The Effect of Molecular Weight on the Antibacterial Activity of N,N,N-Trimethyl Chitosan (TMC)

N,N,N-trimethyl chitosan (TMC) with 93% degree of trimethylation was synthesized. TMC and the chitosan starting material were subjected to acidic hydrolysis to produce 49 different samples with a reduced average molecular weight (M(w)) ranging from 2 to 144 kDa. This was done to allow the investigation of the relationship between antibacterial activity and M(w) over a wide M(w) range. NMR investigation showed that hydrolysis did not affect the degree of trimethylation (DS(TRI)) or the structure of the polymer backbone. The activity of TMC against Staphylococcus aureus (S. aureus) increased sharply with M(w) until a certain M(w) value (critical M(w) for high activity, CMW) was reached. After the CMW, the activity was not affected by a further increase in the M(w). A similar pattern of activity was observed for chitosan. The CMW was determined to be 20 kDa for TMC and 50 kDa for chitosan.