Chitosan (Chi; 0.5 g) in 69.66 mM aqueous acetic acid was mixed with 312.4 mM methionine (methi) at 0.01 mL/s to disperse and cause optimum collisions for supporting condensation reactions through ANH 2 of Chi and ACOOH groups of methi. The functionalized chitosan (f-Chi) product with methi developed an amide bond, which was represented as methi-functionalized chitosan [Chi-NHAC(@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 8C and 750 rpm with an approximately 72% yield. The Chi-NHAC(@O)-methi was characterized by 1 H-NMR spectroscopy and Fourier transform infrared stretching frequencies. The onset temperature of 280 8C recorded by thermogravimetric analysis/differential scanning calorimetry analysis, confirmed the high stability of the covalent bonds in Chi-NHAC(@O)-methi. The synthesis was repeated with other series members of sulfur (S) atoms containing a-amino acids: homocysteine, ethionine, and propionine. The shielding of terminal ACH 3 was enhanced on elongation of the terminal alkyl chain in the case of propionine. The peak for the ANH 2 of Chi at a d value of 4.73 ppm shifted to 5.36 ppm in Chi-NHAC(@O)-methi because of the involvement of ANH 2 in ANHAC(@O)A. Theoretically, the value of ANH 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 d value of 3.26 ppm also confirmed Chi functionalization.

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