Activity of Bromelain with Cationic Surfactants and the Correlation with the Change of ¹H NMR Signals

Abstract

This study observed the activities of bromelain in the presence of various cationic surfactants at different temperatures and the conformational changes in bromelain by 1 H NMR spectroscopy. We found that the bromelain activity was enhanced by tens to hundreds of micromoles per liter of the surfactant. In the presence of the surfactants, bromelain exhibited good tolerance to a range of substrate temperatures and its thermal stability was also increased. The 1 H NMR experiments indicated that when the temperature was increased from 25.0 to 45.0 C, the protons of bromelain having chemical shifts (δ) between 3.7 and 5.2 ppm moved upfield, while those having δ values between 3.2 and 3.7 ppm moved downfield. In the bromelain/cationic surfactant mixture, the values of δ for the protons in both bromelain and the cationic surfactants decreased, accompanied by the broadening of the half-peak width of the surfactant protons. These results indicated that both increasing temperature and adding a cationic surfactant made the bromelain chain more flexible and hence, increased the bromelain activity. To the best of our knowledge, this was the first time that the relationship between the protein activity and the 1 H NMR data was expounded.