DIRECT OBSERVATION OF ULTRAFAST ENERGY-TRANSFER PROCESSES IN LIGHT-HARVESTING COMPLEX-II

A femtosecond fluorescence upconversion apparatus was used to study th e isotropic and anisotropic fluorescence dynamics in the chlorophyll a /b light harvesting complex II of a PS I-PS II double-deficiency strai n, C2, of Chlamydomonas reinhardtii. The fluorescence depolarization m easurements reveal nonexponential behavior that is well described by a sum of two exponentials with time constants of 250-300 fs and 5-12 ps . The nonexponential behavior likely arises from intracomplex heteroge neity in the structural and spectral factors that control energy-trans fer dynamics. Isotropic fluorescence measurements reveal a rise time o f 250-300 fs at both 700 and 710 nm, but not at 730 nm. We discuss the implications of our data for LHC II structural organization.