Collagen, as a thermal-sensitive protein, is the most abundant structural protein in animals. Native collagen has been widely applied in various fields due to its specific physicochemical and biological properties. The beneficial properties would disappear with the collapse of the unique triple helical structure during heating. Understanding thermal stability of collagen is of great significance for practical applications. Previous studies have shown the thermal stability would be affected by the different sources, extraction methods, solvent systems in vitro and modified methods. Accordingly, the factors affecting thermal stability of collagen are discussed in detail in this review.

The economic and sustainable use of fish-processing by-products has received considerable attention. Large amounts of fish skin discarded by industry processes can be a source of collagen extraction. This study developed a method to extract collagen with high purity and yield from Southern catfish skins through improved pretreatment methods. Multiple degreasing methods effectively removed fat from fish skins and obtained a maximum degreasing rate (90.24%). The results of electrophoretic, hydroxyproline-content, and extraction-rate analyses revealed that the collagen extracted from defatted skin, which decoloured in 0.5% hydrogen peroxide solution (pH 10), exhibited higher purity and the highest yield (23.14% wet weight and 78.57% dry weight, respectively) as compared with other decolouration conditions. The histological appearance of pretreated fish skin indicated that the non-collagenous substances were removed effectively and the fibres loosened. Amino acid analysis, ultraviolet spectra, Fourier transform infrared (FTIR) spectroscopy, and thermal-stability analysis indicated that the collagen isolated from fish skin under optimal pretreated conditions was classified as type I. Our method for the optimal pretreatment of collagen extracted from fish skin following degreasing and decolouring procedures resulted in improved collagen purity and yield.

The thermal stability of collagen has an important effect on its practical applications. Many believe that hydroxyproline (Hyp) improves the structural stability of collagen molecules. In this study, for the first time, a method of building natural collagen molecular models was described. We constructed a collagen model with typical triple-helix structure and calculated the hydrogen bond energy between collagen α chains. The calculated hydrogen bond energy was consistent with the experimental results of differential scanning calorimetry. After the calculation simulation, we verified that the hydrogen bond energy between collagen chains was positively correlated with Hyp content in the models and an increased Hyp content in the model was beneficial in improving the thermal resistance of the structure. In addition, we found that thermal unfolding did not occur simultaneously along the entire molecule but started in the regions with less Hyp content. This study provides a collagen model with a natural collagen amino acid sequence, which will be helpful for further investigation of the physical and chemical properties of natural collagen.

Collagen self-assembly and simultaneous mineralization by incubating a mixture containing collagen, calcium, and phosphate ions under physiological conditions, is an effective method to prepare bone-like biomimetic materials. The formation of fibrils and minerals is related to pH of system. In the present work, we evaluated the effect of pH (7.9-10.4) on biomineralization process and synthesized composites. Turbidity kinetics and X-ray diffraction (XRD) measurements revealed that increasing pH delayed the crystallization process from nucleation phase to plateau phase because of promoting chelation of Ca 2+ with collagen. Typical peaks of phosphate in Fourier transform infrared spectroscopy coupling with characteristic peaks of hydroxyapatite (HAp) in XRD spectra illustrated the formation of HAp after biomineralization. Scanning electron microscopy measurements indicated that the increase of pH promoted the deposition of spherical minerals in fibrils. Especially, the minerals tended to form cluster-like structure at pH 10.4. The hyp content, Ca and P contents, and gel strength measurements suggested that higher pH promoted the formation of HAp with a Ca/P closed to 1.67 and the prolongation of crystallization gave the time for collagen self-assembly leading to the increase of gel strength at higher pH (9.4-10.4). These results might provide some new ideas for designing biomimetic materials.

Collagen solutions in the presence of 0.1-2.0M acetic acid (AA) were investigated to understand the interaction between collagen molecules and an acidic solvent. Fluorescence measurements of pyrene showed that the critical aggregation concentration (CAC) of collagen increased from 0.518 to 1.581 mg/mL for AA concentrations ranging from 0.1 to 2.0M, indicating that the aggregated state of collagen molecules was associated with AA concentration. The size of the collagen aggregates, determined by dynamic light scattering, demonstrated that their disaggregation was enhanced with increasing electrostatic repulsion between the collagen chains. The variations in the intrinsic viscosity and Huggins coefficient depended on the molecular interaction among the collagen molecules. Furthermore, the increased AA concentration has a different influence on the rigidity of collagen molecules above and below the CAC. This observation was manifested by the changes in the morphology of collagen molecules observed by atomic force microscopy.