Protein solubility, secondary structure and microstructure changes in two types of undenatured type II collagen under different gastrointestinal digestion conditions.Sodium dodecyl sulfonate polyacrylamide gel electrophoresis (SDS-PAGE), circular dichroism spectroscopy (CD), transmission electron microscope (TEM) and atomic force microscope (AFM) were used to analyze the structural properties and gastrointestinal digestive characteristics of natural insoluble undenatured type II collagen (IC II) and pepsin soluble undenatured type II collagen (SC II). 
SDS-PAGE
Protein solubility, secondary structure and microstructure changes in two types of undenatured type II collagen under different gastrointestinal digestion conditions.Sodium dodecyl sulfonate polyacrylamide gel electrophoresis (SDS-PAGE), circular dichroism spectroscopy (CD), transmission electron microscope (TEM) and atomic force microscope (AFM) were used to analyze the structural properties and gastrointestinal digestive characteristics of natural insoluble undenatured type II collagen (IC II) and pepsin soluble undenatured type II collagen (SC II).  Results showed that, after gastric digestion, especially under low pH conditions, some thick and short collagen fibers were dissolved from IC II, which was accompanied by the release of soluble protein with triple helix structure.  As to SC II, when gastric digestion was performed under higher pH conditions, collagen fibers were mildly thinned and curved, and the triple helix structure was slightly destroyed. 
CD
Protein solubility, secondary structure and microstructure changes in two types of undenatured type II collagen under different gastrointestinal digestion conditions.Sodium dodecyl sulfonate polyacrylamide gel electrophoresis (SDS-PAGE), circular dichroism spectroscopy (CD), transmission electron microscope (TEM) and atomic force microscope (AFM) were used to analyze the structural properties and gastrointestinal digestive characteristics of natural insoluble undenatured type II collagen (IC II) and pepsin soluble undenatured type II collagen (SC II). 
TEM
Protein solubility, secondary structure and microstructure changes in two types of undenatured type II collagen under different gastrointestinal digestion conditions.Sodium dodecyl sulfonate polyacrylamide gel electrophoresis (SDS-PAGE), circular dichroism spectroscopy (CD), transmission electron microscope (TEM) and atomic force microscope (AFM) were used to analyze the structural properties and gastrointestinal digestive characteristics of natural insoluble undenatured type II collagen (IC II) and pepsin soluble undenatured type II collagen (SC II). 
AFM