Reinvestigation of the triplet-minus-singlet spectrum of chloroplasts

Direct observation of triplet energy transfer between chlorophylls and carotenoids in the core antenna of photosystem I from Thermosynechococcus elongatus

Quenching of chlorophyll triplet states by carotenoids is an essential photoprotective process, which prevents formation of reactive singlet oxygen in photosynthetic light-harvesting complexes. The process is usually very efficient in oxygenic organisms under physiological conditions, thus preventing any observable accumulation of chlorophyll triplets. However, it subsequently prevents also the determination of the triplet transfer rate. Here we report results of nanosecond transient absorption spectroscopy on photosystem I core complexes, where a major part of chlorophyll a triplet states (~60 %) accumulates on a nanosecond time scale at ambient temperature. As a consequence, the triplet energy transfer could be resolved and the transfer time was determined to be about 24 ns. A smaller fraction of chlorophyll a triplet states (~40 %) is quenched with a faster rate, which could not be determined. Our analysis indicates that these chlorophylls are in direct contact with carotenoids. The overall chlorophyll triplet yield in the core antenna was estimated to be ~0.3 %, which is a value two orders of magnitude smaller than in most other photosynthetic light-harvesting complexes. This explains why slower quenching of chlorophyll triplet states is sufficient for photoprotection of photosystem I. Nevertheless, the core antenna of photosystem I represents one of only few photosynthetic complexes of oxygenic organisms in which the quenching rate of the majority of chlorophyll triplets can be directly monitored under physiological temperature.

Unraveling the electronic origin of a special feature in the triplet-minus-singlet spectra of carotenoids in natural photosystems

The influence of carotenoid triplet states on the Qy electronic transitions of chlorophylls has been observed in experiments on light-harvesting complexes over the past three decades, but the interpretation of the resulting spectral feature in the triplet minus singlet (T-S) absorption spectra of photosystems is still debated, as the physical-chemical explanation of this feature has been elusive. Here, we resolve this debate, by explaining the T-S spectra of pigment complexes over the Qy-band spectral region through a comparative study of chlorophyll-carotenoid model dyads and larger pigment complexes from the main light harvesting complex of higher plants (LHCII). This goal is achieved by combining state-of-the-art time-dependent density functional theory with analysis of the relationship between electronic properties and nuclear structure, and by comparison to the experiment. We find that the special signature in the T-S spectra of both model and natural photosystems is determined by singlet-like triplet excitations that can be described as effective singlet excitations on chlorophylls influenced by a stable electronic triplet on the carotenoid. The comparison with earlier experiments on different light-harvesting complexes confirms our theoretical interpretation of the T-S spectra in the Qy spectral region. Our results indicate an important role for the chlorophyll-carotenoid electronic coupling, which is also responsible for the fast triplet-triplet energy transfer, suggesting a fast trapping of the triplet into the relaxed carotenoid structure. The gained understanding of the interplay between the electronic and nuclear structures is potentially informative for future studies of the mechanism of photoprotection by carotenoids.

Temperature-induced zeaxanthin overproduction in Synechococcus elongatus PCC 7942

The exogenous crtZ gene from Brevundimonas sp. SD212, coding for a 3,3' β-car hydroxylase, was expressed in Synechococcus elongatus PCC 7942 under the control of a temperature-inducible promoter in an attempt to engineer the carotenoid metabolic pathway, to increase the content of zeaxanthin and its further hydroxylated derivatives caloxanthin and nostoxanthin. These molecules are of particular interest due to their renowned antioxidant properties. Cultivation of the engineered strain S7942Z-Ti at 35 °C, a temperature which is well tolerated by the wild-type strain and at which the inducible expression system is activated, led to a significant redistribution of the relative carotenoid content. β-Carotene decreased to about 10% of the pool that is an excess of a threefold decrease with respect to the control, and concomitantly, zeaxanthin became the dominant carotenoid accounting for about half of the pool. As a consequence, zeaxanthin and its derivatives caloxanthin and nostoxanthin collectively accounted for about 90% of the accumulated carotenoids. Yet, upon induction of CrtZ expression at 35 °C the S7942Z-Ti strain displayed a substantial growth impairment accompanied, initially, by a relative loss of carotenoids and successively by the appearance of chlorophyll degradation products which can be interpreted as markers of cellular stress. These observations suggest a limit to the exploitation of Synechococcus elongatus PCC 7942 for biotechnological purposes aimed at increasing the production of hydroxylated carotenoids.

