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 Ca(2+), 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 Sr(2+)/Ca(2+) ATR-FTIR difference bands that reflect changes in the polypeptide backbones and amino acid residues upon the replacement of native Ca(2+) with Sr(2+). The difference bands also appeared in the following Ca(2+)/Sr(2+) 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 faintly detected in the mesophilic counterpart Allochromatium vinosum . A comparative analysis using LH1 complexes lacking the RCs strongly indicated that the metal-sensitive bands originate from polypeptide backbones and amino acid residues near the putative Ca(2+)-binding site at the C-terminal region of the Tch. tepidum LH1 complexes. Structural changes induced by Sr(2+) and Ba(2+) substitutions were essentially identical. However, Cd(2+) substitution exhibited unique structural modifications, which may be responsible for the severely deteriorated thermal stability of Cd(2+)-substituted complexes. Possible assignments for the present ATR-FTIR signals and their relation with the molecular mechanism of enhancing the thermal stability of Tch. tepidum LH1-RC proteins are discussed on the basis of the recent structural information on the Ca(2+)-binding site.