The concentration-, pH- and temperature-responsive self-assembly of undenatured type II collagen: Kinetics, thermodynamics, nanostructure and molecular mechanism

In this work, the undenatured type II collagen (UC-II) with an intact triple helix structure was obtained from chicken breast cartilage by the enzymatic extraction method. The self-assembly kinetics of UC-II at different parameters of concentration, pH and temperature was characterized by ultraviolet–visible spectroscopy (UV–vis). It occurred at pH 4.5–7.0 and was promoted by rising concentration and temperature in a certain range, and conformed to the first-order dynamics equation. The activation enthalpy (ΔH = 55.76 kJ mol−1) and entropy transition (ΔG = 69.30–71.40 kJ mol−1) revealed that the self-assembly of UC-II was a non-spontaneous heat absorption process. It was observed by atomic force microscopy (AFM) that, regulated by concentration, pH and temperature, the UC-II self-assemblies displayed diverse nanostructure, such as protofibrils, proto- and nano-filaments, nanofibrils of varying thickness and dense degree. Based on the results of circular dichroism (CD) and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), the UC-II self-assembly was mainly triggered by hydrophobic effects driven by a partial transition of the left-handed polyproline II (PP II) conformation, and both hydrogen bonding and electrostatic interactions were also involved in the self-assembly process. These results would lay the theoretical foundation for the fabrication and application of controllable UC-II self-assemblies.