Metal Cations Induced αβ-BChl a Heterogeneity in LH1 as Revealed by Temperature-Dependent Fluorescence Splitting

Two spectral forms of the core light-harvesting complex (LH1) of the purple bacterium Thermochromatium (Tch.) tepidum, the native Ca²⁺-binding and the Ba²⁺-substituted one, exhibit different fluorescence (FL) emission spectra at low temperature (T). While Ca-LH1 exhibits one emission band, an unusual splitting of the fluorescence is observed for Ba-LH1. These two sub-bands display the same spectral-width dependence according to T, but their intensity evolves differently with T. Based on the crystal structures, we propose that the FL splitting originates from a large αβ-BChl a transition energy heterogeneity, ≈600 cm⁻¹, which is much larger compared with other LH1 and LH2 complexes (80–200 cm⁻¹). This large heterogeneity is induced by the inhomogeneous Coulomb (and possibly hydrogen-bonding) interactions exerted by Ba²⁺. The energy levels of the two LH1s were compared using exciton calculations in combination with Redfield theory. To simulate the FL splitting, an electronic transition containing two resonant bands was considered. This work shows how metal cations incorporated into the polypeptide modulate the electronic properties of BChl a aggregates.