Supramolecular chirality transfer from biopolymeric templates to achiral metalloporphyrin aggregates

Misfolding or unfolding of the native structure of proteins can have major physiological implications, especially for neurodegenerative diseases. The role of achiral building blocks able to self-assemble onto homochiral templates has received particular attention, and many examples are now available of the formation of organized supramolecular assemblies of chromophores in which different types of chiral templates are exploited to achieve control of their size and morphology. Porphyrins are appealing candidates because of their rich spectroscopic features which are markedly affected by selfaggregation and by the specific microenvironment. An interesting application of synthetic cationic porphyrins (and their metalloderivatives) in biophysics is represented by their use as probes of the structure and conformational dynamics of nucleic acids and helical polypeptides. We have previously investigated the use of the aggregating cationic metalloporphyrin trans-bis(N-methylpyridinium-4-yl)diphenylporphinato Cu(II) (t-CuPagg) on polyglutamate (PGA) for the detection of the protein conformations as a function of plI and of the order of mixing of the reagentsIS] The chiral scaffold can interact with the achiral dye present in solution as a dimer or in an extensively aggregated state, and it can transfer the chiral information to the chromophores with the appearance of an induced circular dichroism (ICD) signal in the absorption region typical for the porphyrin. Here we report on the systematic study of the t-CuPagg/PGA system with respect to key parameters, e.g. porphyrin to protein ratio or ionic strength, allowing to identify the different supramolecular assemblies forming by the polypeptide in its two conformations. This investigation has been carried out through the combined use of UV/vis extinction, circular dichroism (CD), and Resonance Light Scattering (RLS) techniques.