Molecular origins of absorption wavelength variation among phycocyanobilin-binding proteins.

Phycocyanobilin (PCB)-binding proteins, including cyanobacteriochromes and phytochromes, function as photoreceptors and exhibit a wide range of absorption maximum wavelengths. To elucidate the color-tuning mechanisms among these proteins, we investigated seven crystal structures of six PCB-binding proteins: Anacy_2551g3, AnPixJg2, phosphorylation-responsive photosensitive histidine kinase, RcaE, Sb.phyB(PG)-PCB, and Slr1393g3. Employing a quantum chemical/molecular mechanical approach combined with a polarizable continuum model, our analysis revealed that differences in absorption wavelengths among PCB-binding proteins primarily arise from variations in the shape of the PCB molecule itself, accounting for a ∼150 nm difference. Remarkably, calculated excitation energies sufficiently reproduced the absorption wavelengths of these proteins spanning ∼200 nm, including 728 nm for Anacy_2551g3. However, assuming the hypothesized lactim conformation resulted in a significant deviation from the experimentally measured absorption wavelength for Anacy_2551g3. The significantly red-shifted absorption wavelength of Anacy_2551g3 can unambiguously be explained by the significant overlap of molecular orbitals between the two pyrrole rings at both edges of the PCB chromophore without the need to hypothesize lactim formation.