v2i3-makita

Research Paper

Nanosecond Time-Resolved Absorption Difference Spectroscopy for the Study of Carotenoid Triplet States in Photosystem I Photosynthetic Reaction Centers

Hiroki Makita and Gary Hastings*

Department of Physics and Astronomy, Georgia State University, Atlanta, Georgia 30303, USA.

*Corresponding author, Gary Hastings, Email: ghastings@gsu.edu

Received 21 April, 2018, revised 31 May, 2018, accepted 1 June, 2018

Publication Date (Web): June 1, 2018

© Frontiers in Science, Technology, Engineering and Mathematics

Abstract

Nanosecond to microsecond time-resolved spectroscopy in the 420 - 520 nm spectral range has been used to study the formation and decay of antenna carotenoid triplet states in photosystem I particles from Synechocystis sp. PCC6803 at 298 K.  From global analysis of the transient absorption data, three decay phases with lifetimes of 30, 300, and 6000 ns, were obtained.  The flash intensity dependence of the phases, along with the spectral profiles indicate that the 30 ns and 6 µs phases are associated with the formation and decay of an antenna carotenoid triplet state, respectively.  The 300-ns phase is associated with electron transfer from A1– to FX.  In menB– PSI particles from S6803 the carotenoid composition is altered compared to that of wild type PSI, and the carotenoid triplet state decay time decreases to ~2.3 ms.

Keywords

Time-resolved, photosystem I, triplet state, carotenoid, chlorophyll

Introduction

In oxygen-evolving photosynthetic organisms, solar energy is captured and converted by large pigment-protein complexes called photosystem I and photosystem II (PSI and PSII).  In each photosystem, light induces the transfer of electrons via a series of protein-bound pigments across a biological membrane (the thylakoid membrane). 

In PSI, the protein-bound pigments involved in ET are organized into two nearly-symmetric branches (Jordan et al. 2001; Malavath et al. 2018; Mazor et al. 2017).  The two branches are termed the A- and B-branch, and both are active in ET (Guergova-Kuras et al. 2001).  Light energy impinging on PSI is absorbed by antenna chlorophyll pigments, raising these molecules to an excited state.  Energy transfer between pigments in the antenna occur until this excited state energy is transferred to a centralized electron donor species called P700, so-called because of its optical absorption at 700 nm.  P700 is dimeric chlorophyll-a (Chl-a) species that when excited rapidly transfers an electron to an acceptor molecule called A0.  A0 is a monomeric Chl-a molecule. From A0–, an electron is transferred to A1, which is a phylloquinone molecule.  Beyond A1, the two ET branches converged on a [4Fe-4S] cluster termed FX.  Two peripherally bound iron-sulfur clusters, termed FA and FB, serve as terminal electron acceptors.  A variety of modifications have previously been introduced to PSI for the study of structural and functional properties.  In menB deletion mutant of Synechocystis sp. PCC6803 (S6803), the cyanobacterium lacks the ability to synthesize phylloquinone for A1, and plastoquinone-9 occupies the A1 binding site instead (Johnson et al. 2000).

In PSI following light excitation of P700, an electron is transferred, resulting in the formation of the P700+A1– state within ~50 ps (Hastings et al. 1995; Hastings et al. 1994).  The charge separation is further stabilized by ET from A1– to FX.  The rate of ET from A1– to FX depends on the branch; A1B– to FX is characterized by a lifetime of ~15 – 25 ns, while A1A– _ FX ET is 250 – 310 ns (Agalarov and Brettel 2003; Makita et al. 2015).  In PSI isolated from menB mutant of S6803 (menB– PSI), the rates are modified to ~13 µs and ~200 µs for A1B– _ FX and A1A– _ FX ET, respectively (Makita et al. 2015; Semenov et al. 2000).  ET from FX– to FA and FB occurs on a nanosecond timescale (Byrdin et al. 2006). 

