Magnetic MOF derivatives from Tunable MIL-88A for enhanced lipase immobilization and phosphatidyl EPA/DHA synthesis

Metal-organic frameworks (MOFs) with tunable porosity are promising carriers for enzyme immobilization, but their practical application is limited by structural instability and difficult recovery. We previously developed an oxidation-reduction (O-R) pyrolysis strategy for mesoporous MIL-88 A, yielding a magnetic carrier (O-R-Meso) with excellent lipase immobilization and recyclable catalysis. Herein, we verify the applicability of this O-R pathway for Fe-based MIL-88 A with tailored pore structures (micro-, meso-, macroporous) by extending it to microporous C-MIL-88 A and macroporous SOM-MIL-88 A. The O-R derivatives (O-RC, O-R-Meso and O-R-SOM) retained the original morphology and pore channel integrity, with improved specific surface area and sufficient magnetization (13-23 emu/g) for magnetic separation. Notably, after O-R pyrolysis, MIL-88 A with various pore sizes essentially maintained or even enhanced the specific activity of immobilized lipase (Candida antarctica lipase B, CalB), successfully deriving into magnetic carriers with enhanced catalytic efficiency and recyclability. This superiority originates from the preserved pore structure and biocompatible γ-Fe₂O₃ surface of the O-R derivatives, which effectively retain the native conformation of CalB. Besides, this work systematically compares the secondary structure of free and immobilized CalB on MOF-derived carriers with distinct pore architectures, providing direct structural evidence for the superior enzyme-carrier compatibility of mesoporous materials. In the transesterification synthesis of phosphatidyl EPA/DHA, CalB@O-R-Meso showed the highest EPA/DHA total incorporation, prominent sn-1 positional selectivity, and optimal reusability, outperforming CalB@O-R-C and CalB@O-R-SOM. Furthermore, kinetic studies on the CalB@O-R-Meso-catalyzed system confirmed a Ping-Pong BiBi mechanism and revealed competitive product inhibition by phosphatidyl EPA/DHA (K_i = 29.8 mM) with negligible internal mass transfer resistance (η ≈ 1). This work establishes a O-R pyrolysis strategy for tunable porosity Fe-MOFs, clarifies pore size-dependent enzyme -carrier compatibility, and provides a fundamental kinetic basis for phosphatidyl EPA/DHA synthesis.