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

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.

Molecular mechanisms involved in plant photoprotection

Photosynthesis uses sunlight to convert water and carbon dioxide into biomass and oxygen. When in excess, light can be dangerous for the photosynthetic apparatus because it can cause photo-oxidative damage and decreases the efficiency of photosynthesis because of photoinhibition. Plants have evolved many photoprotective mechanisms in order to face reactive oxygen species production and thus avoid photoinhibition. These mechanisms include quenching of singlet and triplet excited states of chlorophyll, synthesis of antioxidant molecules and enzymes and repair processes for damaged photosystem II and photosystem I reaction centers. This review focuses on the mechanisms involved in photoprotection of chloroplasts through dissipation of energy absorbed in excess.

Quenching of chlorophyll triplet states by carotenoids in algal light-harvesting complexes related to fucoxanthin-chlorophyll protein

We have used time-resolved absorption and fluorescence spectroscopy with nanosecond resolution to study triplet energy transfer from chlorophylls to carotenoids in a protective process that prevents the formation of reactive singlet oxygen. The light-harvesting complexes studied were isolated from Chromera velia, belonging to a group Alveolata, and Xanthonema debile and Nannochloropsis oceanica, both from Stramenopiles. All three light-harvesting complexes are related to fucoxanthin-chlorophyll protein, but contain only chlorophyll a and no chlorophyll c. In addition, they differ in the carotenoid content. This composition of the complexes allowed us to study the quenching of chlorophyll a triplet states by different carotenoids in a comparable environment. The triplet states of chlorophylls bound to the light-harvesting complexes were quenched by carotenoids with an efficiency close to 100%. Carotenoid triplet states were observed to rise with a ~5 ns lifetime and were spectrally and kinetically homogeneous. The triplet states were formed predominantly on the red-most chlorophylls and were quenched by carotenoids which were further identified or at least spectrally characterized.

A comparison between plant photosystem I and photosystem II architecture and functioning

Oxygenic photosynthesis is indispensable both for the development and maintenance of life on earth by converting light energy into chemical energy and by producing molecular oxygen and consuming carbon dioxide. This latter process has been responsible for reducing the CO2 from its very high levels in the primitive atmosphere to the present low levels and thus reducing global temperatures to levels conducive to the development of life. Photosystem I and photosystem II are the two multi-protein complexes that contain the pigments necessary to harvest photons and use light energy to catalyse the primary photosynthetic endergonic reactions producing high energy compounds. Both photosystems are highly organised membrane supercomplexes composed of a core complex, containing the reaction centre where electron transport is initiated, and of a peripheral antenna system, which is important for light harvesting and photosynthetic activity regulation. If on the one hand both the chemical reactions catalysed by the two photosystems and their detailed structure are different, on the other hand they share many similarities. In this review we discuss and compare various aspects of the organisation, functioning and regulation of plant photosystems by comparing them for similarities and differences as obtained by structural, biochemical and spectroscopic investigations.

Carotenoid triplet states in photosystem II: coupling with low-energy states of the core complex

The photo-excited triplet states of carotenoids, sensitised by triplet-triplet energy transfer from the chlorophyll triplet states, have been investigated in the isolated Photosystem II (PSII) core complex and PSII-LHCII (Light Harvesting Complex II) supercomplex by Optically Detected Magnetic Resonance techniques, using both fluorescence (FDMR) and absorption (ADMR) detection. The absence of Photosystem I allows us to reach the full assignment of the carotenoid triplet states populated in PSII under steady state illumination at low temperature. Five carotenoid triplet ((3)Car) populations were identified in PSII-LHCII, and four in the PSII core complex. Thus, four (3)Car populations are attributed to β-carotene molecules bound to the core complex. All of them show associated fluorescence emission maxima which are relatively red-shifted with respect to the bulk emission of both the PSII-LHCII and the isolated core complexes. In particular the two populations characterised by Zero Field Splitting parameters |D|=0.0370-0.0373 cm(-1)/|E|=0.00373-0.00375 cm(-1) and |D|=0.0381-0.0385 cm(-1)/|E|=0.00393-0.00389 cm(-1), are coupled by singlet energy transfer with chlorophylls which have a red-shifted emission peaking at 705 nm. This observation supports previous suggestions that pointed towards the presence of long-wavelength chlorophyll spectral forms in the PSII core complex. The fifth (3)Car component is observed only in the PSII-LHCII supercomplex and is then assigned to the peripheral light harvesting system.

