ATR-FTIR detection of metal-sensitive structural changes in the light-harvesting 1 reaction center complex from the thermophilic purple sulfur bacterium Thermochromatium tepidum.

Thermochromatium tepidum grows at the highest temperature among purple bacteria, and the light-harvesting 1 reaction center (LH1-RC) complex enhances the thermal stability by utilizing Ca2+, although the molecular mechanism has yet to be resolved. Here, we applied perfusion-induced attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy to highly purified LH1-RC complexes from Tch. tepidum and detected for the first time metal-sensitive fine structural changes involved in the enhanced thermal stability of this complex. The Tch. tepidum LH1-RC complex exhibited Sr2+/Ca2+ ATR–FTIR difference bands that reflect changes in the polypeptide backbones and amino acid residues upon the replacement of native Ca2+ with Sr2+. The difference bands also appeared in the following Ca2+/Sr2+ difference spectra with almost identical intensities but inverse signs, demonstrating that the structural changes induced by the metal exchange are fully reversible. In contrast, these ATR–FTIR signals were fa...