Modeling dendrimers

Numerical calculations inspired in the ZK method for stars have also been applied for the description of the dynamics of model dendrimers. La Ferla [232] used a freely-rotating model, including a topology-dependence stiffness parameter and preaveraged HI. With this model, he obtained a complex analytical expression for the mean size. Cai and Chen [233] used a Gaussian model without HI and performed a detailed analysis of the relaxation motions. They investigated the diffusion of the center of mass, the relaxation of the center of mass position relative to the core monomer, and also the rotational and internal modes. 

Mansfield ML. Dendron segregation in model dendrimers. Pol3mier 1994 35 1827-1830. 

In their theoretical considerations of molecular structure, de Gennes and Her-vet assumed an ideal dendrimer with extended branches with all terminal groups arranged at its periphery in a kind of outer ring around the dendrimer core [9]. According to this model, dendrimers should exhibit a lower segment density at the core, which increases to a maximum value on moving to the periphery. This concept is known as the dense-shell model (Fig. 1.17). 

The shape deduced from the analysis of the radius of gyration tensor also changes as the generation number increases, as shown in Table 1.3. The entries in this table were calculated from the results for the moments of inertia that were reported by Mansfield and Klushin J24 Even as soon as the second generation, these model dendrimers (1) are closer to being spherically symmetric than are any of the conformations listed in Table 1.2. 

Table 1.3 Ratios of the averaged principal moments of the radius of gyration tensor, asphericity, acylindricity, and anisotropic shape factor for model dendrimers (1, Fig. l.l)a).

Figure 1.2. Asphericity of model dendrimers, as calculated from the moments of inertia described by Mansfield and Klushin.1241...

Mansfield 79 performed Monte Carlo calculations on model dendrimers and determined that as a result of the unique architecture of the branches, even when similar chemically, they are well segregated. Further, he concluded that dendrimers are fractal (D ranges from 2.4 to 2.8) and self-similar only over a rather narrow scale of lengths. 

The field of synthetic enzyme models encompasses attempts to prepare enzymelike functional macromolecules by chemical synthesis [30]. One particularly relevant approach to such enzyme mimics concerns dendrimers, which are treelike synthetic macromolecules with a globular shape similar to a folded protein, and useful in a range of applications including catalysis [31]. Peptide dendrimers, which, like proteins, are composed of amino acids, are particularly well suited as mimics for proteins and enzymes [32]. These dendrimers can be prepared using combinatorial chemistry methods on solid support [33], similar to those used in the context of catalyst and ligand discovery programs in chemistry [34]. Peptide dendrimers used multivalency effects at the dendrimer surface to trigger cooperativity between amino acids, as has been observed in various esterase enzyme models [35]. 

An interesting case in the perspective of artificial enzymes for enantioselective synthesis is the recently described peptide dendrimer aldolases [36]. These dendrimers utilize the enamine type I aldolase mechanism, which is found in natural aldolases [37] and antibodies [21].These aldolase dendrimers, for example, L2Dl,have multiple N-terminal proline residues as found in catalytic aldolase peptides [38], and display catalytic activity in aqueous medium under conditions where the small molecule catalysts are inactive (Figure 3.8). As most enzyme models, these dendrimers remain very far from natural enzymes in terms ofboth activity and selectivity, and at present should only be considered in the perspective of fundamental studies. 

Anionic poly(amidoamine) (PAMAM) dendrimer was selected as a model of the soluble acidic-rich proteins to prepare CaC03 film on a poly(ethylenimine) film [49]. The CaCOj/polylethylenimine) composite film was obtained in the... 

Fig. 2. Model of a toroid-like complex between plasmid DNA and dendrimers. Dendrimer molecules (spheres) are located both inside and outside the coiled DNA molecules, and more than one plasmid DNA molecule can be present in one toroid-like complex. The left part of the graphic shows the most probable situation the dendrimers on the outside of the complex are not shown in the remainder of the graphic in order to give a better view of the inner part of the complex. The diameter of such a complex is approximately 50 to 100 nm...

Dendrimers are attractive nanosize model compounds because of their globular architecture and their highly functionalized surface. These hyperbranched compounds are synthesized in a repetitive reaction sequence of nearly quantitative reactions. The synthetic route can either be divergent, starting from the nucleus toward the surface, or convergent, where dendrons or wedges are covalently linked to a polyfunctional nucleus. The number of metallodendrimers is still limited.493-506... 

These researchers also described [93] the design and synthesis of iron(II) porphyrin dendrimers with triethylene glycol monomethyl, ether surface groups (e.g., 31) which render them soluble in a wide range of organic solvents and water. The potential difference between the first (1 FeCl) and second generation (2 FeCl) Fe-porphryin dendrimers was found to increase more in water than in dichloromethane (0.42 vs 0.08 V). This remarkable potential difference between 2 FeCl and 1 FeCl in water was comparable with that found between cytochrome c and a similarly ligated, more solvent-exposed cytochrome c heme model compound. 

Fig. 26. a Specific optical rotations for the model building blocks for core 78, interior building block 79 and peripheral unit 80 of dendrimers of type 77 b calculated average optical rotations for the doubly branching dendrimers of Oth to 4th generation c measured optical rotations in CHC13 at different temperatures and concentrations [94]... 

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