Limits...
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.

Show MeSH

Related in: MedlinePlus

Light-minus-dark absorbance difference spectraof PS I trimers of T. elongatus at5 K measured after selective excitation at 754 nm, 785 or 808 nm anddifferent illumination periods. The difference spectra were obtainedby subtracting the absorbance spectra in the dark-adapted state fromthose measured after selective excitation. The curves denoted “dark”show the difference between two absorbance spectra of the dark-adaptedsample measured directly one after the other. The curve denoted “LED455 nm” is the difference between the absorbance spectra inthe dark-adapted state and those measured after illumination by 455nm LED light.
© Copyright Policy
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC3959156&req=5

fig9: Light-minus-dark absorbance difference spectraof PS I trimers of T. elongatus at5 K measured after selective excitation at 754 nm, 785 or 808 nm anddifferent illumination periods. The difference spectra were obtainedby subtracting the absorbance spectra in the dark-adapted state fromthose measured after selective excitation. The curves denoted “dark”show the difference between two absorbance spectra of the dark-adaptedsample measured directly one after the other. The curve denoted “LED455 nm” is the difference between the absorbance spectra inthe dark-adapted state and those measured after illumination by 455nm LED light.

Mentions: Figure 9 showslight-minus-dark absorbance difference spectra of trimeric PS I complexesfrom T. elongatus measured at 5 K inducedby illumination of various periods of time with far-red light at 754nm (top), 785 nm (middle) and 808 nm (bottom). The curves denoted“dark” show the difference between two absorbance spectraof the dark-adapted sample measured directly one after the other.These dark-minus-dark spectra are identical within the error limitsto the zero line. This clearly demonstrates that photochemical reactionscaused by the measuring light in the spectrophotometer are negligible.The light-minus-dark difference spectra were obtained by subtractingthe absorbance spectrum of PS I in the dark-adapted state (with P700reduced) from those after far-red illumination for the period indicated.Already after 10 min illumination, the characteristic features assignedto the formation of P700+•FA/B–• at 5 K are visible:a broad bleaching at 703 nm, a narrow positive band at 698.5 nm, asmall negative band at 696 nm, and a strong absorbance increase at690 nm.3,29


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)

Light-minus-dark absorbance difference spectraof PS I trimers of T. elongatus at5 K measured after selective excitation at 754 nm, 785 or 808 nm anddifferent illumination periods. The difference spectra were obtainedby subtracting the absorbance spectra in the dark-adapted state fromthose measured after selective excitation. The curves denoted “dark”show the difference between two absorbance spectra of the dark-adaptedsample measured directly one after the other. The curve denoted “LED455 nm” is the difference between the absorbance spectra inthe dark-adapted state and those measured after illumination by 455nm LED light.
© Copyright Policy
Related In: Results  -  Collection

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

fig9: Light-minus-dark absorbance difference spectraof PS I trimers of T. elongatus at5 K measured after selective excitation at 754 nm, 785 or 808 nm anddifferent illumination periods. The difference spectra were obtainedby subtracting the absorbance spectra in the dark-adapted state fromthose measured after selective excitation. The curves denoted “dark”show the difference between two absorbance spectra of the dark-adaptedsample measured directly one after the other. The curve denoted “LED455 nm” is the difference between the absorbance spectra inthe dark-adapted state and those measured after illumination by 455nm LED light.
Mentions: Figure 9 showslight-minus-dark absorbance difference spectra of trimeric PS I complexesfrom T. elongatus measured at 5 K inducedby illumination of various periods of time with far-red light at 754nm (top), 785 nm (middle) and 808 nm (bottom). The curves denoted“dark” show the difference between two absorbance spectraof the dark-adapted sample measured directly one after the other.These dark-minus-dark spectra are identical within the error limitsto the zero line. This clearly demonstrates that photochemical reactionscaused by the measuring light in the spectrophotometer are negligible.The light-minus-dark difference spectra were obtained by subtractingthe absorbance spectrum of PS I in the dark-adapted state (with P700reduced) from those after far-red illumination for the period indicated.Already after 10 min illumination, the characteristic features assignedto the formation of P700+•FA/B–• at 5 K are visible:a broad bleaching at 703 nm, a narrow positive band at 698.5 nm, asmall negative band at 696 nm, and a strong absorbance increase at690 nm.3,29

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