Extent of shielding by counterions determines the bactericidal activity of N,N,N-trimethyl chitosan salts

• N,N,N-trimethyl chitosan salts (TMCs) with different counterions were obtained.
• The full ion exchange was verified by the EDS analysis.
• The bactericidal effect of different TMCs was evaluated against Escherichia coli and Staphylococcus aureus.
• TMCS and TMCAc presented the highest antibacterial activity.
• This action is correlated with the “availability” of −+N(CH3)3 groups.

In this study, we show that the bactericidal activity of quaternized chitosans (TMCs) with sulfate, acetate, and halide counterions against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) correlates with the “availability” of N-quaternized groups [−+N(CH3)3] in the TMCs backbones. N,N,N-trimethyl chitosan sulfate (TMCS) and N,N,N-trimethyl chitosan acetate (TMCAc) displayed the highest activities, probably due to their delocalized π system. Among TMCs with halide counterions, activity was higher for N,N,N-trimethyl chitosan chloride (TMCCl), whereas N,N,N-trimethyl chitosan iodide (TMCI) and N,N,N-trimethyl chitosan bromide (TMCBr) exhibited lower, similar values to each other. This is consistent with the shielding of −+N(CH3)3 groups inferred from chemical shifts for halide counterions in 1HNMR spectra. We also demonstrate that TMCs with distinct bactericidal activities can be classified according to their vibrational spectra using principal component analysis. Taken together, these physicochemical characterization approaches represent a predictive tool for the bactericidal activity of chitosan derivatives.

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