Metal chelating cores

Each step in dendrimer synthesis occurs independent of the other steps therefore, a dendrimer can take on the characteristics defined by the chemical properties of the monomers used to construct it. Dendrimers thus can have almost limitless properties depending on the methods and materials used for their synthesis. Characteristics can include hydrophilic or hydrophobic regions, the presence of functional groups or reactive groups, metal chelating properties, core/shell dissimilarity, electrical conductivity, hemispherical divergence, biospecific affinity, photoactivity, or the dendrimers can be selectively cleavable at particular points within their structure. 

Scrimin and co-workers have extensively studied the ester-cleavage abilities of various Cu(II)-chelating bidentate ligand [(2-hydroxymethyl)pyridine] amphi-phile, 9, and related bolaphile, 10, in micellar media [32], The corresponding metallomicelles are powerful catalysts for the cleavage of substrates, e.g. p-nitrophenyl alkanoates, that do not coordinate with the metal-complex core. Subsequent studies demonstrated that tridentate ligand amphiphiles such as 9,... 

As previously mentioned (see also Fig. 3), the initiator core/core cell acts as the primary template which permits the introduction of very important molecular information (size, shape, multiplicity [Nc]) and specific function. For example, introduction of a specific function such as a chromophore [2] or metal chelator [132] in the initiator core represents stored information which will become buried... 

First, one can envision the expanded porphyrins being useful in a number of applications involving metal chelation The increased core size of these macrocycle could prove particularly useful for the coordination of large cations. This would suggest their utility as detoxification chelators in medical situations. One could also conceive that the expanded porphyrins could prove of particular use in the... 

Because of their basic resemblance to porphyrins, it was initially expected that the sapphyrins would mimic, at least on some level, the rich coordination chemistry displayed by the porphyrins. However, the larger core size ca. 5.5 A inner N-N diameter vs. ca. 4.0 A for porphyrins), the greater number of potentially chelating heteroatom centers, and the fact that pentaazasapphyrins when fully deprotonated are potentially trianionic ligands made sapphyrin a likely candidate for large metal chelation, particularly as a potential ligand for the trivalent lanthanides and actinides. Unfortunately, in spite of extensive effort, this hope remains largely unrealized. Nonetheless, some metal complexes of sapphyrins and heterosapphyrins have been successfully prepared and characterized. Their preparation and properties are reviewed in this section. 

Bombesin was derivatized at the C terminus with bidentate chelators (Smith et al. 2003 a, b). The labeled peptide fully retained the biological activity and was stable in vitro and in vivo. Since the bidentate coordination is not optimal with respect to stability (pharmacokinetics), the coordination sphere of the metal tricarbonyl core has been saturated with a highly hydrophilic phosphine. This additional coordination is an example of the 2-Hl approach mentioned in the previous section. The mixed-ligand approach resulted in significantly higher hydrophilicity of the radioconjugates and an improved biodistribution labeled with Tc-99m and also with Re-188 (Smith et al. 2003). 

A closer look at the system, however, does pique curiosity. The initial pH within the chamber is not 7 but 2-3, and the reactions are non-equilibrium, often irreversible, and involve other intermediates that can become important end products. The acidic pH represents a problem in that thiolates, not thiols, are the operative reductants, thus cannot reduce at pH values below their typical i.e. 8-9. This is resolved by proteins, including mfp-6, by sequence specific effects such as flanking cationic groups that reduce the Cys pK, e.g. redox active Cys-59 in DsB-A has a p Tg of 3.5. Several Cys residues in mfp-6 are acidic, but specific p Tg values have yet to be measured. The non-equilibrium, irreversible nature of the oxidation reactions is a particular problem with Dopa and other catechols. Indeed, the chemical fate of catechols in mussel byssus is highly dependent on their location. In the cuticle, the fate of Dopa appears to be tris catecholato-Fe complexes in the thread and plaque core, Dopa forms covalent cross-links after oxidation to quinones, whereas at the plaque-substratum interface, it is some combination of metal chelates and reduced H-bonded Dopa on metal oxide surfaces. The reducing capacity of mfp-6 plays a role in maximizing the latter and is astonishingly sustained, i.e. >21 days. ... 

The photochemistry of transition metal 1,3-diketone chelate complexes has been known for some time [30,31], and their photophysical and photochemical properties and photocatalytic activity in different chemical reactions were reviewed in 1990 by Marciniak and Buono—Core [32]. Further discussion on the photochemistry of meta] chelate will not take place here since this subject is out of the scope of this chapter. 

Halet )-F, Saillard )-Y (1997) Electron Count Versus Structural Arrangement in Clusters Based on a Cubic Transition Metal Core with Bridging Main Group Elements. 87 81-110 Hall DI, Ling JH, Nyholm RS (1973) Metal Complexes of Chelating Olefin-Group V Ligands. 15 3-51... 

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