In PSI, the pigments that participate in the ET reactions are surrounded by Chl-a and carotenoid pigments which are also bound to the PsaA and B protein subunits.  Isolated cyanobacterial PSI particles contain ~90 Chl-a molecules and ~23 carotenoid molecules (Jordan et al. 2001; Malavath et al. 2018).  The carotenoids in PSI serve both structural (Vajravel et al. 2017) and functional roles (Bautista et al. 2005; Cazzaniga et al. 2016), being involved in both light-harvesting and photo-protection.  In light-harvesting carotenoids are antenna pigments that absorb light in the 450 – 550 nm range, a range where Chl-a lacks absorbance.  Carotenoids are involved in a number of photo-protection mechanisms (Cazzaniga et al. 2016), and the mechanism focused upon here is the quenching of Chl-a triplet states (3Chl-a).  3Chl-a can be formed in the PSI antenna under high light intensities.  3Chl-a in PSI can lead to the formation of highly reactive and toxic singlet oxygen species (via triplet-triplet annihilation).  Carotenoids in PSI can quench 3Chl-a through triplet transfer to form the carotenoid triplet state (3Car) (Cogdell 1985).  Recently, the x-ray crystal structure of trimeric PSI isolated from S6803 was resolved at 2.5 Å, and carotenoid compositions were identified (Malavath et al. 2018).    

In this study, time-resolved visible absorption difference spectroscopy (DS) has been applied to characterize the lifetime and spectral features of 3Car in PSI from wild type (WT) and in a menB deletion mutation of S6803.  Previously, an increased absorption had been observed for menB– PSI in the spectral range of carotenoid absorption.  (Chauvet et al. 2012).  However, whether if this difference indicates a modification of carotenoid compositions in menB– PSI has not been addressed. 

The lifetime of the 3Car state is obtained, as is the triplet-minus-singlet (T – S) decay-associated spectrum (DAS).  The (T – S) DAS is compared to the (T – S) DS of carotenoids in PSI from Thermosynechococcus elongatus (T. elongatus) (Schlodder 2001) and in light-harvesting complex-I (LHCI) associated with PSI from Arabidopsis thaliana (A. thaliana) (Croce et al. 2007).  The similarities and differences are discussed.

Materials and Methods

Sample preparation

Trimeric PSI particles from WT and menB– S6803 were prepared as described previously (Johnson et al. 2000).  For spectroscopic measurements PSI particles were suspended in 25 mM Tris buffer (pH 8.0) with 0.04 % n-dodecyl-_-D-maltoside in a 1 cm path-length cuvette as described previously (Makita and Hastings 2016).  For rapid re-reduction of P700+, sodium ascorbate (Asc) (20 mM) and phenazine methosulfate (PMS) (10 µM) were added.  All chemicals were from Sigma-Aldrich (St. Louis, MO), and used as received.  Sample concentration in the cuvette was adjusted so the absorption at 680 nm, measured using Shimadzu UV-1601 spectrometer (Shimadzu Scientific Instruments, Columbia, MD), is ~1.6.

Time-resolved visible absorption DS 

Transient absorption kinetic data on ns to µs timescales, at 298 K, were obtained using an Edinburgh Instruments LP920 flash photolysis spectrometer (Edinburgh Instruments, Livingston, UK), as described previously (Makita and Hastings 2016).  Pump pulses (~6 ns in duration) at 532 nm were provided by a Surelite III Nd:YAG laser operating at 10 Hz repetition rate (Continuum, San Jose, CA).  The probe wavelength was selected by a TMc 300 monochromator (Bentham Instruments Ltd., Reading, UK) placed between the sample and a detector, and a set of interference filters with 10 nm bandwidth.  One interference filter was placed between the probe light source and the sample to reduce actinic effects, and another interference filter was placed between the sample and monochromator to attenuate scattered photons from reaching the detector.  All measurements were at ~298 K.  The wavelength of the probing light was altered by placing interference filters at an angle, altering their passband.  The wavelength selected by the rotated interference filters were confirmed using the monochromator.

Spectral and kinetic analysis

Second-derivative spectra of absorption spectra in the visible range were calculated using the Savitzky-Golay algorithm as implemented in OPUS software (Bruker Optics, Billerica, MA).  The 25-point smoothing was applied.

Transient absorption data were fitted, individually or globally, to a sum of exponential functions and a constant, using the Levenberg-Marquardt algorithm, as implemented within Origin 2016 software (OriginLab Corporation, Northampton, MA).

