Chapter 6
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 21, 2017 | http://pubs.acs.org Publication Date: August 28, 2006 | doi: 10.1021/bk-2006-0935.ch006
κ-Carrageenan Interaction with Bovine and Caprine Caseins as Shown by Sedimentation and NMR Spectroscopic Techniques Implication of Surface Charge by a Homologous Three-Dimensional Model for α -Casein: κ-Carrageenan-Casein Interaction S2
1
Harold M. Farrell, Jr. and Adela Mora-Gutierrez
2
1
2
Eastern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, 600 East Mermaid Lane, Wyndmoor, PA 19038 Cooperative Agricultural Research Center, Prairie View A & M University, Prairie View, TX 77446
The solubility and hydration characteristics of κ-carrageenan -casein systems from bovine and caprine milk with incorporated salt (NaCl) were determined by means of sedimentation and O nuclear magnetic resonance ( N M R ) experiments. Relative salt interaction parameters both for caseins alone and in mixtures with κ-carrageenan were assessed by nonlinear regression analysis from the characteristics of solubilization of the systems. The κ-carrageenan-casein interactions appear to depend largely on the ratios of κ- to α -casein and possibly α -casein. Second virial coefficients (B values) and hydration products derived from O N M R data suggest that while soluble at high salt, the caprine casein mixtures exhibit strong interactions, whereas the bovine counterparts do not. A t lower salt concentrations 17
s1
s2
o
17
© 2006 American Chemical Society
Fishman et al.; Advances in Biopolymers ACS Symposium Series; American Chemical Society: Washington, DC, 2006.
93
94 17
the solubility data and the O NMR data are in agreement. Thus, a structural dependence upon protein components in salt containing κ-carrageenan-casein solutions from bovine and caprine milk has been demonstrated. The evidence suggested a role for α -casein in the interactions observed. A homologous three dimensional model for α -casein was developed to test this hypothesis. The model was produced by the use of a template model derived from a crystal structure of a human chloride intracellular channel (CLIC)-1 and demonstrates a large positively charged surface potential for interactions with the negatively charged the negatively charged κ-carregeenan. S2
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 21, 2017 | http://pubs.acs.org Publication Date: August 28, 2006 | doi: 10.1021/bk-2006-0935.ch006
S2
17
Key Words: solubility; NMR, 0; salt binding; water binding; κ-carrageenan; bovine casein; caprine casein; caprine oisi-casein; bovine a -casein; molecular modeling. Abbreviation Key: NMR = Nuclear magnetic resonance s2
One of the most important properties of polysaccharide hydrocolloids in food systems (e.g., carrageenans, dextrans, starches) is their ability to complex protein to form modified food structures. In model systems complex formation has been observed, and although both polymers carry a net negative charge the interaction has been generally recognized to be electrostatic in nature {1-3). In milk systems κ-carrageenan is an important determiner of sensory texture, Theological properties and functional properties {4, 5). Salt cations and/or anions may not only affect protein-hydrocolloid electrostatic interactions, but may also alter water binding in carrageenan systems (6). Snoeren et al. (7) demonstrated that, at the pH and ionic strength prevailing in milk, it is mainly the casein micelles (and perhaps κ-casein in particular) that are involved in κcarrageenan-protein interactions. The amino acid sequence of the κ-casein molecule suggests that in addition to the highly negatively charged macropeptide it has also areas of 'net' positive charge, which have been speculated to be on the surface of the casein micelle (8). Such an accumulation of positive charges is thought to be lacking in a and β-casein, where positive and negative amino acid side chains appear to be evenly distributed along the polypeptide chain (9, 10). Of all the protein fractions in milk, κ-casein is the most reactive through normal food processing (//). s r
Caprine caseins, in contrast to bovine caseins vary considerably in the types of casein present, some are poor in -casein and some arericherin a -casein (12). The a^-casein has a primary structure quite different than the a i - and β-caseins. In a linear array the a -casein displays a cluster of 'net' positive s2
s
s2
Fishman et al.; Advances in Biopolymers ACS Symposium Series; American Chemical Society: Washington, DC, 2006.
95 charge for residues 170 through 207 at the C-terminal end (13). Physical chemical studies of a -casein by Snoeren et al. (14) suggested a model in which this positively charged tail participates in the isodesmic self-association of the protein, this argues for a surface position for the positively charged cluster. Thus, caprine caseins rich in a -casein may offer enhanced sites for interactions with κ-carrageenan, provided that this positive cluster is on the surface in associated whole casein. Moreover, sodium casemates from bovine and caprine milks containing varying casein components have not been characterized comparatively relative to their interaction with κ-carrageenan. This is especially true with regard to the elevated a -casein content of some caprine milks. Therefore, the objective of this work is to examine the effect of NaCl on solubility and the hydration behavior of bovine casein and two caprine caseins of known casein distribution following complex formation with κ-carrageenan in the absence of calcium ions by use of sedimentation and 0 nuclear magnetic resonance (NMR) techniques. s2
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 21, 2017 | http://pubs.acs.org Publication Date: August 28, 2006 | doi: 10.1021/bk-2006-0935.ch006
s2
s2
17
Materials and Methods
Materials All reagents used were of analytical grade or 'ACS certified' from Aldrich, Baker, Sigma (St. Louis, MO), κ-carrageenan was obtained from Sigma Chemical Co. 99.8% deuterium oxide (D 0) was obtained from Sigma (St. Louis, MO). κ-carrageenan was exhaustively dialyzed against deionized water, which had been adjusted to pH 7.0 with 0.5 M sodium hydroxide, and then lyophilized 2
Preparation of Bovine and Caprine Caseins Bovine casein was obtained from the milk of a Jersey cow. The caprine caseins characterized by high and low content of the
