Red antenna states of photosystem I from Synechocystis PCC 6803

Single-molecule spectroscopy at low temperatures was used to elucidate spectral properties, heterogeneities, and dynamics of the red-shifted chlorophyll a (Chl a) molecules responsible for the fluorescence from photosystem I (PSI). Emission spectra of single PSI complexes from the cyanobacterium Synechocystis PCC 6803 show zero-phonon lines (ZPLs) as well as broad intensity distributions without ZPLs. ZPLs are found most frequently on the blue side of the broad intensity distributions. The abundance of ZPLs decreases almost linearly at longer wavelengths. The distribution of ZPLs indicates the existence of at least two pools with maxima at 699 and 710 nm. The pool with the maximum at 710 nm is assigned to chlorophylls absorbing around 706 nm (C706), whereas the pool with the maximum at 699 nm (F699) can be assigned to chlorophylls absorbing at 692, 695, or 699 nm. The broad distributions dominating the red side of the spectra are made up of a low number of emitters assigned to the red-most pool C714. The properties of F699 show close relation to those of F698 in Synechococcus PCC 7002 and C708 in Thermosynechococcus elongatus. Furthermore, a high similarity is found between the C714 pool in Synechocystis PCC 6803 and C708 in Synechococcus PCC 7002 as well as C719 in T. elongatus.     

Assignment of red antenna states in photosystem I from Thermosynechococcus elongatus by single-molecule spectroscopy

Single-molecule spectroscopy at cryogenic temperatures was used to elucidate spectral properties, heterogeneities, and dynamics of the chlorophyll a (Chla) molecules responsible for the fluorescence in photosystem I (PSI) from the cyanobacteria Thermosynechococcus elongatus. Absorption and hole burning data suggest the presence of three pools absorbing at wavelengths greater than 700 nm with their absorption maxima at 708, 715, and 719 nm. The responsible Chla molecules are termed C708, C715, and C719. In the emission spectra of single PSI complexes, zero-phonon lines (ZPLs) were observed over the whole red emission range of PSI. The spectral region of the C708 pool is dominated by intense ZPLs; on the other hand, the broad emission of C715/C719 is unstructured and ZPLs are seen in this region much less frequently. Spectral jumps of ZPLs were observed. The dynamics as well as the spectral range covered by such jumps differ for C708 and C715/C719. This heterogeneity is likely caused by differences in the close environment of the chromophores. A tentative assignment of C708 and C715/C719 to Chla dimers and a Chla trimer is discussed, which is based on the remarkable structural differences in the environment of the most probable candidates for the red-most fluorescence.     

Polarized site-selective fluorescence spectroscopy of the long-wavelength emitting chlorophylls in isolated Photosystem I particles of Synechococcus elongatus

Isolated trimeric Photosystem I complexes of the cyanobacterium Synechococcus elongatus have been studied with absorption spectroscopy and site-selective polarized fluorescence spectroscopy at cryogenic temperatures. The 4 K absorption spectrum exhibits a clear and distinct peak at 710 nm and shoulders near 720, 698 and 692 nm apart from the strong absorption profile located at 680 nm. Deconvoluting the 4 K absorption spectrum with Gaussian components revealed that Synechococcus elongatus contains two types of long-wavelength pigments peaking at 708 nm and 719 nm, which we denoted C-708 and C-719, respectively. An estimate of the oscillator strengths revealed that Synechococcus elongatus contains about 4–5 C-708 pigments and 5–6 C-719 pigments. At 4 K and for excitation wavelengths shorter than 712 nm, the emission maximum appeared at 731 nm. For excitation wavelengths longer than 712 nm, the emission maximum shifted to the red, and for excitation in the far red edge of the absorption spectrum the emission maximum was observed 10–11 nm to the red with respect to the excitation wavelength, which indicates that the Stokes shift of C-719 is 10–11 nm. The fluorescence anisotropy, as calculated in the emission maximum, reached a maximal anisotropy of r=0.35 for excitation in the far red edge of the absorption spectrum (at and above 730 nm), and showed a complicated behavior for excitation at shorter wavelengths. The results suggest efficient energy transfer routes between C-708 and C-719 pigments and also among the C-719 pigments.Abbreviations Chl chlorophyll - FWHM full width at half maximum - PS I Photosystem I     

Red antenna states of Photosystem I trimers from Arthrospira platensis revealed by single-molecule spectroscopy

Single-molecule fluorescence spectroscopy at 1.4K was used to investigate the spectral properties of red (long-wavelength) chlorophylls in trimeric Photosystem I (PSI) complexes from the cyanobacterium Arthrospira platensis. Three distinct red antenna states could be identified in the fluorescence spectra of single PSI trimers from A. platensis in the presence of oxidized P700. Two of them are responsible for broad emission bands centered at 726 and 760nm. These bands are similar to those found in bulk fluorescence spectra measured at cryogenic temperatures. The broad fluorescence bands at ?726 and ?760nm belong to individual emitters that are broadened by strong electron-phonon coupling giving rise to a large Stokes-shift of about 20nm and rapid spectral diffusion. An almost perpendicular orientation of the transition dipole moments of F726 and F760 has to be assumed because direct excitation energy transfer does not occur between F726 and F760. For the first time a third red state assigned to the pool absorbing around 708nm could be detected by its zero-phonon lines. The center of the zero-phonon line distribution is found at ?714nm. The spectral properties of the three red antenna states show a high similarity to the red antenna states found in trimeric PSI of Thermosynechoccocus elongatus. Based on these findings a similar organization of the red antenna states in PSI of these two cyanobacteria is discussed.     