Results

 Absorption spectra of WT and menB– PSI

Fig. 1 shows visible absorption spectra for WT and menB– PSI, normalized at 680 nm to an absorption of 1.6, and the corresponding second-derivative spectra.  The spectra are near identical in the 530 – 800 nm range, with second-derivative peaks within 1 nm.  Below 530 nm, distinct differences are observed in both absorption and peak positions.  menB– PSI exhibits increased absorption in the 450 – 510 nm range.  For WT PSI peaks are observed at 473 and 503 nm, which shift to 471 and 500 nm in menB– PSI.  For PSI samples absorption changes in the 500 – 470 nm range are often associated with carotenoids, so Fig. 1 suggests differences in the carotenoid distribution in the different PSI particles. 

Figure 1.  Absorption spectra of WT (blue) and menB– (red) PSI, normalized to Qy peak at 680 nm.  Second-derivative spectra are also shown (scaled by a factor of 50).  Spectra and peak labels are color coded.

Time-resolved visible absorption changes for WT PSI

Fig. 2A and B shows transient absorption changes for WT PSI on various timescales, probing at 465 and 520 nm, respectively.  In Fig. 2A, probing at 465 nm, a nanosecond phase with a positive amplitude and microsecond phase with negative amplitude are observed.  These phases with opposite sign make visualization straightforward.  In Fig. 2B, probing at 520 nm, a µs decay phase is observed along with a minor ns phase. To further characterize the µs phase, transient absorption data probing at 450, 460, 465, 470, 480, 487, 500, 505, 510, and 520 nm, on a 10-µs timescale, were obtained (Fig. 3a).  These kinetic data were globally analyzed with the data being fitted to two exponential functions and a constant.  The lifetimes obtained from the fitting were 152 ns and 5.91 µs.  The DAS obtained from the fitting are shown in Fig. 3b.  The ~6 µs DAS closely resembles previously obtained carotenoid (T – S) DS (Croce et al. 2007; Schlodder 2001).

To better resolve the ns phase, transient absorption changes probing at 460, 465, 470, 480, 487, 500, 505, 510, and 520 nm on a 1-µs timescale were undertaken (Fig. 4a).  From global analysis of the data in Fig. 4a the previously observed 150-ns phase was shown to be composed of two separate ns phases, with lifetimes of 28.7 and 303.4 ns.  The fitted functions derived from global analysis are also shown in Fig. 4a, The DAS of the ~29 ns, ~303 ns and n.d. components are shown in Fig. 4b.

Figure 2. Transient absorption changes at (A) 465 and (B) 520 nm, observed for WT PSI from S6803 at 298 K. Flash intensity is 2.5 mJ per pulse. Absorption changes on a) 400 ns, b) 2 µs, c) 10 µs, and d) 200 µs timescale are shown.

Forward ET from A1– to FX (more specifically A1A– to FX) is well known to occur in ~300-ns (Agalarov and Brettel 2003; Makita et al. 2015), so an immediate suggestion is that this process is responsible for the 300-ns phase observed in the data in Fig. 4.  However, a ~6 µs phase (Fig. 3) is not expected given this interpretation.  The shape of the 6 µs DAS in Fig. 3b suggests that it is due to triplet states of antenna carotenoids (Schlodder 2001).  To test the hypothesis that the phase is associated with antenna pigments transient absorption data was collected at 465 nm for PSI samples without the addition of ascorbate and PMS to the reaction medium.  Exposure to repetitive actinic flashes at 10 Hz in the absence of these reducing agents will lead to the accumulation of P700+, effectively “closing” the PSI RC, making light-induced ET impossible.  In the absence of ET any remaining absorption changes are most likely associated with antenna pigments.  Fig. 5 shows transient absorption changes measured using “closed” PSI particles at 465 (d, e) and 703 nm (f).  Comparing absorption changes at 703 nm obtained using “open” and “closed” PSI (Fig. 5c and f) shows that the P700 ground state bleaching is absent for “closed” PSI, indicating little or no light-induced ET in PSI in this state.  At 465nm, however, for both “closed” and “open” PSI, µs and ns phases with similar lifetimes and amplitudes are observed (compare Fig 5a with 5d, and Fig, 5b with 5e).  The time constants obtained from the kinetics for the “closed” PSI were 40 – 60 ns and 2.46 µs.  While the faster ns agrees with the “open” PSI in time constant,

 ~2.5 µs is faster than ~6 µs observed for the “open” PSI.  487 nm, the 300-ns phase was present in the “open” but absent in “closed” PSI (data not shown).  The data in Fig. 5 demonstrates that the ~6 µs phase is independent of ET, and is therefore consistent with the idea that it is associated with antenna pigments. 

