. 2024 Nov 16;16(22):3187. doi: 10.3390/polym16223187

Table 6.

Mechanism of chitin isolating with deep eutectic solvents.

Chitin Type	Resource	Proposed Mechanism		Ref.
Chitin Type	Resource	Demineralization	Deproteinization	Ref.
α-chitin	Lobster shells	Lactic acid was used to remove CaCO₃. In addition, the elevated viscosity impedes the penetration of DESs into lobster shells, leading to incomplete removal of CaCO₃ and consequently reducing the purity of the extracted chitin. Due to the partial removal of CaCO₃ by DESs, the linkages in the inner structural organization of lobster shells were weakened.	Proteins are rich in carboxylic and hydroxyl groups, serving as HBD that compete for chloride anions through electrostatic interaction by inducing H⁺, resulting in most lactic acid being attracted. Hence, new hydrogen bonds were formed between choline chloride and protein, disrupting the hydrogen bonds within the protein–chitin fibers.	[94]
α-chitin	Shrimp shells	Malic acids carried out demineralization. The malic acid removed the shrimp shells and minerals, which are mostly in the form of crystalline CaCO₃, leaving the proteins and chitin. The spacing between the chitin–protein fibers was filled with proteins and minerals; thus, removing minerals weakened the linkages within the inner structural organization of the shrimp shells.	Competing hydrogen bond formation between DES and carbohydrates breaks the intramolecular hydrogen bond network, weakening the shrimp shells’ hydrogen bond interactions. As a result, chitin is dissolved in DES and separated from the proteins.	[103]
α-chitin	Shrimp shells	The pH of DES was about 1.5, indicating that H⁺ can be released from DES. The released H⁺ reacted with calcium carbonate to produce CO₂ and water-soluble calcium salts, removing CaCO₃.	The protein in the shrimp shells in DES was degraded to amino acids or a water-soluble protein, which can be dissolved in DES to remove it.	[96]
α-chitin	Shrimp shells	The removal of minerals was mainly accomplished through the lactic acid of DES. The pH value of DES was about 0.88. The reaction of minerals in shrimp shells in the form of crystalline CaCO₃ with H+ is released from DES to produce CO₂ and water-soluble calcium salts.	The H⁺ present in DES forms ammonium salts with proteins by electrostatic forces that attract large amounts of chlorine and lactic acid, thus forming new hydrogen bonds. The appearance of competing hydrogen bonds weakened the covalent bond between the protein and chitin in the shrimp shell. The protein was dissolved into DES and separated from the chitin.	[100]
α-chitin	Crab shells	The weak acidity of reagents was the main driving force of demineralization. Many CO₂ bubbles were rapidly generated when DES were added to the crab shells. More importantly, free Ca²⁺ was detected in the solution after DES treatment.	The protein in the crab shell was removed in two ways: one was converted into soluble protein, and the other was degraded into soluble amino acids. Sixteen common amino acids were present in the supernatant after DES treatment. Simultaneously, the content of nine amino acids increased after HCl treatment.	[107]
α-chitin	Crab shells	Because the minerals in crab shells are mostly in the form of crystalline CaCO₃, when DES were applied to the crab shells, minerals were removed by malic acid, leaving chitin and protein. The strong internal structure of crab shells was weakened after removing minerals.	The strong hydrogen-bond network between chitin and proteins was weakened due to competing hydrogen bonds formed between the Cl⁻ of DES and the hydroxyl groups, and the proteins were removed by dissolution because of the hydrogen-bond interaction with DES.	[106]
α-chitin	Prepupae	The ability of the released H⁺ from the acidic solvents of DES was attributed to removing crystalline CaCO₃. This decreased the linkages between protein and chitin, which facilitates the soaked behaviors between DES and protein–chitin fibers. The release of H⁺ was the key factor in the removal of minerals.	The new hydrogen bonds were generated between DES and the protein, which damaged the hydrogen bond formed in protein–chitin fibers. On the other hand, the amounts of peptide bonds hydrolyzed by the released H⁺ resulted in the formation of free amino acids during deproteinization.	[115]
β-chitin	Squid pens	The proteins and minerals were removed from squid pens through the synergistic action of alkaline DES formed by K₂CO₃ and glycerol. As a component of squid pens, minerals were distributed between chitin–protein fibers and removed with the degradation and dissolution of proteins. Hence, the alkaline DESs also played the role of demineralization.	Alkaline K₂CO₃ and glycerol played the role of protein degradation and dissolution, respectively, and the hydrogen bond interaction with DES further weakened the binding between ß-chitin and protein and increased protein dissolution. In addition, the efficient dissolution of proteins accelerated protein dissolution, when the protein removal efficiency was increased by increasing glycerol content.	[121]