Effects of modified beta-cyclodextrins on the hydrolysis reaction of p-nitrophenyl alpha-methoxyphenylacetate

Abstract

Effects of beta-cyclodextrin (beta-CD) 1 and its derivatives 2-7 on the deacylation reaction of p-nitrophenyl (R or S)-alpha-methoxyphenylacetate were studied. The beta-CD derivatives used were 6-alpha-D-glucosyl-beta-CD 2, sulfated beta-CD (7-11 sulfate groups/CD ring) 3, dimethylated beta-CD 4, carboxymethylated beta-CD (3.5 carboxymethyl groups/CD ring) 5, 2-tri(2-hydroxypropyl)-beta-CD 6, and beta-CD appended on poly(allylamine) 7. The rate constant (k(psi)(CD)) of the substrate/beta-CD complexes and the formation constants (K) of the complexes were determined from the dependence of the pseudo-first order rate constants of the deacylation reaction on the concentration of beta-CDs. The order of k(psi)(CD) for the R-enantiomer at pH 8.0 is 4 << 5 < H2O < 3 congruent to 6 < 1 congruent to 2 < 7, while that for the S-enantiomer is 4 < 5 congruent to 6 < H2O congruent to 1 congruent to 2 3 < 7: H2O denotes the rate in the absence of beta-CDs. The order of K values is 3 < 7 < 6 congruent to 2 congruent to 1 < 4 < 5. This work indicates that, though the secondary hydroxyl groups of beta-CD play critical roles in the deacylation reactions of the esters complexed with beta-CDs, the reactivity of the ester/beta-CD complexes depends highly on the nature of the substituents at the secondary face of beta-CD. It also suggests that the substrates inserted from the secondary side as well as the primary side of beta-CD of poly(allylamine)-bound beta-CD undergo the reaction by attack of amino groups on the polymer chain.