Determination of Calcium Binding to Beta-Casein at Various Ph Values Using the Calcium Murexide Method
Wei, Tsao-Ming
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https://hdl.handle.net/2142/70088
Description
Title
Determination of Calcium Binding to Beta-Casein at Various Ph Values Using the Calcium Murexide Method
Author(s)
Wei, Tsao-Ming
Issue Date
1986
Department of Study
Food Science
Discipline
Food Science
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Agriculture, Food Science and Technology
Abstract
Calcium binding to casein affects the formation of casein micelles and the behavior of the casein proteins during processing. Since calcium binding is pH dependent, it is important to understand the binding properties of the non-micellar caseins with calcium ions under the influence of pH. The calcium murexide method which is based on the dissociation of calcium salt of murexide was found to be useful for the calcium binding study in a casein system of known composition.
The results reported in this thesis show that the absorption spectra of calcium murexide solutions were pH dependent while that of murexide solutions were pH independent; thus a shift in the maximum absorption occured towards higher wavelength for calcium murexide complex without any change in the maximum absorption at 520 nm for murexide ion alone as the pH was increased. The spectra recorded in the pH range from 6.25 to 8.5 showed a common intersection, the isosbestic point. The linear relationship between the absorbance at the isosbestic point and the absorbance maxima in the absence and presence of excess calcium ion was established for the pH range from 6.25 to 8.5.
From the study, the apparent dissociation constant, K(,d)', of calcium murexide at constant ionic strength ((mu) = 0.1) and 2(DEGREES)C could be evaluated and it was found to be a quadratic function of the calcium ion concentration in the pH range from 6.25 to 8.5. Therefore, the free calcium ion concentration can be determined by the calcium murexide method in this pH range.
Studies of calcium binding by beta-casein (1%) in calcium-potassium chloride solutions at various pH values, (mu) = 0.1, and 2(DEGREES)C were made by employing the calcium murexide procedure. It was found that the calcium murexide method can be used for determining free calcium and calcium bound to beta-casein in the pH range from 6.25 to 8.5. The isotherm for calcium binding to beta-casein indicates that the casein-bound calcium not only increases with increasing ionic calcium concentration but also with increasing pH. The calculated number of binding sites per molecule are presented. For (mu) = 0.1 and 2(DEGREES)C at various pH values, it was found that beta-casein has only one type of binding site for calcium and the free carboxyl groups of the dicarboxylic acids are the most likely sites of calcium binding. The result indicates that the phosphoseryl groups may be buried within the core of the beta-casein molecule, or they are already saturated at minimum calcium concentration which remains to be found. (Abstract shortened with permission of author.)
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