Abstract Far-red absorption in eukaryotic light-harvesting complexes (LHCs) has been associated with strongly excitonically coupled chlorophyll a clusters exhibiting mixing with charge-transfer states, yet the structural rules enabling this spectral tuning remain unclear. Previous studies have highlighted the importance of the amino acid ligating Chl a 603 in providing the pigment orientation required for the formation of a red-shifted Chl a 603- a 609 cluster. More recently, it has been suggested that the steric properties of the residue at the i -4 position from the ligand may also play a crucial role. Here, we test this hypothesis through targeted mutagenesis of two light-harvesting complexes, Lhca4 and CP29, which host the Chl a 603- a 609 pair, but differ in their protein environment and spectral properties. In Lhca4, introduction of steric constraints at the i -4 position relative to the Chl a 603 ligand (A43L) abolishes far-red absorption, indicating that steric crowding at this position destabilizes the strongly coupled pigment configuration. In CP29, substitution of the Chl a 603 ligand (H111N) is required for far-red absorption, while the additional mutation at i -4 position (H111N/C107A) modulates the magnitude of the red-shift. Together these results highlight the importance of both axial ligand identity and local protein environment in controlling far-red absorption in Chl a clusters.
Agostini et al. (Mon,) studied this question.