The electronic excitation population and coherence dynamics in the
chromophores of the photosynthetic light harvesting complex 2 (LH2) B850 ring
from purple bacteria (Rhodopseudomonas acidophila) have been studied
theoretically at both physiological and cryogenic temperatures. Similar to the
well-studied Fenna-Matthews-Olson (FMO) protein, oscillations of the excitation
population and coherence in the site basis are observed in LH2 by using a
scaled hierarchical equation of motion (HEOM) approach. However, this
oscillation time (300 fs) is much shorter compared to the FMO protein (650 fs)
at cryogenic temperature. Both environment and high temperature are found to
enhance the propagation speed of the exciton wave packet yet they shorten the
coherence time and suppress the oscillation amplitude of coherence and the
population. Our calculations show that a long-lived coherence between
chromophore electronic excited states can exist in such a noisy biological
environment.Comment: 21 pages, 9 figure
