Ca²⁺-Driven Protein Compaction Tunes NIR Absorption in the Thermophilic Purple Bacterial LH1

The Ca²⁺-binding light-harvesting 1 (LH1) complex from the thermophilic purple phototrophic bacterium Thermochromatium tepidum exhibits an unusually redshifted absorption at 915 nm. To establish how Ca²⁺ regulates spectral tuning, we employed a multimer exciton model incorporating charge transfer (CT) interactions to reproduce the spectrum (calculated maximum: 892 nm) and decompose the redshift, attributing 56.3% to CT and 28.8% to excitonic coupling (85.1% combined), whereas the electrostatic embedding (including Ca²⁺) accounts for 1.1%. Consistent with this, removing the Ca²⁺ point charge shifts the band by ∼0.005 eV, indicating a negligible electrostatic effect. Instead, Ca²⁺-induced compaction of the protein scaffold yields two indirect contributions: shortening interbacteriochlorophyll a separations (mean Mg-Mg of 8.80 Å, 0.12 Å shorter than in Rhodospirillum rubrum LH1), which strengthens interpigment electronic interactions, and enhancing intramolecular π-conjugation via macrocycle flattening. These concerted structural changes promote CT mixing and excitonic coupling, rationalizing the extreme long-wavelength absorption of T. tepidum LH1.