Dendrimers with Porphyrin Cores: Synthetic models for globular heme proteins

Abstract

Dendritic iron porphyrins were synthesized as functional mimics of globular electron-transfer heme proteins.The cascade molecules 1 * Zn-3 -Zn of first to third generation were obtained starting from the (meso-diarylporphyrin) zinc 6 * Zn which contains four carboxylate arms for attachment of the poly(ether-amide) dendritic branches by peptide-coupling methodology (Scheme I). Generation 3 compound 3 . Zn with 108 methyl-carboxylate end groups has a molecular weight of 19054 D, and computer modeling suggests that its structure is globular and densely-packed, measuring ea. 4 nm in diameter and, therefore, similar in dimensions to the electron-transfer protein cytochrome-c. Starting from the generation 1 poly(carboxy1ic acid) 11 * Zn and the generation 2 analog 12 * Zn the dendritic Zn" porphyrins 4 * Zn and 5 . Zn, respectively, were obtained by esterification with triethyleneglycol monomethyl ether (Schemes 3 and 4). Demetallation followed by insertion of Fe" and in situ oxidation afforded the water-soluble dendritic iron porphyrins 4 * FeCl and 5 * FeCl. The electrochemical behavior of esters 1 * Zn -3 . Zn in organic solvents changed smoothly with increasing dendritic generation (Table f).Progressing from 1 * Zn to 3 . Zn in THF, the first porphyrin-centered oxidation and reduction potentials become more negative by 320 and 210 mV, respectively. These changes were attributed to strong microenvironmental effects imposed on the electroactive core by the densely packed dendritic surroundings. The electrochemical properties of 4 * FeCI and 5 -FeCl were investigated by cyclic voltammetry in both CH,CI, and H,O (Tables 2 and 3). Progressing from 4 . FeCl to 5 * FeCl in CH,CI,, the redox potential of the biologically relevant Fe"'/Fe" couple remained virtually unchanged, whereas in aqueous solution, 5 * FeCl exhibited a potential 420 mV more positive than did 4 . FeCI. The large difference between these potentials in H,O was attributed to differences in solvation of the core electrophore. Whereas the relatively open dendritic branches in 4 -FeCl do not impede access of bulk solvent to the central core, the densely packed dendritic superstructure of 5 * FeCl significantly reduces contact between the heme and external solvent. As a result, the more charged Fe"' state is destabilized relative to Fe", and the redox potential is strongly shifted to a more positive value.

A Redox-Controlled Molecular Switch Based on the Reversible C−C Bond Formation in Octamethoxytetraphenylene

“…Similarly, the simple dissolution of a 2:1 mixture of the wellknown one-electron oxidant NO SbCl 6 À and neutral OMT in dichloromethane at À 20 8C immediately results in a red solution with an identical absorption spectrum; and the quantitative formation of OMT 2 according to Equation (3) is determined spectrophotometrically. [13] OMT + OMT 2+ + 2 NO 2 NO + (3) It is important to note that the transformation of OMT to OMT 2 is initiated by well-known one-electron oxidants [see Eq. (2) and (3)].…”

Dendrimers: synthetic strategies, properties and applications

“…This kind of site isolation was used to mimic different natural systems by introduction of the functional groups (i.e., porphyrins) at the core. For example, Diederich et al prepared zinc porphyrin dendrimers ( Figure 3) as a model to mimic the electron-transfer proteins (e.g., cytochrome C) 34,35,[48][49][50] .…”

Supramolecular chemistry of dendrimers with functional cores

“…Examples are hemoglobin-and myoglobin-like gas binding ability (34)(35)(36), heme monooxygenase activity (14,37), electron acceptor capacity in light-harvesting antenna systems (38), and shell-modulated redox potentials as found in cytochromes (39)(40)(41). We discuss here the model character of dendritic iron porphyrins with tethered axial ligands for cytochromes (42,43).…”

“…The heme microenvironment, created by the surrounding protein shell, has been identified as a major factor influencing the redox potentials that are substantially anodically shifted (by 300-400 mV) compared with simple heme model systems lacking the peptide coverage. We had prepared a series of dendritic iron porphyrins as model compounds for cytochromes (39)(40)(41), in which the protein shell around the buried electroactive core was mimicked by the dendritic superstructure. However, the nature of the axial ligation to the iron center, which is known to have a very strong influence on the redox properties (45), was not controlled in these earlier systems.…”