Dendrimers with Porphyrin Cores: Synthetic models for globular heme proteins

Abstract

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 1 ). 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 ca. 4 nm in diameter and, therefore, similar in dimensions to the electron‐transfer protein cytochrome‐ c. Starting from the generation 1 poly(carboxylic acid) 11 · Zn and the generation 2 analog 12 · Zn the dendritic Zn II 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 II 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 1 ). Progressing from 1 · Zn to 3 · Zn in THF, the first porphyrin‐centered oxidation and reduction potentials become more negative by 320 and 210mV, 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 · FeCl and 5 · FeCl were investigated by cyclic voltammetry in both CH 2 Cl 2 and H 2 O ( Tables 2 and 3 ). Progressing from 4 · FeCl to 5 · FeCl in CH 2 Cl 2 , the redox potential of the biologically relevant Fe III /Fe II couple remained virtually unchanged, whereas in aqueous solution, 5 FeCl exhibited a potential 420 mV more positive than did 4 FeCl. The large difference between these potentials in H 2 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 III state is destabilized relative to Fe II , and the redox potential is strongly shifted to a more positive value.