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Long-wavelength limit of photochemical energy conversion in Photosystem I.

Schlodder E, Lendzian F, Meyer J, Çetin M, Brecht M, Renger T, Karapetyan NV - J. Am. Chem. Soc. (2014)

Bottom Line: Therefore, it is concluded that electron transfer through PS I is induced by direct excitation of a proposed charge transfer (CT) state in the reaction center.The present findings suggest that nature can exploit CT states for extending the long wavelength limit in PSI even beyond that of LWC.Similar mechanisms may work in other photosynthetic systems and in chemical systems capable of photoinduced electron transfer processes in general.

View Article: PubMed Central - PubMed

Affiliation: Max-Volmer-Laboratorium für Biophysikalische Chemie, Technische Universität Berlin , Strasse des 17. Juni 135, 10623 Berlin, Germany.

ABSTRACT
In Photosystem I (PS I) long-wavelength chlorophylls (LWC) of the core antenna are known to extend the spectral region up to 750 nm for absorbance of light that drives photochemistry. Here we present clear evidence that even far-red light with wavelengths beyond 800 nm, clearly outside the LWC absorption bands, can still induce photochemical charge separation in PS I throughout the full temperature range from 295 to 5 K. At room temperature, the photoaccumulation of P700(+•) was followed by the absorbance increase at 826 nm. At low temperatures (T < 100 K), the formation of P700(+•)FA/B(-•) was monitored by the characteristic EPR signals of P700(+•) and FA/B(-•) and by the characteristic light-minus-dark absorbance difference spectrum in the QY region. P700 oxidation was observed upon selective excitation at 754, 785, and 808 nm, using monomeric and trimeric PS I core complexes of Thermosynechococcus elongatus and Arthrospira platensis, which differ in the amount of LWC. The results show that the LWC cannot be responsible for the long-wavelength excitation-induced charge separation at low temperatures, where thermal uphill energy transfer is frozen out. Direct energy conversion of the excitation energy from the LWC to the primary radical pair, e.g., via a superexchange mechanism, is excluded, because no dependence on the content of LWC was observed. Therefore, it is concluded that electron transfer through PS I is induced by direct excitation of a proposed charge transfer (CT) state in the reaction center. A direct signature of this CT state is seen in absorbance spectra of concentrated PS I samples, which reveal a weak and featureless absorbance band extending beyond 800 nm, in addition to the well-known bands of LWC (C708, C719 and C740) in the range between 700 and 750 nm. The present findings suggest that nature can exploit CT states for extending the long wavelength limit in PSI even beyond that of LWC. Similar mechanisms may work in other photosynthetic systems and in chemical systems capable of photoinduced electron transfer processes in general.

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Yields of P700+• photoaccumulationupon selective far-red excitation in PS I trimers from T. elongatus (a) and PS I monomers and trimers from A. platensis (b) detected by EPR at 30 K dividedby the maximum yield obtained with white light illumination as a functionof the duration of excitation. The solid lines are the result of abiexponential fit. In a control experiment (785 nm control), an interferencefilter (F34-786 from AHF Analysentechnik) was placed directly in frontof the resonator.
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fig7: Yields of P700+• photoaccumulationupon selective far-red excitation in PS I trimers from T. elongatus (a) and PS I monomers and trimers from A. platensis (b) detected by EPR at 30 K dividedby the maximum yield obtained with white light illumination as a functionof the duration of excitation. The solid lines are the result of abiexponential fit. In a control experiment (785 nm control), an interferencefilter (F34-786 from AHF Analysentechnik) was placed directly in frontof the resonator.

Mentions: Similar measurements as shown in Figure 6ahave been performed using laser diodes with an emission wavelengthof 754 and 808 nm for excitation (not shown). Figure 7a shows the yield of P700+• formation dividedby the maximum yield obtained with white light illumination as a functionof the duration of excitation. The blue triangles show the resultof a control experiment, whereby a narrow band (fwhm =18 nm) lasercleanup filter (F34-786 from AHF Analysentechnik) with a central wavelengthat 785 nm and a maximum transmission of about 90% was placed directlyin front of the resonator. The result excludes the possibility thatlaser sidebands or UV–VIS stray light distorted the measurementsand confirms that the formation of P700+•FA–• is solely induced by far-red 785 nm light. The slightly lower yieldis the result of the somewhat lower photon flux reaching the sample.The rise kinetics can be fitted with two exponentials (see solid linesin Figure 7a). The extrapolated relative yieldsare 0.64 for 754 nm, 0.61 for 785 nm, and 0.4 for 808 nm. Half ofthe extrapolated yields are reached after 19 min (754 nm), 7 min (785nm), and 9 min (808 nm).


