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

Abstract

Two spectral forms of the core light-harvesting complex (LH1) of the purple bacterium Thermochromatium (Tch.) tepidum, the native Ca -binding and the Ba -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 , which is much larger compared with other LH1 and LH2 complexes (80-200 cm ). This large heterogeneity is induced by the inhomogeneous Coulomb (and possibly hydrogen-bonding) interactions exerted by Ba . 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.

Structure of photosynthetic LH1–RC supercomplex at 1.9 Å resolution

“…The structural stability induced by the binding of Ca 2+ may therefore contribute to the thermophilic stability of LH1 as well as the red shift of the absorption peak, two unique features of the thermophilic bacterium. These results are in agreement with those of recent spectral measurements [24][25][26] .…”

Cryo-EM structure of a Ca2+-bound photosynthetic LH1-RC complex containing multiple αβ-polypeptides

“…Much of the insight gained from the thoroughly investigated Tch. tepidum LH1-RC 9 – 11 , 14 , 15 , 18 , 20 , 28 – 30 has paved the way for understanding the structural basis of the spectroscopic behavior of the Trv . strain 970 LH1-RC because the Ca 2+ -binding sites are similar in the two complexes.…”

“…strain 970 LH1 complex is structurally more homogeneous and less mobile than the Tch. tepidum LH1 14 , 30 , 36 , especially as regards the C-terminal domains that are close to the BChl a -binding sites. By contrast, the Trv .…”