The shape of the 6 µs DAS in Fig. 3, with a negative peak near 460 nm and a positive peak near 520 nm, is similar to that observed previously for carotenoid triplet states (Schlodder 2001), and on the basis of this comparison the 6 µs DAS observed here can be assigned to a carotenoid triplet state.

To further characterize the ~6 µs phase, absorption changes as a function of actinic laser flash intensity were studied.  Fig. 6 shows absorption changes at 703 and 465 nm obtained using “open” PSI samples with three different flash excitation intensities.

The absorption changes in Fig. 6 indicate that, with a decrease in the actinic intensity, the µs phase (observed at 465 nm (Fig. 6b)) diminishes in amplitude faster than the bleaching at 703 nm (Fig. 6a).  The result further demonstrates that the ~6 µs phase is not associated with ET in PSI.

Figure 3. (a) Transient absorption changes obtained following flash excitation of PSI particles from S6803, at 450, 460, 465, 470, 480, 487, 500, 505, 510, and 520 nm on a 10-µs timescale.  Flash intensity is 2.5 mJ per pulse.  All of the kinetic data was fitted simultaneously to a sum of two exponential functions and a constant.  The fitted functions are shown in red.  (b) DAS of the three phases obtained from fitting the experimental data in (a).  The lifetimes obtained for the three phases are 5.91 µs, 152 ns, and non-decaying (n.d.).

Figure 4. (a) Transient absorption changes at 460, 465, 470, 480, 487, 500, 505, 510, and 520 nm on a 1-µs timescale, obtained following flash excitation of PSI particles from S6803.  Flash intensity is 2.5 mJ per pulse. The kinetic data was fitted to a sum of two exponential functions and a constant.  The fitted functions are also shown (red).  (b) DAS of the 28.7, 303.4 and n.d. phases obtained from global analysis.  (c) Close-up view of the 303.4 ns phase in (b)

Figure 5. Flash induced absorption changes obtained using “open” (a-c) and “closed” (d-f) PSI particles, probing at 465 (a, b, d, e) and 703 nm (c and f).  Flash intensity is 2.5 mJ per pulse.  The kinetics in (d) and (e) were fitted to single- and two-exponential functions and a constant, respectively.  The time constants obtained from the fitting were 38 ns for (d) and 60 ns and 2.46 µs for (e).  Fitted curves are in red.  See Fig. 3 and 4 for the fitting of kinetics in (a) and (b). 

Figure 6.  Flash induced absorption changes at a) 703 nm and b) 465 nm triggered by actinic laser flashes with an intensity of 2.5 mJ (black), 2.0 mJ (blue), and 1.0 mJ (red) per pulse.

Figure 7. Flash induced absorption changes obtained using “open” and “closed” menB– PSI, probed at 487 nm (a) and 703 nm (b). Flash intensity is 2.5 mJ per pulse.

Figure 8. (a) Flash induced absorption changes at 510 nm for “closed” menB– PSI, with an excitation intensity varied from 2.5 mJ to 0.05 mJ per pulse.  (b) Flash induced absorption changes probed at 703 nm for “open” menB– PSI triggered by actinic flashes with an intensity of 2.5 mJ and 0.05 mJ per pulse.

Transient absorption changes for menB– PSI

In the absorption spectra for WT and menB– PSI (Fig. 1), notable differences were observed in the 510 – 450 nm range, which is in the region that carotenoids generally absorb.  So the suggestion is that the carotenoid distribution is different in menB–- PSI compared to WT PSI.  This suggests that the microsecond decay phase observed for WT PSI might thus be altered for in menB–- PSI.  To investigate this possibility time-resolved visible absorption DS measurements were undertaken using menB– PSI. 

