title: Calcium-induced changes to the molecular conformation and aggregate structure of beta-casein at the air-water interface.
abstract: The influence of calcium on interactions of beta-casein at the air-water interface has been studied by several techniques, including interfacial rheology, atomic force microscopy (AFM), infrared reflectance-absorbance spectroscopy (IRRAS), and zeta potential measurements. In the absence of calcium, a weak interfacial gel forms after about 2.5 h. Also in the absence of calcium, the adsorbed beta-casein film exhibits some degree of both intra- and intermolecular structural organization. For example, IRRAS spectra show a measurable amount of alpha-helix content, and AFM images indicate the presence of interfacial aggregates with a characteristic lateral length scale of 20-30 nm, which we interpret as hemimicelles. Upon the addition of calcium, particularly at Ca:beta-casein molar ratios above approximately 5:1, a stronger interfacial gel forms more quickly; for example, the interfacial shear moduli increase twice as rapidly. Also under these conditions (5:1 Ca:beta-casein ratio) there is little evidence of structural organization; i.e., the alpha-helix peaks are very weak, and AFM images show a disordered, but continuous film, without distinct hemimicelles. On the basis of these findings, we hypothesize that calcium binding destabilizes the coupled intra- and intermolecular structural organization, and that the loss of organization permits more rapid interfacial gelation. These phenomena are characteristic of the air-water interface; they are not accompanied by analogous structural changes in bulk solution.
authors.affiliation: Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309, USA.
authors.forename: Christina R
authors.initials: CR
authors.lastname: Vessely
authors.forename: John F
authors.initials: JF
authors.lastname: Carpenter
authors.forename: Daniel K
authors.initials: DK
authors.lastname: Schwartz
mesh_terms: D000327:Adsorption; D000388:Air; D001672:Biocompatible Materials / Q000737:chemistry*; D002118:Calcium / Q000737:chemistry* / Q000378:metabolism; D002138:Calibration; D002364:Caseins / Q000737:chemistry*; D002942:Circular Dichroism; D055666:Lipopeptides; D046911:Macromolecular Substances / Q000737:chemistry; D008823:Micelles; D018625:Microscopy, Atomic Force; D008968:Molecular Conformation; D010456:Peptides, Cyclic / Q000737:chemistry; D011312:Pressure; D011485:Protein Binding; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D012212:Rheology; D013050:Spectrometry, Fluorescence; D013053:Spectrophotometry; D013055:Spectrophotometry, Infrared; D013499:Surface Properties; D013696:Temperature; D013997:Time Factors; D014466:Ultraviolet Rays; D014867:Water / Q000737:chemistry*
chemical_list: D001672:Biocompatible Materials; D002364:Caseins; D055666:Lipopeptides; D046911:Macromolecular Substances; D008823:Micelles; D010456:Peptides, Cyclic; D014867:Water; C009365:surfactin peptide; D002118:Calcium