Effect of the molecular weight of chitosan on the enzymatic properties and wettability of its polyelectrolyte complex with lipase

Chitosan-lipase polyelectrolyte complexes are promising stabilizers for Pickering emulsion catalysis. In this work, three commercial chitosan species with low (50–190 kDa), medium (190–310 kDa), and high (310–375 kDa) molecular weights (MW), designated as LCS, MCS, and HCS respectively, were selected to coacervate with the lipase from Aspergillus oryzae (AOL) and the impacts of their MWs on the attributes of AOL were compared. It was found that MCS contributed the strongest interaction with AOL and its protein recovery and AOL loading ratios reached 81.40 and 91.82 % respectively. The complexed AOLs exhibited the same optimum pH and temperature, but MCS conferred the greatest substrate affinity decrease by 2.7 folds and highest V max increase by 0.65 folds compared with free AOL. Coacervation with chitosan enhanced the thermostability of the lipase and the most profound effect occurred in MCS, which contributed an increase in E a by 87.6 %. Circular dichroism results indicated that MCS induced the greatest α-helix content decrease in AOL by 45.6 %. Surface hydrophobicity and contact angle measurements demonstrated that the chitosan-AOL complexes possessed improved wettability. Hence, the MW of chitosan is a crucial parameter for the application of its AOL complex in Pickering emulsion catalysis and a medium MW favors this purpose.