Probing the interior of peptide amphiphile supramolecular aggregates.

We present a study of the aqueous solvation within self-assembled structures formed from peptide amphiphiles. We have placed tryptophan and pyrene chromophores onto the peptide backbone to enable spectroscopic examinations of the interior of the resulting supramolecular objects. Self-assembly constrains the chromophores to a defined location within an aggregate, and they experience differing degrees of quencher penetration reflective of their depth within the nanostructure. Tryptophan fluorescence indicates that the interiors remain well-solvated, suggesting that the supramolecular aggregates maintain high degrees of free volume. The Stern-Volmer quenching constants and the fractional accessibility (of covalently bound pyrene) progressively increase as the chromophore is placed closer to the aggregate exterior. Furthermore, these aggregates encourage chromophore uptake from aqueous solution as evidenced by the solubilization of free pyrene chromophores. Our findings demonstrate that covalently bound fluorophores within an aggregate can interact with the external environment. Studies with small molecular probes indicate that these self-assembled architectures may represent viable vehicles to sequester hydrophobic, insoluble organic molecules (within the interior) and to present signaling protein epitopes to cells (on the periphery).