Long-wavelength limit of photochemical energy conversion in Photosystem I.

Schlodder E, Lendzian F, Meyer J, Çetin M, Brecht M, Renger T, Karapetyan NV - J. Am. Chem. Soc. (2014)

Yields of P700+• photoaccumulationupon selective far-red excitation in PS I trimers from T. elongatus (a) and PS I monomers and trimers from A. platensis (b) detected by EPR at 30 K dividedby the maximum yield obtained with white light illumination as a functionof the duration of excitation. The solid lines are the result of abiexponential fit. In a control experiment (785 nm control), an interferencefilter (F34-786 from AHF Analysentechnik) was placed directly in frontof the resonator.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3959156&req=5

fig7: Yields of P700+• photoaccumulationupon selective far-red excitation in PS I trimers from T. elongatus (a) and PS I monomers and trimers from A. platensis (b) detected by EPR at 30 K dividedby the maximum yield obtained with white light illumination as a functionof the duration of excitation. The solid lines are the result of abiexponential fit. In a control experiment (785 nm control), an interferencefilter (F34-786 from AHF Analysentechnik) was placed directly in frontof the resonator.
Mentions: Similar measurements as shown in Figure 6ahave been performed using laser diodes with an emission wavelengthof 754 and 808 nm for excitation (not shown). Figure 7a shows the yield of P700+• formation dividedby the maximum yield obtained with white light illumination as a functionof the duration of excitation. The blue triangles show the resultof a control experiment, whereby a narrow band (fwhm =18 nm) lasercleanup filter (F34-786 from AHF Analysentechnik) with a central wavelengthat 785 nm and a maximum transmission of about 90% was placed directlyin front of the resonator. The result excludes the possibility thatlaser sidebands or UV–VIS stray light distorted the measurementsand confirms that the formation of P700+•FA–• is solely induced by far-red 785 nm light. The slightly lower yieldis the result of the somewhat lower photon flux reaching the sample.The rise kinetics can be fitted with two exponentials (see solid linesin Figure 7a). The extrapolated relative yieldsare 0.64 for 754 nm, 0.61 for 785 nm, and 0.4 for 808 nm. Half ofthe extrapolated yields are reached after 19 min (754 nm), 7 min (785nm), and 9 min (808 nm).

Bottom Line: Therefore, it is concluded that electron transfer through PS I is induced by direct excitation of a proposed charge transfer (CT) state in the reaction center.The present findings suggest that nature can exploit CT states for extending the long wavelength limit in PSI even beyond that of LWC.Similar mechanisms may work in other photosynthetic systems and in chemical systems capable of photoinduced electron transfer processes in general.

View Article: PubMed Central - PubMed

Affiliation: Max-Volmer-Laboratorium für Biophysikalische Chemie, Technische Universität Berlin , Strasse des 17. Juni 135, 10623 Berlin, Germany.

ABSTRACT
In Photosystem I (PS I) long-wavelength chlorophylls (LWC) of the core antenna are known to extend the spectral region up to 750 nm for absorbance of light that drives photochemistry. Here we present clear evidence that even far-red light with wavelengths beyond 800 nm, clearly outside the LWC absorption bands, can still induce photochemical charge separation in PS I throughout the full temperature range from 295 to 5 K. At room temperature, the photoaccumulation of P700(+•) was followed by the absorbance increase at 826 nm. At low temperatures (T < 100 K), the formation of P700(+•)FA/B(-•) was monitored by the characteristic EPR signals of P700(+•) and FA/B(-•) and by the characteristic light-minus-dark absorbance difference spectrum in the QY region. P700 oxidation was observed upon selective excitation at 754, 785, and 808 nm, using monomeric and trimeric PS I core complexes of Thermosynechococcus elongatus and Arthrospira platensis, which differ in the amount of LWC. The results show that the LWC cannot be responsible for the long-wavelength excitation-induced charge separation at low temperatures, where thermal uphill energy transfer is frozen out. Direct energy conversion of the excitation energy from the LWC to the primary radical pair, e.g., via a superexchange mechanism, is excluded, because no dependence on the content of LWC was observed. Therefore, it is concluded that electron transfer through PS I is induced by direct excitation of a proposed charge transfer (CT) state in the reaction center. A direct signature of this CT state is seen in absorbance spectra of concentrated PS I samples, which reveal a weak and featureless absorbance band extending beyond 800 nm, in addition to the well-known bands of LWC (C708, C719 and C740) in the range between 700 and 750 nm. The present findings suggest that nature can exploit CT states for extending the long wavelength limit in PSI even beyond that of LWC. Similar mechanisms may work in other photosynthetic systems and in chemical systems capable of photoinduced electron transfer processes in general.

Show MeSH
Related in: MedlinePlus