The influence of beta- and gamma-cyclodextrin cavity size on the association constant with decanoate and octanoate anions

This work evaluates the influence of the beta- and gamma -cyclodextrin (CD) cavity size on the association constant (K-CDA) with decanoate (C-10) and octanoate (C-8) anions. The spectral displacement technique with phenolphth alein was used to obtain the 1:1 association constant (K-CDA) in NaHCO3/NaO H buffer pH 10.5 at 25 degreesC. The K-CDA value obtained were 2.6 (+/-0.2) x 10(3), 2.5 (+/-0.5) x 10(2), for beta CD-C-10 and gamma CD-C-10 inclusio n complexes, and 5.1 (+/-0.2) x 10(2) and 4.7 (+/-0.2) x 10(1) for beta CD- C-8 and gamma CD-C-8 inclusion complexes, respectively. The K-CDA values of either acid with beta CD is approximately 10 times higher than for the sam e acid with gamma CD, where as for the same cyclodextrin, the K-CDA value i s 5 times higher for the C-10 association than for the C-8. The data demons trate that the cyclodextrin cavity size exerts a greater influence on the a ssociation constant than the chain length of the acid for these compounds. H-1 NMR studies show that fatty acid protonation has a distinct effect on t he chemical shift of CD protons.