Chitosan functionalization with a series of sulfur‐containing α‐amino acids for the development of drug‐binding abilities

Chitosan (Chi; 0.5 g) in 69.66 m M aqueous acetic acid was mixed with 312.4 m M methionine (methi) at 0.01 mL/s to disperse and cause optimum collisions for supporting condensation reactions through NH 2 of Chi and COOH groups of methi. The functionalized chitosan (f‐Chi) product with methi developed an amide bond, which was represented as methi‐functionalized chitosan [Chi–NHC(O)–methi]. Both the 1‐Ethyl‐3‐(3‐dimethylaminopropyl)carbodiimide (EDC) and Dean–Stark methods were followed for Chi functionalization. Sulfonation with chlorosulfonic acid in a dimethylformamide medium was conducted at 90 °C and 750 rpm with an approximately 72% yield. The Chi–NHC(O)–methi was characterized by 1 H‐NMR spectroscopy and Fourier transform infrared stretching frequencies. The onset temperature of 280 °C recorded by thermogravimetric analysis/differential scanning calorimetry analysis, confirmed the high stability of the covalent bonds in Chi–NHC(O)–methi. The synthesis was repeated with other series members of sulfur (S) atoms containing α‐amino acids: homocysteine, ethionine, and propionine. The shielding of terminal CH 3 was enhanced on elongation of the terminal alkyl chain in the case of propionine. The peak for the NH 2 of Chi at a δ value of 4.73 ppm shifted to 5.36 ppm in Chi–NHC(O)–methi because of the involvement of NH 2 in NHC(O). Theoretically, the value of NH 2 of Chi was 5.11 ppm, with a difference of 0.38 ppm as compared to the experimentally determined value of 4.73 ppm. Additionally, a new peak at a δ value of 3.26 ppm also confirmed Chi functionalization. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135 , 46000.