Quantitative spectrophotometry using integrating cavities

Absorption spectrophotometry, a standard tool for quantitative analysis, suffers from two major drawbacks: lack of sensitivity and vulnerability to scattering. It has been pointed out earlier that the solution to these problems lies in using a reflecting cavity as a sample holder. Due to multiple reflections at the cavity wall, the effective pathlength becomes considerably larger than the diameter of the cavity, and scattering losses are eliminated because scattered light is prevented from escaping the detector. Though much effort has been spent in analysing and improving the performance of such a device, often called an integrating cavity absorption meter (ICAM), a simple strategy for deducing the absorbance of the sample is still lacking. It is shown here that the absorbance A' measured by using an ICAM exhibits a sublinear increase with the solute concentration C. The physical reason for this departure from linearity is explained, and a straightforward procedure for converting A' to the true absorbance A (proportional to C) is established. The reliability of the procedure is demonstrated by comparing the ICAM absorption spectrum of dilute dye solutions with the spectra of more concentrated solutions recorded in a conventional spectrophotometer. The ability of the device to cope with scattering was tested by filling the ICAM with a suspension of chloroplasts, and the spectrum was found, as expected, to be free from scattering artefacts.

A fluorescence detected magnetic resonance investigation of the carotenoid triplet states associated with photosystem II of isolated spinach thylakoid membranes

The carotenoid triplet populations associated with the fluorescence emission chlorophyll forms of Photosystem II have been investigated in isolated spinach thylakoid membranes by means of fluorescence detected magnetic resonance in zero field (FDMR). The spectra collected in the 680-690 nm emission range, have been fitted by a global analysis procedure. At least five different carotenoid triplet states coupled to the terminal emitting chlorophyll forms of PS II, peaking at 682 nm, 687 nm and 692 nm, have been characterised. The triplets associated with the outer antenna emission forms, at 682 nm, have zero field splitting parameters |D| = 0.0385 cm-1, |E| = 0.00367 cm-1; |D| = 0.0404 cm-1, |E| = 0.00379 cm-1 and |D| = 0.0386 cm-1, |E| = 0.00406 cm-1 which are very similar to those previously reported for the xanthophylls of the isolated LHC II complex. Therefore the FDMR spectra recorded in this work provide insights into the organisation of the LHC II complex in the unperturbed environment represented by thylakoid membranes. The additional carotenoid triplet populations, detected by monitoring the chlorophyll emission at 687 and 692 nm, are assigned to carotenoids bound to inner antenna complexes and hence attributed to beta-carotene molecules.

Carotenoid Triplet States Associated with the Long-Wavelength-Emitting Chlorophyll Forms of Photosystem I in Isolated Thylakoid Membranes

The carotenoid triplet populations associated with the long-wavelength-emitting chlorophyll forms of photosystem I (PS I)(dagger) have been investigated in isolated spinach thylakoids by means of fluorescence-detected magnetic resonance in zero field. The spectra collected in the 730-800 nm emission range can be globally fitted assuming the presence of four different carotenoid triplet states coupled to long-wavelength-emitting forms of PS I, having zero-field-splitting parameters /D/ = 0.0359 cm(-1) and /E/ = 0.00371 cm(-1), /D/ = 0.0382 cm(-1) and /E/ = 0.00388 cm(-1), /D/ = 0.0395 cm(-1) and /E/ = 0.00397 cm(-1), and /D/ = 0.0405 cm(-1) and /E/ = 0.00411 cm(-1). On the basis of the triplet-associated fluorescence emission profile, it is suggested that those triplets are associated with light-harvesting complex I, the peripheral antenna complex of PS I.