Induction and memory of chirality in porphyrin hetero-aggregates: the role of the central metal ion.

The anionic H2TPPS porphyrin and its copper derivative, CuTPPS, form in aqueous solution hetero-aggregates with the cationic H2T4 porphyrin and its copper derivative, CuT4. In the presence of poly-L-glutamate, at pH 4.0, a CD signal appears in the Soret region of the spectrum, indicating that the polypeptide has induced chirality into the structure of the aggregates. These species exhibit remarkable inertness due to the strength and number of the coulombic interactions between the anionic and the cationic porphyrins. This property allows them to preserve the chiral structure, even when the matrix changes or loses its chiral conformation, demonstrating that these aggregates are capable of memorizing the chiral information. The remarkable properties of the title systems may find various applications (chiral amplification, discrimination, and separation) that, on the other hand, require a more strict control of the aggregate dimension. Here, we show that the central copper of these macrocycles is crucial for determining the aggregate dimension.

Hierarchical porphyrin self-assembly in aqueous solution.

In aqueous solution meso-tetrakis(4-phosphonatophenyl)porphyrin shows self-aggregation processes controlled by the "sergeant-soldier rule". After partial protonation of the external phosphonic groups, it is possible (i) to further protonate the inner nitrogen atoms of the molecules or (ii) to allow, over time, the system to aggregate. Therefore, the two procedure lead to a different system evolution, producing species with different chemico-physical properties.