Functional core

A wide range of dendrimers with functional core is described in the literature. Thus chromophores [7], electrochemically active, redox active [8], and catalyti-cally active [9] or also self-associating and chiral units as well as polymerisable monomers and polymers have been successfully introduced into the centre of dendrimers. However, the core unit not only has a determining effect on the function, but also has a decisive influence on the multiplicity, size, and shape of the dendrimer. 

The long-term goal is to be able to tune the properties of the functional core unit to the technical requirements of specific applications by attachment of appropriate dendrons. Numerous publications devoted to the elucidation of these interrelationships are bringing the day when this goal will be realised closer and closer, in spite of the many factors which still have to be considered. 

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The ability of a dendritic shell to encapsulate a functional core moiety and to create a specific site-isolated microenvironment capable of affecting the molecular properties has been intensively explored in recent years [19]. A variety of experimental techniques have been employed to evidence the shielding of the core moiety and to ascertain the effect of the dendritic shell [19, 20]. Dendrimers with a fullerene core appear to be appealing candidates to evidence such effects resulting from the presence of the surrounding dendritic branches. Effectively, the lifetime of the first triplet excited state of fullerene derivatives... 

Literature reports on synthetic methods for the construction of the pyrimidinone core were very limited. Most of the synthetic strategies toward the densely functionalized core fell into two methodologies, which start from the same amidoxime 13 (Scheme 6.3). Route A is a three-step sequence that involves hydrogenation of 13 to prepare amidine 14. Claisen condensation of commercially available a-benzyloxy acetate and methyl tert-butyl oxalate provides the dihydroxyfumarate... 

Scheme 4.17 Generation of secondary amine 33 by deprotection of N-allyl group enables access to functionalized core structures...

Nayak, S. Lyon, L. A., Ligand functionalized core/shell microgels with permselective shells, Angew. Chem. Int. Ed. 2004, 43, 6706 6709... 

The potential influence of the dendrylation on the functional core unit includes sometimes a drastically increased molecule size as well as a steric shielding (encapsulation) and a micro-environment different and isolated from its external surroundings, eg., unimolecular micellar structures, electron-rich shells, solubilization. It is even possible to activate the core unit by both energy and electron transfer processes. In the following subsections, these design possibilities will be dealt with in more detail. 

In agreement with Flory s predictions, hyperbranched polymers based on A,jB monomers reported in the literature exhibit a broad molecular weight distribution (typically 2-5 or more). The polydispersity of a hyperbranched polymer is due to the statistical growth process. A strategy to overcome this disadvantage is to add a By-functional core molecule, or a chain terminator, which Hmits the polydispersity and also provides a tool to control the molecular weight of the final polymer. The concept of copolymerizing an A2B monomer with a B3 functional core molecule was first introduced by Hult et al. [62] and more recently also utilized by Feast and Stainton [63] and Moore and Bharathi [64]. 

The dendrimer framework also plays an important role. The catalytic performance measured by activity, selectivity, stability, and recyclability depends on the dendritic architecture, and it is important to distinguish periphery-functionalized, core-functionalized, and focal point-functionalized dendrimers (Fig. 1). Periphery-functionalized dendrimers have catalytic groups located at the surface where they are directly available to the substrate. In contrast, when a dendrimer is functionalized at its core, the substrate has to penetrate the dendrimer support before it reaches the active center, and this transport process can limit the rate of a catalytic reaction if large and congested dendrimers are involved. 

Connal LA, Vestberg R, Hawker CJ, Qiao GG (2007) Synthesis of dendron functionalized core cross-linked star polymers. Macromolecules 40 7855-7863... 

The medial and caudal portions of the midbody of the CC contain interhemispheric projections from the auditory cortices, posterior cingulate, retrosplenial cortex, and insula, and somatosensory and visual cortices to a lesser extent. It also includes connections from the inferior parietal lobe to the contralateral superior temporal sulcus, cingulate, and parahippocampal gyrus (Pandya and Seltzer, 1986). Several of the regions with interhemispheric projections through the medial and caudal portions of the midbody of the CC are involved in the processing of emotional stimuli and various memory functions—core disturbances frequently observed in children with a history of early trauma. 

