Effect of Oxygen Exposure on the Triplet Excitation Dynamics of the Monomeric LHCII Complex from Spinach.

Light-harvesting complex IIs (LHCIIs) are the major antenna in higher plants, balancing light capture through photoprotection. While it naturally forms trimers, stress conditions can induce monomerization, altering pigment interactions. Here, we explored how molecular oxygen affects triplet excited-state dynamics in LHCII monomers using time-resolved transient absorption spectroscopy under aerobic and anaerobic conditions. Two chlorophyll-to-carotenoid (Chl → Car) triplet energy transfer (TET) pathways were identified: a fast subnanosecond and a slow tens of nanosecond process. Under anaerobic conditions, inefficient quenching leads to 3Chl* accumulation, with Chl a604 and Chl a602 proposed as sites of long-lived triplet Chl in monomer. In aerobic condition, 3Car* formation is strongly suppressed despite similar 1Chl* lifetimes, suggesting rapid quenching of 3Chl* by oxygen and potential singlet oxygen (1O2) formation. Chl a614 is proposed as an additional site promoting oxygen access and early 1O2 generation. Notably, monomerization enhances the fast TET pathway, possibly compensating for the disrupted pigment coupling at Chl a602/a603 in the Lut621 domain. These results suggest that LHCII trimerization structurally limits oxygen diffusion and promotes efficient triplet quenching via carotenoids, thereby minimizing 1O2 production. Thus, trimeric organization is essential for protecting the photosynthetic apparatus from photooxidative damage under stress.