Unfolding/Refolding Study on Collagen from Sea Cucumber Based on 2D Fourier Transform Infrared Spectroscopy

We aimed to explore the differences of thermal behaviors between insoluble collagen fibrils (ICFs) and pepsin-solubilized collagens (PSCs) from sea cucumber Stichopus japonicus. The unfolding/refolding sequences of secondary structures of ICFs and PSCs during the heating and cooling cycle (5 -> 70 -> 5 degrees C) were identified by Fourier transform infrared spectrometry combined with curve-fitting and 2D correlation techniques. ICFs showed a higher proportion of alpha-helical structures and higher thermostability than PSCs, and thus had more-stable triple helical structures. The sequences of changes affecting the secondary structures during heating were essentially the same between ICFs and PSCs. In all cases, alpha-helix structure was the most important conformation and it disappeared to form a beta-sheet structure. In the cooling cycle, ICFs showed a partially refolding ability, and the proportion of beta-sheet structure rose before the increasing proportion of alpha-helix structure. PSCs did not obviously refold during the cooling stage.