How to Squeeze a Sponge Casein Micelles under Osmotic Stress, a SAXS Study

By combining the osmotic stress technique with small angle x ray scattering measurements we followed the structural response of the casein micelle to an overall increase in concentration When the aqueous phase that separates the micelles is extracted they behave as polydisperse repelling spheres and their internal structure is not affected When they are compressed the micelles lose water and shrink to a smaller volume Our results indicate that this compression is non affine i e some parts of the micelle collapse whereas other parts resist deformation We suggest that this behavior is consistent with a spongelike casein micelle having a triple hierarchical structure The lowest level of the structure consists of the CaP nanoclusters that serve as anchors for the casein molecules The intermediate level consists of 10 to 40 nm hard regions that resist compression and contain the nanoclusters Those regions are connected and/or partially merged with each other thus forming a continuous and porous material The third level of structure is the casein micelle itself with an average size of 100 nm In our view such a structure is consistent with the observation of 10 to 20 nm casein particles in the Golgi vesicles of lactating cells upon aggregation those particles would rearrange fuse and/or swell to form the spongelike micelle