Browsing by Author "Basilio, Nuno"
Now showing 1 - 3 of 3
Results Per Page
Sort Options
- Cucurbit[7]uril: surfactant host-guest complexes in equilibrium with micellar aggregatesPublication . Pessêgo, Márcia; Basilio, Nuno; Moreira, José; García-Río, LuísIn order to compare the formation of host–guest complexes between b-cyclodextrin (b-CD) or cucurbit[7]uril (CB7) and cationic surfactants we studied the hydrolysis of 4-methoxybenzenesulfonyl chloride (MBSC). The selected surfactants allowed the length of the hydrocarbon chain to be varied between 6 and 18 carbon atoms. Contrary to the expected behaviour, the values of the binding constants between CB7 and surfactants are independent of the alkyl chain length of the surfactant. In the case of b-CD, however, a clear dependence of the binding constant on the hydrophobic character of the surfactant was observed. The values obtained with CB7 are significantly higher than those obtained with b-CD and these differences are explained to be a consequence of electrostatic interactions of the surfactants with the portals of CB7. It was found that a small percentage of uncomplexed CB7 was in equilibrium with the cationic micelles and this percentage increased on increasing the hydrophobic character of the surfactant.
- Supramolecular catalysis by cucurbit[7]uril and cyclodextrins: similarity and differencesPublication . Basilio, Nuno; García-Río, Luís; Moreira, José; Pessêgo, MárciaTo understand the analogies and differences between the cucurbituril and cyclodextrin cavities different solvolytic reactions have been studied in the presence of cucurbit[7]uril, CB7, and β-CD or its methylated derivative, DM-β-CD. Solvolysis of 1-bromoadamantane has been used as a test to evaluate the ability of the cavities to solvate the Br- leaving group. Obtained results show that in both cases the polarity inside the cavity is similar to that of a 70% ethanol:water mixture. Solvolysis of substituted benzoyl chlorides shows a great difference between the CB7 and DM-β-CD cavity. Solvolysis of electron withdrawing substituted benzoyl chlorides (associative mechanism) is catalyzed by DM-β-CD and inhibited by CB7. However, solvolysis of electron donating substituted benzoyl chlorides(dissociative mechanism) is catalyzed by CB7 and inhibited by DM-β-CD. These experimental behaviors have been explained on the basis of different solvolytic mechanisms. Participation of the hydroxyl groups of the cyclodextrin as a nucleophile can explain the catalytic effect observed for solvolysis of benzoyl chlorides reacting by an associative mechanism. Solvolysis of benzoyl chlorides reacting by a dissociative mechanism is catalyzed by CB7 due to the ability of the CB7 cavity to stabilize the acylium ion developed in the transition state by electrostatic interactions.
- Unveiling the formation 1: 2 supramolecular complexes between cucurbit[7]uril and a cationic calix[4]arene derivativePublication . Pessego, Marcia; Mendoza, Johan; Da Silva, José Paulo; Basilio, Nuno; Garcia-Rio, LuisThe formation of host-guest complexes between cucurbit[7]uril (CB7) and a tetracationic calix[4]arene derivative in the so-called cone conformation was investigated by H-1 NMR, DOSY NMR, isothermal titration calorimetry and ESI-MS. The results point to the formation of 1: 1, 1: 2 and 2: 1 CB7:calixarene complexes with binding constants of 3 x 10(6) M-1, approximate to 2 x 10(2) M-1, and 9 x 10(4) M-1 respectively. The study demonstrates, on one hand, that despite having four potential recognition sites, the calixarene only binds two CB7 molecules and, on the other, that for sterically crowded binding motifs that prevent CB7 inclusion with optimized hydrophobic and ion-dipole interactions, the formation of 1:2 complexes can be observed most likely due to formation of external binding of the cationic moieties to the CB7 carbonyl portals.