Protein and Solvent Reorganization Drives Radical Pair Stability in Avian Cryptochrome 4a

Area of Science:

• Biophysics
• Quantum Biology
• Avian Physiology

Background:

• Cryptochrome-4a (Cry4a) is implicated as a magnetic compass sensor in migratory birds' retinas.
• European robin Cry4a shows higher magnetic sensitivity than nonmigratory species.
• Blue-light excitation of Cry4a initiates electron transfer, forming radical pairs potentially involved in magnetoreception.

Purpose of the Study:

• To investigate the stabilization, interconversion, and recombination of radical pairs in European robin Cry4a (ErCry4a).
• To computationally explore the role of a "composite" radical pair in avian magnetoreception.
• To identify key residues for targeted mutations to probe magnetoreception mechanisms.

Main Methods:

• First-principles electronic structure calculations.
• Hybrid quantum mechanical/molecular mechanical (QM/MM) simulations.
• Free energy analysis and electronic coupling calculations.

Main Results:

• Protein and solvent reorganization significantly stabilize long-range charge-transfer states.
• Radical pair states become energetically comparable to charge-neutral states.
• Evidence supports a "composite" radical pair involving Trp3 and Trp4 as the functional magnetoreceptor.

Conclusions:

• The "composite" radical pair mechanism is supported by computational findings.
• Specific amino acid residues are identified as crucial for modulating radical pair stability and magnetic sensitivity.
• Mutational studies of identified residues could elucidate Cry4a's role in avian magnetoreception.