Spectral Variability in Phycocyanin Cryptophyte Antenna Complexes is Controlled by Changes in the α-Polypeptide Chains

Quantitative models of light harvesting in photosynthetic antenna complexes depend sensitively on the challenging determination of the relative site energies of the pigments. Herein we analyze the basis of the light harvesting properties of four antennae from cryptophyte algae, phycocyanines PC577, PC612, PC630 and PC645, by comparing two alternative theoretical strategies to derive the excitonic Hamiltonian. The first is based on molecular dynamics simulations and subsequent polarizable quantum/molecular mechanics (QM/MMPol) calculations, whereas the second is based on three-layer QM/MMPol/ddCOSMO calculations performed on optimized geometries of the pigments, where the water solvent is described using the ddCOSMO continuum model. We find the latter approach to be remarkably accurate, suggesting that these four phycobiliproteins share a common energetic ordering PCB₈₂ < PCB₁₅₈ < DBV_51/61 for pigments located in the highly-conserved β chains, whereas bilins in the more divergent α chains cause their spectral differences. In addition, we predict a strong screening of the coupling among central dihydrobiliverdins (DBVs) in “open” form complexes PC577 and PC612 compared to “closed” form ones, which together with the increased interpigment separation explains the attenuation of coherence beatings observed for these complexes.