Table I.

pK_a Values of αS in the Absence and Presence of Salt^a

No salt					150 mM NaCl
Site	pKa	n	δlow	δhigh	pKa	n	δlow	δhigh	Resonance
Asp2	3.61 ± 0.05	1.30 ± 0.21	38.64 ± 0.09	41.99 ± 0.06	3.63 ± 0.04	1.00 ± 0.08	122.81 ± 0.05	124.92 ± 0.03	Cβ/Nb
Glu13c	4.21 ± 0.06	0.90 ± 0.08	33.11 ± 0.08	36.79 ± 0.08	4.32 ± 0.09	1.42 ± 0.35	32.85 ± 0.14	36.24 ± 0.16	Cγ
Glu20	4.07 ± 0.04	0.93 ± 0.06	33.10 ± 0.05	36.74 ± 0.04	4.24 ± 0.04	0.90 ± 0.08	32.77 ± 0.19	36.49 ± 0.18	Cγ
Glu28	4.20 ± 0.06	0.96 ± 0.09	33.16 ± 0.08	36.92 ± 0.08	4.27 ± 0.04	0.85 ± 0.06	32.77 ± 0.06	36.50 ± 0.06	Cγ
Glu35d	4.17±0.06	0.93 ± 0.08	33.11 ± 0.07	36.69 ± 0.07	4.08 ± 0.13	0.88 ± 0.20	30.42 ± 0.41	36.39 ± 0.29	Cγ
Glu46c	4.21±0.06	0.90 ± 0.08	33.11 ± 0.08	36.79 ± 0.08	4.31 ± 0.10	0.86 ± 0.15	32.99 ± 0.18	36.39 ± 0.13	Cγ
His50	6.78 ± 0.04	1.08 ± 0.08	123.11 ± 0.02	125.05 ± 0.06	6.52 ± 0.07	1.52 ± 0.37	123.45 ± 0.02	125.05 ± 0.05	N
Glu57	4.20 ± 0.05	0.94 ± 0.08	33.10 ± 0.07	36.87 ± 0.07	4.33 ± 0.09	0.76 ± 0.11	32.89 ± 0.13	36.49 ± 0.13	Cγ
Glu61	4.04 ± 0.27	0.86 ± 0.35	121.05 ± 0.07	121.70 ± 0.08	3.97 ± 0.06	1.12 ± 0.22	121.35 ± 0.04	122.07 ± 0.02	N
Glu83	4.35 ± 0.05	1.07 ± 0.10	33.14 ± 0.06	36.71 ± 0.07	4.33 ± 0.03	0.91 ± 0.05	32.70 ± 0.05	36.32 ± 0.05	Cγ
Asp98	3.68 ± 0.06	0.87 ± 0.11	38.23 ± 0.10	41.56 ± 0.07	4.00 ± 0.03	1.44 ± 0.16	38.60 ± 0.05	41.01 ± 0.04	Cβ
Glu104	4.39 ± 0.13	1.12 ± 0.26	33.13 ± 0.15	36.74 ± 0.17	4.51 ± 0.11	1.01 ± 0.17	33.02 ± 0.12	36.36 ± 0.14	Cγ
Glu105c	4.21 ± 0.06	0.90 ± 0.08	33.10 ± 0.08	36.79 ± 0.08	4.49 ± 0.17	1.04 ± 0.35	33.85 ± 0.18	36.33 ± 0.20	Cγ
Glu110d	4.50 ± 0.09	0.98 ± 0.19	34.34 ± 0.06	36.71 ± 0.08	4.31 ± 0.07	0.85 ± 0.09	32.65 ± 0.12	36.36 ± 0.10	Cγ
Glu114	4.70 ± 0.05	0.86 ± 0.08	33.09 ± 0.06	36.97 ± 0.09	4.49 ± 0.09	0.64 ± 0.08	32.59 ± 0.13	36.58 ± 0.14	Cγ
Asp115	4.15 ± 0.05	0.76 ± 0.06	38.18 ± 0.06	41.69 ± 0.05	4.18 ± 0.09	0.94 ± 0.16	38.10 ± 0.12	41.11 ± 0.11	Cβ
Asp119	4.02 ± 0.07	0.69 ± 0.06	122.85 ± 0.08	125.87 ± 0.06	3.99 ± 0.02	0.77 ± 0.03	123.27 ± 0.03	126.20 ± 0.02	N
Asp121	4.62 ± 0.06	0.97 ± 0.08	39.11 ± 0.03	41.43 ± 0.06	4.39 ± 0.03	0.78 ± 0.04	117.42 ± 0.03	119.36 ± 0.03	Cβ/Nb
Glu123	4.87 ± 0.05	0.77 ± 0.06	33.16 ± 0.05	36.95 ± 0.08	4.69 ± 0.10	0.78 ± 0.12	32.69 ± 0.13	36.46 ± 0.16	Cγ
Glu126	4.89 ± 0.06	0.97 ± 0.18	32.53 ± 0.08	36.72 ± 0.14	4.55 ± 0.10	0.91 ± 0.16	32.03 ± 0.16	36.12 ± 0.19	Cγ
Glu130	4.53 ± 0.06	0.85 ± 0.09	33.09 ± 0.09	36.82 ± 0.09	4.40 ± 0.09	1.09 ± 0.22	33.05 ± 0.14	36.38 ± 0.15	Cγ
Glu131	4.58 ± 0.05	0.93 ± 0.09	33.26 ± 0.06	36.84 ± 0.07	4.71 ± 0.15	0.61 ± 0.12	32.80 ± 0.22	37.09 ± 0.27	Cγ
Asp135	4.16 ± 0.07	0.70 ± 0.06	38.18 ± 0.08	41.74 ± 0.08	4.05 ± 0.06	0.77 ± 0.07	37.88 ± 0.09	41.16 ± 0.07	Cβ
Glu137	4.82 ± 0.16	0.56 ± 0.11	123.36 ± 0.11	125.77 ± 0.17	4.42 ± 0.03	0.92 ± 0.05	123.88 ± 0.02	125.56 ± 0.02	N
Glu139	4.69 ± 0.04	0.97 ± 0.10	33.14 ± 0.06	36.99 ± 0.08	4.54 ± 0.06	0.84 ± 0.09	32.72 ± 0.09	36.62 ± 0.11	Cγ
α-CO2−	3.65 ± 0.03	1.10 ± 0.07	8.537 ± 0.006	8.081 ± 0.003	3.67 ± 0.03	1.01 ± 0.06	8.541 ± 0.007	8.087 ± 0.005	HN(A140)

^aAll values are for a temperature of 10°C.

^bThe pK_a values of Asp2 and Asp121 were determined from Cβ chemical shifts in the absence of salt and ¹⁵N chemical shifts in the presence of 150 mM salt.

^cGlu13, Glu46 and Glu105 titrate as a single unresolved group and the three residues are assigned the same parameters in the absence of salt. The pH titration of the three residues can be resolved in the presence of 150 mM NaCl.

^dThe assignments for Glu35 and Glu110 are tentative due to spectral overlap and may be reversed.