Deciphering intrinsic deactivation/isomerization routes in a phytochrome chromophore model

Area of Science:

• Photochemistry
• Molecular Biophysics
• Quantum Chemistry

Background:

• Phytochromes are crucial photoreceptors in plants.
• Understanding their chromophore's photophysical properties is key to their function.
• Previous models lacked detailed mechanistic insights into excited-state decay.

Purpose of the Study:

• To elucidate photoinduced molecular motion and decay mechanisms of a phytochrome chromophore model.
• To explore the underlying reasons for its photophysical and photochemical properties.
• To develop a general deactivation model for photoexcited phytochrome chromophores.

Main Methods:

• High-level ab initio correlated (CASPT2) computations were employed.
• Simulations were performed for the chromophore model in vacuo.
• Preliminary hybrid QM/MM computations were used for the protein environment.

Main Results:

• Competitive deactivation routes were identified, revealing primary photochemical events and photoisomerization ability.
• A static reactivity model explains excited-state intermediates, short excited-state lifetimes, and ultrafast emission decay.
• Simulated spectroscopic properties matched experimental data for both isolated and protein-bound conditions.

Conclusions:

• The study proposes a general deactivation scheme for photoexcited phytochrome chromophores, independent of the surrounding environment.
• The findings provide a qualitative rationale for observed photophysical properties like small quantum yield and multiexponential decay.
• The results offer new insights into the photochemistry and photophysics of phytochrome systems.