A 10-A spectroscopic ruler applied to short polyprolines.

Area of Science:

• Biophysics
• Spectroscopy
• Computational Chemistry

Background:

• Fluorescence resonance energy transfer (FRET) is a powerful tool for measuring distances in biomolecules.
• Tryptophan (Trp) and 2,3-diazabicyclo[2.2.2]oct-2-ene (Dbo) form a FRET pair with a short Förster radius, suitable for short peptides.
• Polyproline II (PPII) helices are important secondary structures in peptides.

Purpose of the Study:

• To investigate FRET efficiency and donor-acceptor distances in short peptides with varying lengths.
• To explore the influence of solvent on peptide structure and FRET.
• To assess the applicability of the Trp/Dbo FRET pair as a spectroscopic ruler for short polyprolines.

Main Methods:

• Steady-state and time-resolved fluorescence spectroscopy.
• Circular Dichroism (CD) and Nuclear Magnetic Resonance (NMR) spectroscopy.
• Molecular Dynamics (MD) simulations using the GROMOS96 force field.

Main Results:

• FRET efficiencies varied from 2-72% depending on peptide length (n=1-6).
• Donor-acceptor distances ranged from ~8 Å (n=1) to ~16 Å (n=6) in water.
• Peptides showed early nucleation of PPII helix structure for n ≥ 2, with increased rigidity in propylene glycol.

Conclusions:

• The Trp/Dbo FRET pair enabled distance measurements in short peptides.
• Peptide length and solvent significantly influenced FRET efficiency and structural properties.
• FRET-derived distances correlate with Förster radii, indicating limitations for short polyprolines due to chromophore size and dipole approximation breakdown.