pH-Dependent Vibrational Dynamics Drives Excited-State Quenching in the Phycobiliprotein Complex PC645

Area of Science:

• Photosynthesis research
• Algal biochemistry
• Spectroscopy

Background:

• Phycocyanin 645 (PC645) is a light-harvesting complex in cryptophyte algae.
• Its location in the lumen and the pH drop during photosynthesis are unusual.
• The effect of pH on PC645's light-harvesting function is poorly understood.

Purpose of the Study:

• Investigate the spectroscopy and excitation energy transfer in PC645.
• Understand how pH changes affect PC645's function.
• Determine the role of specific bilin pigments in pH-dependent energy transfer.

Main Methods:

• Multiscale simulations using a density functional tight-binding framework.
• Steady-state and time-resolved spectroscopic measurements at varying pH.
• Computational modeling of pH-dependent fluorescence quantum yield.

Main Results:

• (De)protonation of mesobiliverdins (MBVs) modulates excitation energies, couplings, and spectral densities.
• Excitation transfer rates increase by two to three times with pH changes.
• Energy transfer pathways within PC645 are pH-dependent.

Conclusions:

• MBVs are significant excitation sinks in PC645, with their role influenced by pH.
• Computational and experimental results show quantitative agreement.
• The study reveals pH-tuned, enhanced energy transfer in PC645.