue (His64) [33], These simulations showed that when His64 is protonated at N,5 (denoted the tautomer) it often rotates such that it exposes either the N,>—H bond or the un-protonated N atom to the CO, as depicted in Scheme 3.4. We... 

In proteins, the position of the axial histidine ring determines the inequivalence of protons in the porphyrin plane. Probably as a result of this, a variety of patterns is observed with either one or two of the four methyl substituents of protoporphyrin IX being downfield [2,27,59-62]. The orientation of the Xx and Xy axes in the porphyrin plane depends on the orientation of the axial Tt interaction, with a behavior analogous to the low spin Fe(III) case, but with the x and y axes interchanged. In other words, when the Tt interaction is along a metal-pyrrole nitrogen direction, it defines the Xy and not the Xx direction. When the Tt interaction rotates... 

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