Effects of the environmental factors on the casein micelle structure studied by cryo transmission electron microscopy and small-angle x-ray scattering/ultrasmall-angle x-ray scattering.
Casein micelles are colloidal protein-calcium-transport complexes whose structure has not been unequivocally elucidated. This study used small-angle x-ray scattering (SAXS) and ultrasmall angle x-ray scattering (USAXS) as well as cryo transmission electron microscopy (cryo-TEM) to provide fine structural details on their structure. Cryo-TEM observations of native casein micelles fractionated by differential centrifugation showed that colloidal calcium phosphate appeared as nanoclusters with a diameter of about 2.5 nm. They were uniformly distributed in a homogeneous tangled web of caseins and were primarily responsible for the intensity distribution in the SAXS profiles at the highest q vectors corresponding to the internal structure of the casein micelles. A specific demineralization of casein micelles by decreasing the pH from 6.7 to 5.2 resulted in a reduced granular aspect of the micelles observed by cryo-TEM and the existence of a characteristic point of inflection in SAXS profiles. This supports the hypothesis that the smaller substructures detected by SAXS are colloidal calcium phosphate nanoclusters rather than putative submicelles.
Marchin
Stéphane
S
INRA, Agrocampus Rennes, UMR1253, STLO, F-35042 Rennes, France.
Putaux
Jean-Luc
JL
Pignon
Frédéric
F
Léonil
Joëlle
J
D000818:Animals; D002364:Caseins / Q000737:chemistry* / Q000648:ultrastructure*; D002417:Cattle; D003102:Colloids / Q000737:chemistry*; D020285:Cryoelectron Microscopy; D004777:Environment; D006863:Hydrogen-Ion Concentration; D008823:Micelles; D008892:Milk / Q000737:chemistry*; D049329:Nanostructures / Q000737:chemistry / Q000648:ultrastructure; D010316:Particle Size; D011487:Protein Conformation; D013696:Temperature; D014961:X-Ray Diffraction
D002364:Caseins; D003102:Colloids; D008823:Micelles