Porphyrin-Based Supramolecular Flags in the Thermal Gradients' Wind: What Breaks the Symmetry, How and Why.

Angelo Nicosia,Grazia Messina,Maria De Martino,Valentina Villari,Giovanni Marletta,Norberto Micali,Cristina Satriano,Maaike Schotman,Fabiana Vento,Placido Mineo

doi:10.3390/nano11071673

Abstract

The Spontaneous Symmetry Breaking (SSB) phenomenon is a natural event in which a system changes its symmetric state, apparently reasonless, in an asymmetrical one. Nevertheless, this occurrence could be hiding unknown inductive forces. An intriguing investigation pathway uses supramolecular aggregates of suitable achiral porphyrins, useful to mimic the natural light-harvesting systems (as chlorophyll). Using as SSB probe supramolecular aggregates of 5,10,15,20-tetrakis[p(ω-methoxypolyethyleneoxy)phenyl]porphyrin (StarP), a non-ionic achiral PEGylated porphyrin, we explore here its interaction with weak asymmetric thermal gradients fields. The cross-correlation of the experimental data (circular dichroism, confocal microscopy, atomic force microscopy, and cryo-transmission electron microscopy) revealed that the used building blocks aggregate spontaneously, organizing in flag-like structures whose thermally-induced circular dichroism depends on their features. Finally, thermal gradient-induced enantioselectivity of the supramolecular flag-like aggregates has been shown and linked to their size-dependence mesoscopic deformation, which could be visualized as waving flags in the wind.

Highlights

Considering the lack of literature -reported data about size-dependent studies of supramolecular porphyrin aggregates referring to neutral water-soluble porphyrin derivatives, we focused on the structure-effect correlation between the supramolecular aggregates size and the Symmetry Breaking (SB) phenomenon
Thanks to the PEGylation, this system show a particular behavior in aqueous solution than common ionic porphyrins (i.e., TPPS): it has a non-ionic structure; it is soluble in a large class of solvent; it spontaneously aggregates in water solution, but not in organic solvents
This study was focused on the relation between the supramolecular aggregates features and the SB phenomenon

Summary

Introduction

On every scale of the Universe, the Spontaneous Symmetry Breaking (SSB) phenomenon happens. It is an event through which nature chooses the handedness of one species over the opposite, apparently without any reason [1,2,3,4,5]. In laboratory experiments, when achiral molecules are subjected to self-organization or supramolecular aggregation, the phenomenon has been noted [6,7,8]. The porphyrin derivatives are among the most used types of SSB probes [7,9] because of their well-known characteristics, such as high molar absorptivity and water solubility (induced by peripheral charged groups and/or hydrophilic polymer branches), their ability to self-aggregate by tuning the solution

Methods

Results

Conclusion