Temperature and solvent effects on reaction centers from Chloroflexus aurantiacus and Chromatium tepidum.

Temperature and solvent effects on reaction center structures were examined in two thermophilic photosynthetic bacteria, Chloroflexus aurantiacus and Chromatium tepidum, in order to gain insight into the interactions among the reaction center proteins and pigment systems. Thermal stability of the reaction centers was found to be proportional to the optimum growth temperature. Circular dichroism (CD) spectra in the 250-300 nm region indicated that thermal denaturation destroyed tertiary structures (helix-to-helix interactions or amino acid residue conformation) in the native reaction center, keeping helical structures intact. Absorption and circular dichroism spectral changes showed that alcohol denatured the so-called special pair and the accessory BChl a independently. The alcohol denaturation further indicates that the coordination between BChl a and amino acid residue in the protein is one of the important interactions maintaining the pigment organization of the reaction centers.