Table 1.

Chemical shifts of histidine C^ɛ1—H protons

Sample	Imidazolium ion, ppm	Imidazole form, ppm	Δδ, ppm
α-Lytic protease*	9.22	8.15	1.07
α-Lytic protease + AAPbV†	9.35	—	NA‡
α-Lytic protease + DIFP	8.35§	7.37¶	0.98
Subtilisin BPN′97 (D99K)‖	9.18	—	—
Subtilisin BPN′97 (N155A) + AAPbF**	9.45	—	NA‡
Histidine‡‡	8.60	7.65	0.95
N-acetyl-histidine‡‡	8.40	7.45	0.95

^*13C/¹⁵N His-labeled, 30°C (35). 

^†13C/¹⁵N His (AAPbV, K_i = 6.7 × 10⁻⁹ M, (36), 30°C. 

^‡ Not applicable. Chemical shift constant over pH ≈ 3 to 10.5. 

^§(¹⁵N^δ1, ¹⁵N^ɛ2) His, 35°C. 

^¶13C/¹⁵N His, 35°C. 

^‖13C^ɛ1 His, 20 mM CaCl₂, 0.1 M NaCl, 25°C (37). 

^**13C^ɛ1 His (AAPbF, K_i = 0.1 × 10⁻⁹ M, (33) in 10 mM CaCl₂, 40 mM KCl, 25°C (33). 

^‡‡ An amount equal to 0.2 ppm subtracted to compensate for different reference (38).