Revealing the Control Mechanisms of pH on the Solution Properties of Chitin via Single-Molecule Studies

Abstract

Chitin is one of the most common polysaccharides and is abundant in the cell walls of fungi and the shells of insects and aquatic organisms as a skeleton. The mechanism of how chitin responds to pH is essential to the precise control of brewing and the design of smart chitin materials. However, this molecular mechanism remains a mystery. Results from single-molecule studies, including single-molecule force spectroscopy (SMFS), AFM imaging, and molecular dynamic (MD) simulations, have shown that the mechanical and conformational behaviors of chitin molecules show surprising pH responsiveness. This can be compared with how, in natural aqueous solutions, chitin tends to form a more relaxed spreading conformation and show considerable elasticity under low stretching forces in acidic conditions. However, its molecular chain collapses into a rigid globule in alkaline solutions. The results show that the chain state of chitin can be regulated by the proportions of inter- and intramolecular H-bonds, which are determined via the number of water bridges on the chain under different pH values. This basic study may be helpful for understanding the cellular activities of fungi under pH stress and the design of chitin-based drug carriers.

Keywords: chitin; elasticity; molecular dynamic simulations; non-covalent interactions; single-molecule force spectroscopy.

Figure 1

(A) The chemical structure of chitin. (B) A schematic drawing of the working principle of SMFS.

Figure 2

Typical F-E curves of chitin obtained in DI water. (A) Original F-E curve. (B) Normalized effect of those shown in (A), and the fitting result obtained using the TSQM-FJC model.

Figure 3

Typical normalized F-E curves of chitin obtained in HCl aqueous solutions of pH = 5 (A), pH = 3 (B), and pH = 1 (C). (D) Direct comparison of the F-E curves of chitin obtained in acidic conditions and the fitting result obtained using the TSQM-FJC model.

Figure 4

(A–C) Typical normalized F-E curves of chitin obtained in NaOH solutions with different pH values. (D) Direct comparison of the typical F-E curves shown in (A–C).

Figure 5

(A) Direct comparison of the F-E curves of chitin obtained at pH = 3, in DI water, and at pH = 11. (B–D) Molecular morphology of chitin obtained in DI water, at pH = 3, and at pH = 11, respectively.

Figure 6

MD simulation results of chitin in aqueous solutions with different pH levels. (A) The typical molecular conformation of chitin under the tested conditions. (B) The R_g of a chitin chain under different conditions (blue: pH = 11, black: pure water, and red: pH = 3). (C–E) The normalized $\left[\mathsf{\eta }\right]$ and the total number of intramolecular and intermolecular H-bonds of a chitin chain under different conditions, respectively (blue: pH = 11, cyan: pure water, and pink: pH = 3).