Ca ²⁺ -Driven Protein Compaction Tunes NIR Absorption in the Thermophilic Purple Bacterial LH1

High Resolution Image Download MS PowerPoint Slide The Ca 2+ -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 Ca 2+ 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 Ca 2+ ) accounts for 1.1%. Consistent with this, removing the Ca 2+ point charge shifts the band by ∼0.005 eV, indicating a negligible electrostatic effect. Instead, Ca 2+ -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.