Alternative Approach to Synthesize Methylated Chitosan Using Deep Eutectic Solvents, Biocatalyst and “Green” Methylating Agents

Abstract

Conventional synthesis of N-methylated chitosan involves the use of organic solvents in alkaline conditions, using methyl iodide as the methylating agent. However, the method does not result in selective N-methylation and is known to cause heavy polymer scission. In this investigation, we have reported alternative “green” approaches for methylated chitosan synthesis. Two types of deep eutectic solvents (DESs), viz., DES(Urea) and DES(Gly), either alone or in combination with other solvents, were screened as media for facilitating the methylation of chitosan. Our results indicated that DESs mediated selective N-methylation in the absence of NaOH, with no polymer scission, when compared with the reported methods. 1H NMR spectra confirmed selective N-methylation in the case of products obtained using DES(Urea), while that obtained using DES(Gly) demonstrated some O-methylation. Another green method investigated the unexplored property of biocatalyst lipase for methylating chitosan in the presence of “green” methylating agents in DESs systems. Furthermore, lipase from Burkhorlderia species exhibited ability to methylate the chitosan polymer, while the enzyme from Candida Antartica failed to methylate the polymer. Our investigation also confirmed the possibility of using dimethyl carbonate as a benign methylating agent.