Self-assembly capsules

C. A. Schalley, R, K. Castellano, M. S. Brody, D. M. Rudkevich, G. Siuzdak, J. Rebek, Jr, Investigating Molecular Recognition by Mass Spectrometry Characterization of Calixarene-Based Self-Assembling Capsule Hosts with Charged Guests , J. Am. Chern Soc, 1999, 121, 4568-4579. 

Conn MM, Rebek J. Self-assembling capsules. Chem Rev 1997 97 1647-1668. 

A template effect by solvent was found in the synthesis of self-assembled capsules. Experimental evidence shows that the solvent molecules control the covalent bond formation through molecular recognition within the monomeric tetrahedral intermediate. It is proposed that solvation effects can be treated as a subset of molecular recognition events (Tokunaga et al., 1998). 

An interesting example of less rigid cages is also provided by molecular self-assembling capsules obtained in the Rebek group [56]. 81 was found to form dimers held together by hydrogen bonds between the donors at its ends and the acceptors in the middle. 

Self-assembled capsules are of significant interest as molecular reaction vessels and in the control of confined inner space . 

Entrapment of guests within self-assembled capsules provides a fourth method for the construction of chiral supramolecular aggregates. In fact, confinement of guests into cavities with a proper shape and size limits their available space and allows the control over their reciprocal positioning. This reflects directly into their mutual electronic and steric interaction and, as a consequence, in their stereorecognition (Fig. 4D). 

A molecular capsule can be composed of more than two units also a sphere can be obtained assembling together a generic number n of slices. Rebek and collaborators, in their extensive exploration of the properties and features of self-assembled capsules [36], developed a L-shaped molecule characterized by the presence at one end of the glycoluril hydrogen bond system, and at the other end, separated by an aromatic ring, a sulfamide moiety (Fig. 14). 

The flexibility and mutability of hydrogen bonds, as compared to covalent bonds, make self-assembled capsules unlikely to display good selectivities for closely related... 

The inclusion of guests is not the only interesting aspect of the dimers formed by tetra-urea calixarenes. Numerous other self-assembled capsules are known, larger in volume, different in shape, and able to include more than one guest molecule, and various aspects of guest inclusion have been extensively studied [45]. 

Ajami D, Rebek J (2008) Gas behavior in self-assembled capsules. Angew Chem Int Ed Engl 47 6059-6061... 

An alternative to an extensive inorganic structure is an extensive organic one. Candidates for this could include any number of polymers. One approach is to make self-assembled capsules from complementary polymers that form layer by layer (LbL) vesicle-like structures [26], This has been achieved by templating the co-assembly of polymers around a removable core. The alternation of polymers with opposite charges allows the composition and thickness of the artificial cell walls to be controlled. The size of the core determines if the resulting capsule is a model for a cell or a smaller capsule like an organelle within a cell. The porous nature of the polymer allows chemical species to enter and leave the capsule but the potential for capsule growth and division, even with the presence of polymers in the external solution is very limited. 

Besides these covalently constructed cage-type receptors a number of self-assembled capsules with hydrogen bond donors protruding to the inside have also been described. An early example was reported by Atwood et al. This system makes use of the well known properties of resorcin[4]arene 30a and pyrogallol[4]arene 30b to form hexameric spherical capsules in the solid state and, in the case of 30b, also in solution. 

Haino T, Kobayashi M, Chikaraishi M, Fukazawa Y (2005) A new self-assembling capsule via metal coordination. Chem Commun 2321... 

Figure 24. Schematic representation of conformational interconversions in carcer-ands and structurally similar self-assembled capsules. Left. Interconversion of "twistomers" with a still freely rotation guest. Right Rotation of the guest (symbolized by an arrow) in a frozen twistomer.

Kim SP, Leach AG, Honk KN. The origins of noncovalent catalysis of intermolecular Diels-Alder reactions by cyclodextrins, self-assembling capsules, antibodies, and RNAses. J. Org. Chem. 105. 2002 67(12) 4250-4260. 

Encapsulation of Guest Molecules in Self-assembling Capsules... 

The encapsulation of guest molecules in self-assembling capsules is a topic of great... 

Fig. 5.12. Rebek s softballs — dimeric self-assembling capsules. The cavity volume can be tuned by the length of the spacer introduced between the center piece and glycoluril binding sites of the monomers. The methylated monomer (bottom) is not able to form dimeric capsules. Several hydrogen bonding donors and acceptors (arrows) that are responsible for dimerization, are implemented in each building block. A computer-generated model of an example of a dimeric capsule is shown in the box. Right Typical guest molecules which can be encapsulated in the cavity.

---