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

Cyanobacteriochromes (CBCRs) are bilin-binding photoreceptors that photoconvert between long-lived, thermally stable states; in the red/green subfamily, photoexcitation converts the red-absorbing Pr state to the green-absorbing Pg state with a large blue shift. Here we quantify this Pr-Pg separation using quantum-mechanical/molecular-mechanical geometry optimization and wave function-based excited-state calculations. Calculated spectra reproduce the Pr-Pg trend and show that the shift is dominated by a Pg-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 Pr-Pg gap. Residue-chromophore interaction energies reveal state-dependent rewiring: Trp655 dominates in Pr and regulates the C-D dihedral orientation, whereas in Pg this role shifts to Ile691, and Thr658 becomes a major Pg-specific stabilizer acting at the A-B linkage. These results establish trapped-twist tuning as a geometry-driven mechanism with residue-level handles for engineering CBCR absorption.