Temperature-Dependent Lifetimes and Quantum Yield of the Singlet and Triplet States of the B820 Subunit of LHI Antenna Complex of Purple Bacterium Rhodospirillum rubrum

The quantum yield and lifetimes of fluorescence and triplet state of the B820 subunit of core antenna of photosynthetic purple bacterium Rhodospirillum rubrum are reported at temperature range from 4.2 to 300K. The fluorescence quantum yield of B820 is strongly dependent on temperature. From 4.2 to about 200 K the yield remains nearly constant. A nonradiative decay process becomes activated around 170-200 K. Between 200 K and room temperature the quantum yield drops by a factor of 4. The fluorescence lifetimes and the B820 triplet yield follow the same temperature dependence as the fluorescence quantum yield. It is concluded that a change in the rate of internal conversion from the excited singlet state to the ground state produces the observed change. The change is modeled by using the energy gap law in the strong coupling limit. An activation energy of 65 kJ/mol and average frequency of 450 cm-1 for the promoting vibrational mode of the nonradiative decay are found by fitting the data by energy gap law expression. The results are compared with the bacterial reaction center and the intact bacterial light harvesting antenna. A change in the rigidity of the structure of the B820 unit induced by the isolation procedure is the probable reason for the strong temperature dependence.