Figure 6.

Scheme used to assess the effect of fast exchange averaging on histidine–amide H-bonding and histidine ¹⁵N chemical shifts. The histidine can adopt an H-bonded state in which ^2hJ_NN is active. The open states are characterized by a ^2hJ_NN of 0 and partition according to sample pH and microscopic pK_a values. Limiting ¹⁵Nδ1 and ¹⁵Nε2 chemical shifts^38,47 are listed in parentheses. The limiting ¹⁵N chemical shifts for the H-bonded state are estimates based on globins with large Δ¹⁵N values and low apparent histidine pK_a values.