In the major peripheral plant light-harvesting complex LHCII, excitation energy is transferred between chlorophylls
along an energetic cascade before it is transmitted further into the photosynthetic assembly to be converted into chemical energy.
The efficiency of these energy transfer processes involves a complicated interplay of pigment-protein structural reorganization
and protein dynamic disorder, and the system must stay robust within the fluctuating protein environment. The final,
lowest energy site has been proposed to exist within a trimeric excitonically coupled chlorophyll (Chl) cluster, comprising
Chls a610-a611-a612. We studied an LHCII monomer with a site-specific mutation resulting in the loss of Chls a611and
a612, and find that this mutant exhibits two predominant overlapping fluorescence bands. From a combination of bulk measurements,
single-molecule fluorescence characterization, and modeling, we propose the two fluorescence bands originate from
differing conditions of exciton delocalization and localization realized in the mutant. Disruption of the excitonically coupled terminal
emitter Chl trimer results in an increased sensitivity of the excited state energy landscape to the disorder induced by the
protein conformations. Consequently, the mutant demonstrates a loss of energy transfer efficiency. On the contrary, in the wildtype
complex, the strong resonance coupling and correspondingly high degree of excitation delocalization within the Chls a610-
a611-a612 cluster dampens the influence of the environment and ensures optimal communication with neighboring pigments.
These results indicate that the terminal emitter trimer is thus an essential design principle for maintaining the efficient
light-harvesting function of LHCII in the presence of protein disorder.