The rate constants of thermal (irreversible) damage of bacteriochlorin pigments (bacteriochlorophyll monomer [B], bacteriochlorophyll dimer [P] and bacteriopheophytine [H]) in reaction center [RC] protein from the photosynthetic bacterium Rhodobacter sphaeroides were studied in the dark and during intense (400 mW · cm−2) laser light excitation (wavelengths 488 and 515 nm) under deoxygenated conditions. While the kinetics of degradation of P and B were monoexponential, the decay kinetics of H were overlapped by an initial lag phase at elevated (> 40°C) temperature. This is explained by removal of the central metal ion from the bacteriochlorophylls as part of their degradation processes. At all temperatures, the rates of damage were very similar for all bacteriochlorin pigments and were larger in the light than in the dark. The logarithm of the rate constant of pigment degradation and loss of photochemistry as a function of reciprocal (absolute) temperature (Arrhenius/Esyring plot) showed single phase in the light and double phases in the dark. Below 20°C, the rate of pigment degradation in the RC decreased so dramatically in the dark that it became limited by the natural degradation process of bacteriochlorophyll measured in solution. The function of loss of photochemistry in the dark was also biphasic and had a break point at 40°C. The damage in the dark required high enthalpy change (ΔH‡= 64 kcal/mol for P and ΔH‡= 60 kcal/mol for B) and entropy increase (T·ΔS‡= 38 kcal/mol for P and T·ΔS‡= 34 kcal/mol for B at T= 300 K), whereas significantly smaller enthalpy change (ΔH‡= 21 kcal/mol for P and B and ΔH‡= 13 kcal/mol for H) and practically no (T·ΔS‡=−1 kcal/mol for P and B at T= 300 K) or small (T·ΔS‡=−9 kcal/mol for H at T= 300 K) entropy change was needed in the light. The thermodynamic parameters of activation reveal major steps common in the degradation of all bacteriochlorin pigments: ring opening reactions at C5 or C20 meso-bridges (or both) and breaking/removal of the phytyl chain. Their contribution in the degradation is probably reflected in the observed enthalpy/entropy compensation at an almost constant (ΔG‡= 22–26 kcal/mol at T= 300 K) free energy change of activation.
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