Proton-Transfer-Induced Fluorescence in Self-Assembled Short Peptides

Area of Science:

• * Biophysical Chemistry
• * Computational Chemistry
• * Molecular Spectroscopy

Background:

• * Cyclic dipeptides, such as cyclic Phe-Phe (FF), are fundamental units in peptide nanostructures.
• * Visible fluorescence in peptide assemblies is often experimentally observed but lacks a clear molecular explanation.
• * Hydrogen bonding plays a critical role in the self-assembly and properties of peptide nanostructures.

Purpose of the Study:

• * To investigate the molecular mechanisms underlying the visible fluorescence of cyclic FF molecules.
• * To explore the role of proton transfer in excited states of FF dimers and trimers.
• * To correlate specific hydrogen-bonding configurations with observed fluorescence properties.

Main Methods:

• * Employed molecular dynamics (MD) simulations to model FF dimer and trimer structures.
• * Utilized time-dependent density functional theory (TDDFT) to analyze excited-state properties and proton transfer.
• * Analyzed the influence of geometrical arrangements on excited-state proton transfer (ESPT).

Main Results:

• * Identified intra- and inter-molecular proton transfer in excited S1 and S2 states of specific FF configurations.
• * Demonstrated that excited-state proton transfer (ESPT) within hydrogen bonds causes a significant red-shift in fluorescence.
• * Showed that ESPT is highly sensitive to the precise geometry of hydrogen-bonded FF dimers and trimers.

Conclusions:

• * Excited-state proton transfer in specific hydrogen-bonded configurations of cyclic FF molecules explains observed visible fluorescence.
• * The findings provide a molecular basis for the fluorescence of β-sheet peptide nanostructures.
• * Understanding the geometry-dependent nature of ESPT is crucial for designing fluorescent peptide materials.