Temperature Dependence of Chlorophyll Triplet Quenching in Two Photosynthetic Light-Harvesting Complexes from Higher Plants and Dinoflagellates

Abstract

Chlorophyll (Chl) triplet states generated in photosynthetic light-harvesting complexes (LHCs) can be quenched by carotenoids to prevent the formation of reactive singlet oxygen. Although this quenching occurs with an efficiency close to 100% at physiological temperatures, the Chl triplets are often observed at low temperatures. This might be due to the intrinsic temperature dependence of the Dexter mechanism of excitation energy transfer, which governs triplet quenching, or by temperature-induced conformational changes. Here, we report about the temperature dependence of Chl triplet quenching in two LHCs. We show that both the effects contribute significantly. In LHC II of higher plants, the core Chls are quenched with a high efficiency independent of temperature. A different subpopulation of Chls, which increases with lowering temperature, is not quenched at all. This is probably caused by the conformational changes which detach these Chls from the energy-transfer chain. In a membrane-intrinsic LHC of dinoflagellates, similarly two subpopulations of Chls were observed. In addition, another part of Chl triplets is quenched by carotenoids with a rate which decreases with temperature. This allowed us to study the temperature dependence of Dexter energy transfer. Finally, a part of Chls was quenched by triplet-triplet annihilation, a phenomenon which was not observed for LHCs before.

Photosynthetic Light-Harvesting (Antenna) Complexes—Structures and Functions

“…Currently, no high-resolution structure of these complexes is available. However, these complexes are presumed to be structurally and functionally, also in terms of photoprotection, similar to LHCII [ 75 ].…”

“…Photosynthesizing organisms contain carotenoids as essential constituents of LHCs providing effective photoprotection: (i) carotenoids in Van der Waals contact with Chls are very efficient quenchers of 3 Chl * ; in LHCII, the rate of 3 Chl* quenching by carotenoids is much faster that the formation via ISC, see, e.g., [ 75 , 141 ], and (ii) carotenoids can efficiently defuse singlet oxygen or detoxify other reactive species [ 12 ].…”

Antibacterial Nanoparticles with Natural Photosensitizers Extracted from Spinach Leaves

“…Chlorophylls (Chls) are widely abundant photosensitizers in nature that enable conversion of solar energy to biochemically useful forms. , They are very cheap, commercially available, main group metal-based, and provide excellent efficiency for singlet oxygen-mediated chemistry . The photosensitization process from the S 1 states after excitation with red and/or blue radiation competes with a very efficient intersystem crossing to the T 1 state and the formation of highly reactive singlet oxygen [O 2 ( 1 Δ g )].…”

Photoprotection of Photosynthetic Pigments in Plant One-Helix Protein 1/2 Heterodimers

“…For instance, carotenoid triplet states shorten from ∼10 μs under anaerobic conditions to ∼3 μs in the presence of oxygen. 24,25,31 Chls exhibit even longer lifetimes under anaerobic conditions. For instance, a ∼400 μs lifetime was resolved in pyridine.…”

“…32 However, these triplet excited state lifetimes will be shortened again to ∼3 μs in the presence of oxygen. 24,25,31 The reason for the shortening of the lifetime is an interaction with ground state (triplet) oxygen, which does not lead to the formation of singlet oxygen but instead to quenching of the excited states of pigments. 34−36 The results for OHP1/2 indicate that carotenoids and, more importantly, the sensitive Chls are efficiently shielded from interaction with oxygen under aerobic conditions.…”