Isothermal and non-isothermal characterization of catalytic nylon membranes chemically grafted: dependence on the grafting percentage

The behaviour of five different hydrophobic β-galactosidase derivatives, obtained by grafting different amount of butylmethacrylate (BMA) on planar nylon membranes, has been studied under isothermal and non-isothermal conditions. Under isothermal conditions the effect of the grafting percentage on the enzyme activity has been studied as a function of pH, temperature and substrate concentration. Independently from the parameters under observation, the yield of the catalytic process reaches the maximum value at a grafting percentage value equal to 21%. The apparent Km values result linearly increasing with the increase of the grafting percentage, while the apparent Vmax exhibits a maximum value. Under non-isothermal conditions, a decrease of the apparent Km values and increase of the apparent Vmax has been found in respect to the same values obtained under isothermal conditions. The percentage activity increases induced by the presence of a temperature gradient have been found to decrease with the increase of the percentage of graft BMA. A parameter correlating the percentage increase of enzyme activity under non-isothermal conditions with the hydrophobicity of the catalytic membrane has also been identified. This parameter is the ratio between thermoosmotic and hydraulic permeability. Results have been discussed in terms of reduction of diffusion limitations for substrate and products movement towards or away from the catalytic site by the process of thermodialysis. The usefulness of using non-isothermal bioreactors in industrial biotechnological processes has been confirmed.