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The deamidation reactions of asparagine residues in alpha-helical and beta-turn secondary structural environments of peptides and proteins are reviewed. Both kinds of secondary structure tend to stabilize asparagine residues against deamidation, although the effects are not large. The effect of beta-sheet structures on asparagine stability is unclear, although simple considerations suggest a stabilization in this environment also.

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Mineralized biomaterials are promising for use in bone tissue engineering. Culturing osteogenic cells in such materials will potentially generate biological bone grafts that may even further augment bone healing. Here, we studied osteogenic differentiation of human mesenchymal stem cells (MSC) in an alginate hydrogel system where the cells were co-immobilized with alkaline phosphatase (ALP) for gradual mineralization of the microenvironment. MSC were embedded in unmodified alginate beads and alginate beads mineralized with ALP to generate a polymer/hydroxyapatite scaffold mimicking the composition of bone. The initial scaffold mineralization induced further mineralization of the beads with nanosized particles, and scanning electron micrographs demonstrated presence of collagen in the mineralized and unmineralized alginate beads cultured in osteogenic medium. Cells in both types of beads sustained high viability and metabolic activity for the duration of the study (21 days) as evaluated by live/dead staining and alamar blue assay. MSC in beads induced to differentiate in osteogenic direction expressed higher mRNA levels of osteoblast-specific genes (RUNX2, COL1AI, SP7, BGLAP) than MSC in traditional cell cultures. Furthermore, cells differentiated in beads expressed both sclerostin (SOST) and dental matrix protein-1 (DMP1), markers for late osteoblasts/osteocytes. In conclusion, Both ALP-modified and unmodified alginate beads provide an environment that enhance osteogenic differentiation compared with traditional 2D culture. Also, the ALP-modified alginate beads showed profound mineralization and thus have the potential to serve as a bone substitute in tissue engineering.

Structural biocomposites found in nature often have a well-defined organization on the nanometer scale. For mineralized materials, interactions between organic and inorganic phases are important for controlling crystal size, morphology, and spatial arrangement, which is a requirement when structural biomaterials are designed. In this paper, we studied influence of low concentrations of alginate on calcium carbonate crystallization by seeded and unseeded experiments, at controlled activity-based supersaturations. Crystal growth and nucleation were characterized by scanning electron microscopy (SEM), calcium concentration measurements, and crystal volume distribution measurements through the crystallization experiments. Alginate concentrations as low as 10 ppm were found to have a significant effect on growth of vaterite seeds, resulting in decreased growth rates and extensive agglomeration, compared to the case without alginate. For increased alginate concentrations (100 and 200 ppm), vaterite seed growth rates were decreased further. The decreased growth rates were probably caused by adsorption of alginate onto the active growth sites of the crystal surface. Alginate with 65% G-units (HighG) reduced the growth rate more than alginate with 43% G-units (LowG), which may be accounted for by the greater G-block length, and thus higher affinity to calcium, in HighG alginate. The unseeded experiments showed that mainly small vaterite crystals nucleated with 100 ppm alginate present, after an induction time of 50-80 min, while large calcite crystals were formed after some time by transformation from vaterite. The decreased crystal growth rates and higher nucleation rates caused by increased concentrations of alginate explain how small size mineral particles can be formed in alginate gel networks to form nanostructured composite materials.