Table 1.

pK_a values used to fit the pH-dependent unfolding/refolding kinetics

Residue	pKa, N*	pKa, I‡†	pKa, I§	No.
His-24	>3.5	6.7	6.8	1
His-36	8.5	6.7	6.8	1
His-48	6.0	6.7	6.8	1
His-64	5.5	6.7	6.8	1
His-113	6.5	6.7	6.8	1
His-119	6.5	6.7	6.8	1
Acidic I	3.4	4.0	4.0	9
Acidic II	3.4	3.5	3.6	13

All pK_a values refer to 4.5°C, the temperature of the experiments reported here. The estimated uncertainty of the histidine pK_a values is ±0.1. See text for the procedure used to fix pK_a values of the acidic residues (Asp and Glu): group I corresponds to the unfolded CDEF subdomain of the apoMb molten globule, and group II corresponds to the structured A[B]GH subdomain of the molten globule. 

^*The pK_a values of histidine residues in native apoMb are taken from table 2 of ref. 2, determined at 35°C and corrected to 4.5°C (see text). No correction has been made for the possible influence of D₂O on the histidine pK_a values determined by chemical shift titration. 

^† The average pK_a value of histidine residues in the folding transition state, determined by fitting the plot of log k_obs versus pH for wild type in citrate buffer (Fig. 2A), when the pK_a values in N and in Ib are known. 

^§ The average pK_a of the histidine residues in the molten globule, determined as shown in Fig. 1.