Fig. 7 shows transient absorption changes at 487 and 703 nm obtained using “open” and “closed” menB– PSI particles.  As found for WT PSI, the bleaching at 703 nm disappears for menB– PSI in the absence of ET mediators, indicating that ET has been inhibited (PSI is closed).  Probing at 487 nm, a microsecond phase is observed with similar amplitude for both open and closed samples.  P700+ and electrochromic shift due to A1– are also absorbs at 487 nm, giving rise to a long-lived absorption that is observed for the open PSI samples in Fig. 7.  So the data in Fig. 7 demonstrates a microsecond decay phase that is independent of ET processes for menB– PSI, similar to that found for WT PSI.  As found for WT PSI, the kinetic component that represent forward ET from A1– to FX was absent in closed PSI for menB– PSI (data not shown).

By monitoring the flash-induced absorption changes at 510 nm as a function of excitation intensity, the µs phase for menB– PSI was also found to diminish faster than the bleaching at 703 nm (Fig. 8).  This result also demonstrates that the microsecond phase is independent of ET processes in menB– PSI, and the relationship to the actinic intensity suggests that the origin of the µs phase is most likely the same for menB– and WT PSI.  By simultaneously fitting the absorption changes at 487 nm (Fig. 7a) and 510 nm (Fig. 8a) for menB– PSI The lifetime of the phase for menB– PSI a time constant of ~2.3 µs was obtained, which is considerably shorter that the ~6 µs lifetime obtained for WT PSI.

Discussion 

(T – S) DS of carotenoids in PSI from S6803

All of the time-resolved data presented here support the notion that the ~6 µs phase observed here using WT PSI samples is due to the decay of an antenna carotenoid triplet state (3Car), and the 6 µs DAS (Fig. 3B) corresponds to an antenna carotenoid (T – S) DS.  The triplet state of the antenna carotenoid is most likely formed by triplet-triplet exchange with an antenna chlorophyll triplet state (3Chl) (Cogdell 1985).  This indicates that the reaction will be independent of ET processes in PSI, which is exactly as observed, with the 6 µs phase decay amplitude being independent of whether PSI is in the “open” or “closed” state (Fig. 5). 

Under high light conditions 3Chl states can readily form in the PSI antenna.  3Chl formation is generally bad in biological systems, as this can lead to the production of the highly toxic singlet oxygen species (Cogdell 1985).  Carotenoids often function as photo-protective elements in photosynthetic antenna by quenching 3Chl.  If the 3Chl yield correlates with incident light intensity, then one might expect the 3Car yield to also depend on the light intensity.  In Fig. 6, it was demonstrated that the amplitude of the ~6 µs phase is sensitive to the actinic light intensity, supporting the hypothesis that the 6 µs phase is due to decay of an antenna carotenoid triplet state.  Further support for this conclusion comes from the observation that the shape of the 6 µs DAS (Fig. 3b) closely resembles that of previously reported (T – S) DS of carotenoids in the other antenna systems (Croce et al. 2007; Schlodder 2001).  Given this the data presented here strongly supports the notion that the 6 µs phase is assigned to the decay of an antenna carotenoid triplet state.

The 6 µs lifetime for 3Car states observed here in PSI from S6803 at 298 K compares favorably to that obtained for 3Car states in other systems.  For PSI from T. elongatus, a lifetime is 4.6 µs was reported (Schlodder 2001), while for LHCI and four Lhca subunits isolated from A. thaliana, lifetimes in the 2.25 – 2.67 µs range were reported (Croce et al. 2007).  Finally, for _-carotene in hexane a lifetime of 9 µs was reported (Land E et al. 2008). 

The 6 µs DAS in Fig. 3b exhibits a negative/positive peak near 460/520 nm, respectively, and a cross-over point at around 483 nm.  In the (T – S) DS from T. elongatus and LHCI, similar peaks are observed, although red-shifted 5 - 15 nm, with a cross-over point at ~473-478 nm (Croce et al. 2007; Schlodder 2001).  The overall shape and lifetime similarity suggest a common origin.

