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The Dissociation Constants of Tetrahydrofolic Acid

From the ¹ From the Graduate Department of Biochemistry, Brandeis University, Waltham, Massachusetts 02154

Six dissociable groups exist in tetrahydrofolic acid in the range H₀ -3.0 to pH 14. The assignments in tetrahydrofolic acid by titrimetric and spectrophotometric comparisons with model compounds, p-aminobenzoylglutamic acid and 2-amino-6,7-dimethyl-5,6,7,8-tetrahydro-4-pteridinone, are as follows: acidic ionization of amide, pK' 10.5; N₅, pK' 4.82; -carboxyl, pK' 4.8; -carboxyl, pK' 3.5; N₁, pK' 1.24; and N₁₀, pK' -1.25. These pK' values, with the exception of those of the carboxyl groups, were determined directly for tetrahydrofolic acid at 22–25° and ionic strength 1.0.

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