. Author manuscript; available in PMC: 2013 Jun 14.

Published in final edited form as: J Phys Chem B. 2012 Feb 29;116(23):6665–6669. doi: 10.1021/jp211196r

Table 1.

Values of the chemical shift difference, Δ^ξ

Δ^ξ^a	Canonical^b (ppm)	DFT-Computed^c (ppm)	Observed (ppm)
Δ^δ	82	83	79^d; 77^e; 76^f
Δ^ε	82	94	92^d; 83^e; 83^f

Δ^ξ = ‖ζ_15Nδ1|−|ζ_15Nε2‖ with ζ denoting the average-shielding (σ) DFT-computed value, shown in the Figure Caption of Figure 1a–b, or alternatively, the observed chemical shift (δ) for each of the imidazole ring nitrogens (13); and ξ = δ or ε denoting each of the tautomeric forms of the imidazole ring of His, namely, the N^δ1-H and N^ε2-H form, respectively.

Using ¹⁵N chemical shift limiting values listed by Pelton et al. (13).

From average DFT-computed ¹⁵N shielding values (see Figures 1a–b);

From solid-state NMR, at pH 8.5, from Hu et al. (26);

From Farr-Jones et al.(12) for histidine at −55 °C at pH 11.2 in Ethanol;

From Farr-Jones et al.(12) for N^ξ-methylhistidine at 25 °C in H₂O.