Abstract:
In light harvesting complex II (LHCII) of higher plants and green algae, carotenoids (Cars) have an
important function to quench chlorophyll (Chl) triplet states and therefore avoid the production of
harmful singlet oxygen. The resulting Car triplet states lead to a non-linear self-quenching mechanism
called singlet–triplet (S–T) annihilation that strongly depends on the excitation density. In this work we
investigated the fluorescence decay kinetics of single immobilized LHCIIs at room temperature and
found a two-exponential decay with a slow (3.5 ns) and a fast (35 ps) component. The relative amplitude
fraction of the fast component increases with increasing excitation intensity, and the resulting decrease
in the fluorescence quantum yield suggests annihilation effects. Modulation of the excitation pattern by
means of an acousto-optic modulator (AOM) furthermore allowed us to resolve the time-dependent
accumulation and decay rate (B7 ms) of the quenching species. Inspired by singlet–singlet (S–S) annihilation
studies, we developed a stochastic model and then successfully applied it to describe and explain all the
experimentally observed steady-state and time-dependent kinetics. That allowed us to distinctively identify
the quenching mechanism as S–T annihilation. Quantitative fitting resulted in a conclusive set of parameters
validating our interpretation of the experimental results. The obtained stochastic model can be generalized
to describe S–T annihilation in small molecular aggregates where the equilibration time of excitations is
much faster than the annihilation-free singlet excited state lifetime.
