Octadecyl and Sulfonyl Modification of Diatomite Synergistically Improved the Immobilization Efficiency of Lipase and Its Application in the Synthesis of Pine Sterol Esters

Phytosterols usually have to be esterified to various phytosterol esters to avoid their disadvantages of unsatisfactory solubility and low bioavailability. The enzymatic synthesis of phytosterol esters in a solvent-free system has advantages in terms of environmental friendliness, sustainability, and selectivity. However, the limitation of the low stability and recyclability of the lipase in the solvent-free system, which often requires a relatively high temperature to induce the viscosity, also increased the industrial production cost. In this context, a low-cost material, namely diatomite, was employed as the support in the immobilization of Candida rugosa lipase (CRL) due to its multiple modification sites. The Fe3O4 was also then introduced to this system for quick and simple separation via the magnetic field. Moreover, to further enhance the immobilization efficiency of diatomite, a modification strategy which involved the octadecyl and sulfonyl group for regulating the hydrophobicity and interaction between the support and lipase was successfully developed. The optimization of the ratio of the modifiers suggested that the -SO3H/C18 (1:1.5) performed best with an enzyme loading and enzyme activity of 84.8 mg center dot g-1 and 54 U center dot g-1, respectively. Compared with free CRL, the thermal and storage stability of CRL@OSMD was significantly improved, which lays the foundation for the catalytic synthesis of phytosterol esters in solvent-free systems. Fortunately, a yield of 95.0% was achieved after optimizing the reaction conditions, and a yield of 70.0% can still be maintained after six cycles. To overcome the challenge of the low stability of lipase, a cheap inorganic porous material, namely diatomite was applied as a support for the immobilization of Candida rugosa lipase (CRL). To further enhance its efficiency, the magnetic nano-Fe3O4 was firstly attached to the surface of diatomite via a co-precipitation method. Moreover, the octadecyl and sulfonyl groups were applied as the modifiers for regulating the hydrophobicity, flexibility, and interaction of diatomite. The modified diatomite performed excellent efficiency during immobilization due to the synergistic effect of the electrostatic and hydrophobic interactions, and thus significantly enhanced the stability and reusability of CRL. image