Non-covalent encapsulation

Fig. 12 a Molecular nanocapsule synthesis, structure, and non covalent encapsulation of platinum pincer complexes in the hydrophilic interior... 

Drugs are encapsulated in dendrimers primarily using two methods by covalent dendrimer-drug conjugation or non-covalent encapsulation of drugs. 

MC-540 NaYF4 Yb,Er 980 nm Non-covalent encapsulation Polymeric liposome Folic acid and transactivating transduction protein MCF-7 PDT 51 I 3 0 s St a 3. 

Very few examples have been described for the non-covalent immobilization of chiral porphyrin complexes (Fig. 26). In the first case, the porphyrin-dichlororutheninm complex was encapsulated in silica, which was prepared around the complex by a sol-gel method [78], in an attempt to prevent deactivation observed in solution in the epoxidation of different alkenes with 2,6-dichloropyridine N-oxide. In fact, the heterogeneous catalyst is much more active, with TON up to 10 800 in the case of styrene compared to a maximum of 2190 in solution. Enantioselectivities were about the same imder both sets of conditions, with values aroimd 70% ee. 

Layered inorganic solids have been used for site isolation, for example, nickel phosphine complexes confined within the interlayer spaces of sepiolite have been used as olefin hydrogenation catalysts [63], and similarly there has been the encapsulation of metal complexes into zirconium phosphates [64], The principal idea is illustrated in Figure 5.8. The metal complex can be encapsulated by covalent means (a) or by non-covalent interactions (b). 

Scheme 1 summarizes four different approaches used to characterize dendrimer structures by photophysical and photochemical probes 1. Non-covalent, inter-molecularly bound interior probes - to study the internal cavities and the encapsulation abilities of dendrimers. 2. Non-covalent, intermolecularly bound surface probes - to study surface characteristics of dendrimers. 3. Covalently linked probes on dendrimer surfaces - to study the molecular dynamics of dendrimers. 4. Covalently linked probes at the dendrimer central core - to study the site isolation of the core moiety and define the hydrodynamic volume of dendrimers by the concentric dendrimer shells. Critical literature in these four categories will be described using representative examples. 

In the preceding section, we reviewed the non-covalent dynamic encapsulation of guest probe molecules within dendrimer interiors. The second case, Scheme 2, involves the physical encapsulation of guest molecules wherein, guest molecules are locked inside dendritic containers (so-called dendritic boxes). This concept was originally proposed by Tomalia et al and referred to as unimolecular encapsulation [2]. More recent and well characterized examples have now been demonstrated by Meijer and co-workers [11-15]. 

Using a similar approach, Chechik and Crooks (73), modified the PAMAM dendrimer-encapsulated palladium nanoparticles with perfluoropolyether tails utilizing non-covalent ion-pair interactions. The catalytic hydrogenation of six substrates under biphasic conditions (toluene/ perfluoro-2-butyltetrahydrofuran FC-75) was investigated. Allyl alcohol, methyl acrylate, vinyl isopropenyl ether, and... 

With the development of supramolecular chemistry, there has been a concomitant shift in the mind-set of chemists working in the area. This has involved a change in focus from single molecules, often constructed step by step via the formation of direct covalent linkages, towards molecular assemblies, with their usual (see exception above) non-covalent weak intermolecular contacts. This change in focus is nicely encapsulated in Lehn s description of supramolecular chemistry as the designed chemistry of the intermolecular bond . ... 

As shown in Table 2, the simulated instabilily of BSA at the low pH was matched with the instabihty of BSA encapsulated in PLGA, in terms of time to 50% aggregation ( 10 days), aggregation type, and distribution of hydrolysis fragments. It was foxmd that moisture affected the aggregation rate but not the mechanism. For water/protein weight ratios of 0-0.2 and 0.8-1.0, no aggregates and 70% non-covalent... 

The stabilization effect of insoluble base on protein in PLGA microspheres has also been demonstrated (8), as shown in Table 4. BSA also was shown to form non-covalent aggregates ( 25-70%) when encapsulated in w/o/w PLGA microspheres, confirming that an acidic microclimate also commonly develops in PLGA 50/50 microspheres (41,43). Whereas the BSA... 

In the early days of this research field also covalent based capsules were assigned as supramolecular capsules because of their encapsulation properties. However, because of the developments of the last decade we designate here only non-covalent based capsules as supramolecular capsules . 

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