Energy migration in novel pH-triggered self-assembled beta-sheet ribbons

Area of Science:

• Biophysics
• Supramolecular Chemistry
• Protein Science

Background:

• Self-assembled peptide structures are crucial in biological systems.
• Tryptophan (Trp) fluorescence is sensitive to its microenvironment.
• Understanding energy transfer in peptides informs protein folding and function.

Purpose of the Study:

• To experimentally demonstrate energy migration between tryptophan residues in self-assembled peptide tapes.
• To characterize the structural and dynamic changes of peptide assemblies with pH.
• To investigate the role of the local environment on tryptophan fluorescence anisotropy.

Main Methods:

• Fluorescence spectroscopy (spectra, quenching, anisotropy).
• pH-dependent self-assembly of 11-amino acid peptides with Trp at position 6.
• Time-resolved fluorescence anisotropy measurements in water and sucrose-viscosity mixtures.
• Master equation kinetic modeling of Trp-Trp energy migration.

Main Results:

• Peptide self-assembly into ribbons is pH-sensitive, creating a hydrophobic interior at low pH.
• Tryptophan fluorescence lifetime is shorter at low pH, suggesting quenching.
• Time-resolved fluorescence anisotropy showed minimal change with assembly in water.
• Anisotropy decay increased significantly in viscous media at high pH, but not at low pH.
• Depolarization at low pH is attributed to energy migration, not rotational diffusion.

Conclusions:

• Direct experimental evidence for energy migration between tryptophan residues in self-assembled peptide ribbons.
• Peptide ribbons possess a stable, hydrophobic interior at low pH.
• Energy migration, rather than rotational diffusion, explains fluorescence depolarization at low pH.
• Master equation modeling supports the proposed Trp-Trp energy migration mechanism.