Ph.D Thesis

Ph.D StudentSinha Mantosh
SubjectSupramolecular Chemistry of Cucurbit(6)uril
DepartmentDepartment of Chemistry
Supervisor PROFESSOR EMERITUS Ehud Keinan
Full Thesis textFull thesis text - English Version


Artificial molecular machines that are based on rotaxane-type rotors (rod inside wheel) are of particular interest because such rotary motors can be designed to have minimal friction. These systems consist of a long linear component such as a polyalkyne rod that is terminated by two bulky anchor groups, and a macrocyclic component that hosts the linear component such as the cucurbituril cavitand that could host the linear part.

Cucurbit[6]uril, CB[6] a macrocyclic polyurea, is known to form inclusion complexes with alkyl or aryl ammonium cations due to the charge-dipole interactions and hydrogen bonding between the ammonium cation and the polarized carbonyl oxygens in the cucurbituril portals. This unique feature renders cucurbituril an attractive choice for building rotary motors. To construct building blocks for molecular rotary motors, a number of alkyne, and aryl based ammonium, imidazolium and pyridinium cations guests were synthesized and crystallized to study the binding mode with CB[6]. Isothermal Titration Calorimatry (ITC) was used in order to determine the binding constants between CB[6] and various aryl diammonium, imidazolium and prydinium guests.

4-Aminobipyridine derivatives form strong inclusion complexes with CB[6], exhibiting remarkably large enhancements of fluorescence intensity and quantum yields. The remarkable complexation-induced pKa shift (ΔpKa = 3.1) highlights the strong charge-dipole interaction upon binding. The reversible binding phenomenon can be used for the design of switchable beacons that can be incorporated into cascades of binding networks. This concept is demonstrated here by three different applications:

(a) A switchable fluorescent beacon for chemical sensing of transition metals and other ligands. (b) Direct measurement of binding constants between CB[6] and various non-fluorescent guest molecules. The complexation-dependent fluorescence phenomenon provides an opportunity to quantitatively determine binding constants between CB[6] and other ligands via competitive binding experiments. (c) Another attractive application of the complexation-dependent fluorescence of AB is the ability to rapidly monitor biocatalytic reaction, including quantitative determination of their kinetic parameters. We demonstrated this advantage by quantitatively monitoring the penicillin G acylase (PGA)-catalyzed hydrolysis of amide, and by the antibody 38C2-catalyzed b-elimination reaction of a b-cabamoyloxy ketone.

Knowing that CB[6] catalyzes 1,3-dipolar cycloaddition (“Click” reaction) between properly functionalized alkyne and azide to yield 1,4-disubsititued triazoles, a number of rotaxanes and pseudorotaxanes were prepared. Switchable and conductive rotaxanes were also synthesized via 1,3 -dipolar cycloaddition reaction including bipyridine moiety that shows strong fluorescence in the presence of CB[6].