Collagen entanglement influenced by the addition of acids

• Native collagen forms a finely meshed and elastic network.
• Telopeptide-poor collagen matrices cluster and feature lower viscosities.
• Increased ionic strength and pH enhances collagen entanglement for pH < pI.
• Kosmotropic counter ions increase the viscosity and the elasticity of collagen.
• Chaotropes reinforce collagen–solvent interactions.

Collagen represents a biopolymer widely used in tissue engineering and the food industry. However, pH and acid effects, and the function of non-helical telopeptides on the collagen fibril microstructure and overall network structure are not completely understood. Thus, native and telopeptide-poor collagen type I were examined at different concentrations in order to characterize the pH and acids effects on the mechanical properties and entanglement characteristics of collagen suspensions. Collagen suspended in phosphoric, sulfuric, hydrochloric, or perchloric acid at pH 1, 2 and 3 were examined by rheological measurements and scanning electron microscopy.Collagen entanglement increases with increasing pH values below the isoelectric point. This was confirmed by critical overlap and entanglement concentrations, storage and loss moduli, as well as by scanning electron microscopy. Furthermore, decreased ionic strength, increased Debye screening lengths, increased intrinsic viscosities, and increased radii of gyration accompanied collagen entanglement. Phosphoric acid was the most effective acid in terms of critical overlap and entanglement concentrations. By contrast, oscillation tests assessed the highest storage modulus for perchloric acid, while sulfuric acid reinforced protein–solvent interactions according to the reversed Hofmeister series for pH values below the isoelectric point. Moreover, scanning electron microscopy visualized the acid effects for both, wherein native collagen revealed undefined, but finely meshed amorphous structures and telopeptide-poor collagen indicated crystalline-like and well-ordered structures.We demonstrated that collagen entanglement depends strongly on the pH and acid type, thus highlighting the importance of the preparation step of collagen raw materials for subsequent processing that could offer new insights for food manufacturers.

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