Cesar Marquez

Two Mechanisms of Slow Host–Guest Complexation between Cucurbit[6]uril and Cyclohexylmethylamine: pH-Responsive Supramolecular Kinetics

20 times more rapid is the complexation of the organic ammonium ion receptor cucurbit[6]uril (CB6) with cyclohexylmethylamine than with the cyclohexylmethylammonium species. Within the narrow pH region around the pKa value of the amine, the complexation kinetics are accelerated but the binding constant remains essentially unaffected. In this region, the ammonium complex is formed through binding of the amine form followed by fast protonation and not through direct complexation of the ammonium form. Molecular dynamics calculations suggest that the amine form undergoes a direct inclusion into CB6 while the ingression of the ammonium form is retarded by the formation of an association complex, which reacts through a different, higher lying transition state to the inclusion complex.

Cucurbiturils: molecular nanocapsules for time-resolved fluorescence-based assays

A new fluorescent host-guest system based on the inclusion of the fluorophore 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) into the cavity of the molecular container compound cucurbit[7]uril (CB7) has been designed which possesses an exceedingly long-lived emission (690 ns in aerated water). The large binding constant of (4/spl plusmn/1)/spl times/10/sup 5/ M/sup -1/ along with the resistance of the CB7/spl middot/DBO complex toward external fluorescence quenchers allow the use of CB7 as an enhancer in time-resolved fluorescence-based assays, e.g., to screen enzyme activity or inhibition by using DBO-labeled peptides as substrates. The response of CB7/spl middot/DBO to different environmental conditions and possible quenchers are described.

ESI-MS study on the aldol reaction catalyzed by L-proline

The aldol reaction catalyzed by L-proline has been monitored and the accepted mechanism confirmed by intercepting and characterizing, for the first time, all the proposed intermediates by ESI-MS/MS.