Deciphering excited state evolution in halorhodopsin with stimulated emission pumping.

Area of Science:

• Photochemistry
• Biophysics
• Spectroscopy

Background:

• Halorhodopsins are light-driven ion pumps crucial for cellular homeostasis.
• Understanding their primary photochemical dynamics is key to elucidating energy transduction mechanisms.
• Previous studies on related proteins suggest complex excited-state pathways.

Purpose of the Study:

• To investigate the primary photochemical dynamics of Hb. pharaonis Halorhodopsin (pHR).
• To determine the nature of the fluorescent state and its role in the photocycle.
• To compare the excited-state dynamics of pHR with other halorhodopsins.

Main Methods:

• Femtosecond visible pump-near IR dump-hyperspectral probe spectroscopy.
• Analysis of transient absorption changes with and without stimulated emission pumping (SEP).
• Probing of the "K" intermediate difference spectrum to assess photocycle population.

Main Results:

• The cross-section for stimulating emission in pHR is constant throughout the fluorescent state lifetime.
• Excited state depletion via dumping proportionally reduces photocycle yields.
• Internal conversion (IC) kinetics are nonexponential.

Conclusions:

• The fluorescent state in pHR represents a single photocycle intermediate.
• This contrasts with the proposed model for Hb. salinarum Halorhodopsin (sHR), which suggests a distinct subpopulation.
• The findings provide new insights into excited-state dynamics across different halorhodopsins.