The nanosecond kinetic phases

From global analysis of the transient absorption data in the 460 – 520 nm range, two ns phases were obtained.  The 300 ns phase was assigned to ET from A1A– to FX (Makita et al. 2015).  The DAS shows the absorption maxima at ~475 and ~505 nm (Fig. 4c), both of which agree well with that reported previously (Bautista et al. 2005). 

From global analysis of the transient absorption data a ~30 ns phase is also obtained (Fig. 4b).  A ~25 ns phase is associated with ET down the B-branch (A1B– to FX) in PSI, and at first glance one could therefore assign the ~30 ns phase observed here to A1B– to FX ET. However, the ~30 ns DAS reported here does not correspond well with DS associated with the fast phase of ET in PSI reported previously, which shows positive absorption peaks at ~475 and ~515 nm (Bautista et al. 2005), while the DAS of the ~30 ns phase observed here exhibits a positive peak  near 465 nm and a negative peak near 520 nm.  The ~30-ns DAS in Fig. 4b appears to be approximately the inverse of both the n.d. DAS in Fig. 4b, and the 6 µs DAS in Fig. 3b.  Since the 6 µs DAS in Fig. 3b is due to a (3Car – Car) DS, the fact that the ~30 ns DAS is the inverse of the 6 µs DAS suggests that the 30 ns phase is due to 3Car formation, most likely occurring through triplet transfer from a 3Chl state.  In agreement with this assignment, is the observation that the reaction lifetimes of the 3Chl-a decay reported previously in PSI isolated from T. elongatus was 14 ns (Schlodder 2001).

The microsecond kinetic phase for menB– PSI

The 2.3 µs phase obtained from experiments using menB– PSI seems to have the same origin as the 6 µs phase for WT PSI.  The 2.3 µs phase is independent of ET processes, and its amplitude is dependent on the actinic flash intensity, much like that observed for the 6 µs phase observed in experiments using WT PSI.  Therefore, the 2.3 µs phase for menB– PSI is assigned to the decay of 3Car.  A lifetime of 2.3 ms is considerably shorter than the 6 µs lifetime found for WT PSI.  It is, however, very close to the lifetimes observed for LHCI from A. thaliana (2.25 – 2.67 µs) (Croce et al. 2007).  Interestingly, the lifetime obtained for “closed” WT PSI is 2.5 µs, and is similar to the values observed for menB– PSI and LHCI.   

The decay amplitude of the 2.3 µs phase observed for menB– PSI at 487 nm is also considerably larger than that for WT PSI at the same probe wavelength, while the decay amplitude at 510 nm is smaller for menB– PSI.  These changes in the decay amplitudes suggest that the whole (T – S) DS for menB– PSI is blue-shifted by a few nm relative to the DS for WT PSI.  These changes may be consistent with the differences in the absorption spectra (Fig. 1).  Such a difference in absorption spectra for WT and menB– PSI has also been observed by other groups (Chauvet et al. 2012).  The differences in transient absorption changes, and in the absorption spectra below 530 nm, suggest an altered carotenoid composition in the core antenna of menB– PSI relative to WT PSI.

Conclusions

Time-resolved visible absorption difference spectroscopy was used to study the triplet state of antenna pigments in PSI from S6803.  Through global analysis of transient absorption data, DAS with lifetimes of 6 µs, 30 ns, and 300 ns were obtained.  By considering absorption changes in “closed” and “open” PSI, and the actinic flash intensity dependence, the 6 µs phase was assigned to the triplet state of an antenna carotenoid molecule. The 3Car state forms in ~30 ns most likely via quenching of an antenna chlorophyll triplet state.

Acknowledgment

This work was supported in part by Grant Number DE-SC0017937 from the Department of Energy to GH. HM acknowledges support from the Molecular Basis of Disease Program at Georgia State University. The statements made herein are solely the responsibility of the authors.

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Citation:

Hiroki Makita and Gary Hastings (2018) Nanosecond time-resolved absorption difference spectroscopy for the study of carotenoid triplet states in photosystem I photosynthetic reaction centers, Frontiers in Science, Technology, Engineering and Mathematics, Volume 2, Issue 3, 138-147