Excited-state dynamics of bacteriorhodopsin.

Area of Science:

• Biophysics
• Photochemistry

Background:

• Bacteriorhodopsin exhibits near-infrared emission with a large Stokes shift at neutral pH.
• This phenomenon is pH-dependent, with altered characteristics at acidic pH.

Purpose of the Study:

• To investigate the pH-dependent fluorescence properties of bacteriorhodopsin.
• To elucidate the mechanism behind the large Stokes shift and fluorescence enhancement.

Main Methods:

• Spectroscopic characterization of bacteriorhodopsin's emission.
• Fluorescence polarization measurements.
• Absorption spectroscopy.

Main Results:

• At neutral pH, bacteriorhodopsin shows a large Stokes shift in its near-infrared emission.
• At acidic pH (around 2), fluorescence quantum yield increases over 10-fold, with a red shift in absorption.
• Emission originates from the retinylidene chromophore's first allowed excited state, independent of pH.

Conclusions:

• The large Stokes shift at neutral pH is likely due to immediate post-excitation charge displacement, possibly proton translocation.
• Protonation of an amino acid residue in the ground state prevents this charge displacement at acidic pH.