Table 1.

Protonation of histidines in sperm whale Mb at pH 6.2

Residue	Tautomer	Q	pKa	pKb	pKc
12	ɛ	0.70	—	6.38	6.29
	δ	0.49	6.28	6.39	6.26
24	ɛ	0.00	—	—	—
	δ	1.00	—	—	—
36	ɛ	0.76	7.97	8.18	8.06
	δ	1.00	—	—	7.91
48	ɛ	0.73	6.73	5.48	5.25
	δ	0.96	—	5.52	5.30
64	ɛ	0.00	—	—	<5
	δ	0.94	—	—	<5
81	ɛ	0.88	6.17	6.73	6.68
	δ	0.45	—	6.69	6.53
82	ɛ	0.95	—	<4.8	<5
	δ	0.25	—	<4.8	<5
93	ɛ	0.43	—	—	<5
	δ	0.86	—	—	<5
97	ɛ	1.00	—	—	5.63
	δ	0.29	—	—	—
113	ɛ	0.26	5.49	≈5	5.44
	δ	1.00	—	≈5	5.36
116	ɛ	0.06	6.55	6.60	6.49
	δ	1.00	—	6.59	6.50
119	ɛ	0.64	—	6.09	6.13
	δ	0.84	5.46	6.20	6.13

Q is the deuterium occupancy. The pK^a, pK^b, and pK^c values are derived from titration experiments using NMR by Botelho and Gurd (26), Cocco et al. (27), and Bashford et al. (28), respectively.