FIGURE 19-11 Cytochrome be, complex (Complex III). The complex is a dimer of identical monomers, each with 11 different subunits. (a) Structure of a monomer. The functional core is three subunits cytochrome b (green) with its two hemes (bH and foL, light red) the Rieske iron-sulfur protein (purple) with its 2Fe-2S centers (yellow) and cytochrome ci (blue) with its heme (red) (PDB ID 1BGY). (b) The dimeric functional unit. Cytochrome c, and the Rieske iron-sulfur protein project from the P surface and can interact with cytochrome c (not part of the functional complex) in the intermembrane space. The complex has two distinct binding sites for ubiquinone, QN and QP, which correspond to the sites of inhibition by two drugs that block oxidative phosphorylation. Antimycin A, which blocks electron flow from heme bH to Q, binds at QN, close to heme bH on the N (matrix) side of the membrane. Myxothiazol, which prevents electron flow from... 

Two domains, xl and x2, exist which affect the GR post-DNA binding transcription activity (37). The major (xl) transactivation domain is 185 amino acid residues in length with a 58-residue Ct-helical functional core (38). The Tl domain is located at the N terminus of the protein the minor (x2) transactivation domain residues on the carboxy-terminal side of the DNA binding domain. 

Drijhout and Bloemhoff142 introduced a thiol-functionalized core based on the K2K unit by coupling A-(5-acetylsulfanylacetyl)glutamyl residues to the amino groups (Scheme 9). Subsequently, Baleux and Dubois[92] prepared a thiolate core based on a similar design by... 

Method B Protected Cys(Npys) di-K2K core 39 (4 mg, 8 pmol)1441 was suspended in 0.05 M AcOH (5 mL) and the pH of the mixture adjusted to 8 with 1M NH4OH, and DTT (8 mg) was added. The reaction was spurred by N2 and stirred for 0.5 h. The dark yellow soln was applied to a short Sephadex G10 column and eluted with 0.05 M AcOH. The first eluting fractions, positive by Ellman s test and containing the SH functionalized core, were pooled and Cys(Npys) activated peptide (8.8 pmol, 26 mg) was added. The pH was adjusted to 6 with 1M NaOH. The mixture was purged with N2 and stirred overnight. The liberated 3-nitro-pyridine-2(l//)-thione was eliminated by rapid gel filtration. The MAP was then purified to give 62b by HPLC to eliminate the excess peptide monomer yield 12 mg. 

Opatowski, Y., Chen, C.-C., Campbell, K.P. and Hirsch, J.A. (2004) Structural analysis of the voltage-dependent Ca2+ channel P subunit functional core and its complex with the al interaction domain. Neuron 42, 387-399. 

The oligo- or multi-functional core unit also plays a role in determining the space occupied by a dendrimer. The core itself can exercise a function, as demonstrated by metallodendrimers (see Section 4.1.11), in which the metal ion core in a supramolecular or coordinatively constructed architecture coordinates with the surrounding branching units - and in this way can influence catalytic and photochemical processes. 

Fig. 3.1 a) Convergent, b) supramolecular, and c) divergent synthesis of dendrimers with functional core (schematic)... 

In the synthesis of functional dendrimers, interest has hitherto been focussed on variation of the functional core unit or peripheral groups and the resulting effects on the properties of the dendrimer. For a long time, the only function ascribed to the dendritic branches and their repeating units was that of a scaffold linking periphery and core. It was overlooked that, in the interior of the dendrimer scaffold, an individual characteristic (nano)environment can arise which is largely dependent upon the chemical characteristics and the polarity of the repeating units used to construct the dendrimer. Moreover, they can facilitate cascade processes and serve as a platform for cooperative effects between dendrimer branches [37]. 

Whereas the bifunctionalisation strategies presented so far are relatively straightforward, the synthesis of multifunctional dendrimers with more than two kinds of functional units requires considerable synthetic effort. Preparation of dendrimers with a functional core and additional functional units in the dendrimer scaffold and in the periphery requires de novo synthesis of the entire dendrimer scaffold, with the synthesis conditions having to be tolerable for all groups (Fig. 3.15). 

Convergent, supramolecular or more rarely divergent dendrimer assembly using a functional core unit... 

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