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Calorimetric determination of the enthalpy of ionization of oxidized and reduced horse heart cytochrome c
Author(s) -
Marini M. A.,
Martin C. J.,
Berger R. L.
Publication year - 1980
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.1980.360190413
Subject(s) - chemistry , titration , potentiometric titration , calorimetry , enthalpy , titration curve , amperometric titration , redox titration , thermochemistry , analytical chemistry (journal) , inorganic chemistry , thermodynamics , chromatography , physics , electrode
The heats of ionization of protons, Δ H i , of oxidized and reduced horse heart cytochrome c in 0.15 M KCl at 20°C were determined using a titration calorimeter which simultaneously afforded the potentiometric titration curve. Reproducibility of the thermal titrations is within 2%, and evaluation of the heats observed after the heat loss corrections is estimated to be within 5%. A single titration of oxidized cytochrome c from pH 11 to 3 is in excellent agreement with the thermal titration of this protein obtained with flow calorimetry. The thermal titration, however, is not reversible, due in part to the loss of titratable group(s) in this pH region and to the heat contribution of the acid and alkaline conformational changes which occur. Although of lesser magnitude, the reduced form also indicates similar thermal transitions. These differences are due solely to conformational contributions to the thermal process, since the potentiometric curves are reversible. The nature of the irreversibility for oxidized cytochrome c appears to involve the loss of a group with p K ′ 8.9 and the shift of two groups from p K ′ 5.6 to 4.8. Thermal difference curves for this process indicate that heats of −7.8 and −24.1 kcal/mol are liberated which are centered at pH 9.3 and 3.9, respectively.