Energy Transfer among Chlorophylls in Trimeric Light-harvesting Complex II of Bryopsis corticulans

A study on energy transfer among chlorophylls (Chls) in the trimeric unit of the major light-harvesting complex II (LHC II) from Bryopsis corriculan, was carried out using time-correlated single photon counting. In the chlorophyll Q region of LHC II, six molecules characterized as Chlb₆₂₈, Chlb₆₄₆, Chlb₆₅₂^654,657, Chla₆₆₄⁶⁶⁶, Chla₆₇₄^677,680 and Chla₆₈₂⁶⁸³ were discriminated according to their absorption spectrum and fluorescence emission spectrum. Then, excited by pulsed light of 628 nm, fluorescence kinetics spectra in the chlorophyll Q region were measured. In accordance with the principles of fluorescence kinetics, these kinetics data were analyzed with a multi-exponential model. Time constants on energy transfer were obtained. An overwhelming percentage of energy transfer among chlorophylls undergoes a process longer than 97 picoseconds (ps), which shows that, before transferring energy to another Chl, the excited Chl might convert energy to vibrations of a lower state with different multiplicity (intersystem crossing). Energy transfer at the level of approximately 10 ps was also obtained, which was interpreted as the excited Chls may go through internal conversion before transferring energy to another Chl. Although with a higher standard deviation, time constants at the femtosecond level can not be entirely excluded, which can be attributed to the ultrafast process of direct energy transfer. Owing to the arrangement and direction of the dipole moment of Chls in LHC II, the probability of these processes is different. The fluorescence lifetimes of Chlb₆₅₂^654,657, Chla₆₆₄⁶⁶⁶, Chla₆₇₄^677,680 and Chla₆₈₂⁶⁸³ were determined to be 1.44 ns, 1.43 ns, 636 ps and 713 ps, respectively. The percentages of energy dissipation in the pathway of fluorescence emission were no more than 40% in the trimeric unit of LHC II. These results are important for a better understanding of the relationship between the structure and function of LHC II.