Effects of spacer arm length and benzoation on enantioseparation performance of β-cyclodextrin functionalized cellulose membranes

The enantiomeric resolution performance of cellulose membranes functionalized with chiral layers of beta-cyclodextrin (β-CD) of various spacer arm lengths has been investigated. 6-O-(p-toluenesulfonyl)-β-cyclodextrin (Ts-β-CD) was reacted with ethylenediamine (EDA), diaminopropane (PDA) and diaminobutane (BDA) to synthesize the aminated beta-cyclodextrins EDA-β-CD, PDA-β-CD and BDA-β-CD, respectively, which were subsequently grafted onto partially oxidized cellulose membranes via reductive amination. The enantioselectivity, in racemic tryptophan resolution, increased with decreasing spacer arm length, i.e., α_BDA-β-CD < α_PDA-β-CD < α_EDA-β-CD, while the highest selectivity of 1.20 was obtained when a mixture of chiral selectors was grafted to the membranes. The lower selectivity of membranes grafted with longer spacer arms was attributed to their lower grafting efficiency and, consequently, the presence of more defective regions. Further improvement of enantioselectivity, to ∼1.3-1.5, was realized by substituting the hydroxyl groups on the aminated cyclodextrins with benzoate groups. This increase in enantioselectivity was consistent with molecular modeling predictions of the complex formation energies between the enantiomers and various chiral selectors.