Preferential adsorption of L-tryptophan by L-phospholipid coated porous polymer particles.

Chiral selective adsorption of L-amino acid, tryptophan (Trp) was achieved using phospholipid membrane-coated porous polymer particles (PPPs). PPPs with numerous pores were prepared by in situ polymerization of divinylbenzene, and then coated with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC, L-phospholipid) via the impregnation method. Elemental mapping of energy dispersive X-ray (EDX) analysis revealed that DPPC molecules were distributed to the surface and the inner part of PPPs, where almost all the DPPC molecules applied for impregnation were deposited on PPPs. The phospholipid membrane properties of DPPC-PPPs were characterized using the fluorescence probe 6-lauroyl-2-dimethylaminonaphthalene (Laurdan). The results show that DPPC-PPPs possessed a lipid membrane-like environment similar to pure DPPC liposomes, especially at temperatures below 35 °C. DPPC-PPPs slightly adsorbed L-Trp and D-Trp at 45 °C, while DPPC-PPPs significantly adsorbed L-Trp but not D-Trp at 30 °C: enantio excess (e.e.) was 75.0%. The time course of Trp adsorption was investigated: for both enantiomers, similar adsorption behaviors were observed for 30 h, thus suggesting surface adsorption onto DPPC-PPPs. L-Trp adsorption continued after 30 h, suggesting that L-Trp could be distributed in the inner part of DPPC-PPPs. Interestingly, the reused DPPC-PPPs featured improved adsorption performance, suggesting that the deposited DPPC membranes on PPPs could act as chiral selectors for L-Trp. The optical resolution of L-/D-Trp was performed using DPPC-PPPs, resulting in the e.e. of D-Trp was > 60%. Thus, DPPC-PPPs have the potential of chiral selective adsorption of L-amino acid, which can be used as chiral separation materials.