Because of interference from the pH-dependent behavior of nearby groups in the active site of Escherichia coli thioredoxin, the pKa of the buried carboxyl group of the aspartic acid at position 26 has been difficult to quantitate. We report a direct measurement of this pKa using an NMR method utilizing the correlation between the C beta H proton resonances and the 13CO of the titrating carboxyl group. The experiments show unequivocally that the pKa is 7.3-7.5, rather than the value of 9 or greater recently proposed by Wilson, N. A., et al. [(1995) Biochemistry 34, 8931-8939]. The assignment of the titrating resonances to Asp 26 is unambiguous: the values of the C beta H chemical shifts correspond exactly to those of Asp 26, and their titration in the pH range 5.7-10.0 is the same as that observed previously for the proton resonances alone. In addition, the chemical shift of the carboxyl 13C resonance at pH 5.7 is upfield of those of the other carboxyl and carboxamide resonances, diagnostic for a protonated carboxyl group. The resonances assigned to Asp 26 are the only ones that titrate in the pH range 5.7-10.5. None of the other aspartate and glutamate residues in the molecule are titrated in this pH range, consistent with their positions on the surface of the molecule. The pKa measured for Asp 26 in reduced thioredoxin is identical within experimental error to that measured in the oxidized form of the protein. This is significant for the reductive mechanism of thioredoxin: the buried salt bridged/hydrogen-bonded side chains of Asp 26 and Lys 57 are likely to contribute to the facility of the reaction by providing a convenient source and sink for protons in the hydrophobic environment of the complex between thioredoxin and its substrates.