Temperature-Dependent Fluorescence Measurements in LHCII Aggregates Reveal the Nature of Self-Regulation in Light-Harvesting Antenna

J. Chmeliova,b, A. Gelzinisa,b, E. Songailab, R. Augulisb, C.D.P. Duffyc, A.V. Rubanc, L. Valkunasa,b
aDepartment of Theoretical Physics, Faculty of Physics, Vilnius University, Sauletekio Avenue 9, LT-10222 Vilnius, Lithuania; bDepartment of Molecular Compound Physics, Centre for Physical Sciences and Technology, Sauletekio Avenue 3, LT-10222 Vilnius, Lithuania; cThe School of Biological and Chemical Sciences, Queen Mary, University of London, Mile End Road, London E1 4NS, UK

Non-photochemical quenching (NPQ) is a self-regulatory mechanism utilized by green plants on a molecular level that allows them to operate under varying light conditions and to avoid dangerous over-excitation during intense sunlight. To get more insight into its physical origin, we performed high-resolution time-resolved fluorescence measurements of the major light-harvesting complexes (LHCII) and their aggregates across a wide temperature range (from the room temperature down to 15 K) [1]. The thorough analysis of the collected data indicated the co-existence of at least 3 distinct conformational states of the LHCII complexes, one of which is responsible for the excitation quenching (Fig. 1a).
Based on simulations of the excitation energy transfer in the LHCII aggregate at various temperatures, we were able to associate the physical origin of these states with the underlying molecular mechanisms (Fig. 1b). Particularly, it was shown that the quenching state resembling NPQ is related to the incoherent excitation transfer to the short-lived carotenoid excited state, most probably the one of the lutein pigment [2]. Our results also demonstrate that the required level of photoprotection in vivo can be achieved by a very subtle change in the number of LHCIIs switched to the quenched state.

Figure 1. (a) Model of the LHCII aggregate consisting of LHCII complexes being in 3 distinct conformational states—the dominating 680-nm-emitting one (green), the red-emitting one, having fluorescence maximum at ~700 nm (red), and the quenching one (grey). (b) The proposed molecular origin of the red-emitting and quenching states of LHCII.

References

[1] J. Chmeliov, A. Gelzinis, E. Songaila, R. Augulis, C. D. P. Duffy, A. V. Ruban, L. Valkunas, Nature Plants 2016, 2, 16045.
[2] J. Chmeliov, W. P. Bricker, C. Lo, E. Jouin, L. Valkunas, A. V. Ruban, C. D. P. Duffy, Phys. Chem. Chem. Phys. 2015, 17, 15857–15867.