Ionization constants and thermodynamic parameters of histidine and derivatives
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1983-01-01
Autores
Boschcov, Paulo [UNIFESP]
Seidel, Wolfgang
Muradian, José
Tominaga, Mineko
Paiva, Antonio C.M. [UNIFESP]
Juliano, Luiz [UNIFESP]
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The pKa values for the proton dissociation of carboxyl, imidazolium, and ammonium groups for histidine and ten of its derivatives were determined electrometrically at seven temperatures in the range 10–40°C. The ΔH and ΔS values were estimated from the temperature dependence of the dissociation constants of histidine and its derivatives. These results and the pKa values compared in terms of inductive effect suggest an ion-dipole interaction between the protonated amino group and the unprotonated imidazole ring. The charge and the solvation effects of the neighboring groups are the main factors that determine the imidazole group pKa in histidine and its studied derivatives. The Nτ-H tautomer is favored over the Nπ-H by 1.6 kcal/mol, indicating that the inductive substituent effect at position 4 of the imidazole ring is the major component in determining this tautomeric preference.
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Bioorganic Chemistry. San Diego: Academic Press Inc Jnl-comp Subscriptions, v. 12, n. 1, p. 34-44, 1983.