A Residue-Resolved Mechanism for Trapped-Twist Spectral Tuning in Cyanobacteriochromes

Here we quantify this P_r–P_g separation using quantum-mechanical/molecular-mechanical geometry optimization and wave function-based excited-state calculations.

Calculated spectra reproduce the P_r–P_g trend and show that the shift is dominated by a P_g-specific increase in phycocyanobilin A–B inter-ring torsion, which truncates π-conjugation and widens the HOMO–LUMO gap. Protein electrostatics red-shifts both states similarly and thus does not set the P_r–P_g gap. Residue–chromophore interaction energies reveal state-dependent rewiring: Trp655 dominates in P_r and regulates the C–D dihedral orientation, whereas in P_g this role shifts to Ile691, and Thr658 becomes a major P_g-specific stabilizer acting at the A–B linkage.