Electron density maps histidine side chain

There is also a conformational ambiguity characteristic of the histidine side chain as it is built into electron density maps The two possible orientations about the Cfi-Cy bond are indistinguishable, so the crystal-lographer must select one of these orientations based on chemical intuition [e.g., the availability of hydrogen bond interaction(s) in one particular orientation and not the other]. Additionally, the proper histidine tautomer is likewise selected by chemical intuition. Reynolds et al. (1973) demonstrated that the Ne-H tautomer is the predominant form of free histidine in solution, and these investigators showed that the ratio Ne-H tautomer/N8-H tautomer is approximately 80% 20% (Fig. 13). 

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