Excitation wavelength dependence of the fluorescence kinetics in Photosystem I particles from Synechocystis PCC 6803 and Synechococcus elongatus

The excitation-wavelength dependence of the excited-state dynamics of monomeric and trimeric Photosystem I (PSI) particles from Synechocystis PCC 6803 as well as trimeric PSI particles from Synechococcus elongatus has been studied at room temperature using time-resolved fluorescence spectroscopy. For aselective (400 nm), carotenoid (505 nm), and bulk chlorophyll (approximately 650 nm) excitation in all species, a downhill energy-transfer component is observed, corresponding to a lifetime of 3.4-5.5 ps. For selective red excitation (702-719 nm) in all species, a significantly faster, an approximately 1-ps, uphill transfer component was recorded. In Synechococcus PSI, an additional approximately 10-ps downhill energy-transfer component is found for all wavelengths of excitation, except 719 nm. Each of the species exhibits its own characteristic trap spectrum, the shape of which is independent of the wavelength of excitation. This trap spectrum decays in approximately 23 ps in both monomeric and trimeric Synechocystis PSI and in approximately 35 ps in trimeric Synechococcus PSI. The data were simulated based on the 2.5 A structural model of PSI of Synechococcus elongatus using the F?rster equation for energy transfer, and using the 0.6-1-ps charge-separation time and the value of 1.2-1.3 for the index of refraction that were obtained from the dynamics of a hypothetical PSI particle without red chls. The experimentally obtained lifetimes and spectra were reproduced well by assigning three of the chlorophyll-a (chla) dimers observed in the structure to the C708/C702RT pool of red chls present in PSI from both species. Essential for the simulation of the dynamics of Synechococcus PSI is the assignment of the single chla trimer in the structure to the C719/C708RT pool present in this species.     

Aggregation of 8,12-diethyl farnesyl bacteriochlorophyll c at low temperature

The effect of temperature on the aggregation of 3^lR-8,12-diethyl farnesyl bacteriochlorophyll c in a mixture of n-pentane and methylcyclohexane (1/1, v/v) was studied by means of absorption, circular dichroism and fluorescence spectroscopy. At room temperature essentially only two aggregate species, absorbing at 702 nm (A-702) and 719 nm (A-719), were present. Upon cooling to 219 K, A-702 was quantitatively converted to A-719. Further lowering of the temperature led to the stepwise formation of larger aggregates by the conversion of A-719 to aggregate species absorbing at 743 nm (A-743) and 755 nm (A-755). All absorption changes were reversible. A-719 was highly fluorescent (maximum at 192 K: 744 nm), while A-743 and especially A-755 were weakly fluorescent. Below 130 K the mixture solidified, and no major changes in the absorption spectrum were observed upon further cooling. At 45 K, however, a relatively strong emission at 775 nm was observed. Below 200 K, the absorption, fluorescence and circular dichroism spectra resembled that of the chlorosome. These results open up the possibility to study higher aggregates of BChl c as models for the chlorosome by various methods at low temperature, thus avoiding interference by thermal processes.Abbreviations A-680, A-702, A-719, A-743 and A-755- BChl c aggregates absorbing at the wavelengths indicated - BChl- bacteriochlorophyll - R[E,E] BChl c _F- the 3¹ R isomer of 8,12-diethyl BChl c esterified with farnesol (F), analogously - M- methyl - Pr- propyl - S- stearol (see Smith 1994) - CD- circular dichroism     

Spectral inhomogeneity of photosystem I and its influence on excitation equilibration and trapping in the cyanobacterium Synechocystis sp. PCC6803 at 77 K.

Ultrafast transient absorption spectroscopy was used to probe excitation energy transfer and trapping at 77 K in the photosystem I (PSI) core antenna from the cyanobacterium Synechocystis sp. PCC 6803. Excitation of the bulk antenna at 670 and 680 nm induces a subpicosecond energy transfer process that populates the Chl a spectral form at 685--687 nm within few transfer steps (300--400 fs). On a picosecond time scale equilibration with the longest-wavelength absorbing pigments occurs within 4-6 ps, slightly slower than at room temperature. At low temperatures in the absence of uphill energy transfer the energy equilibration processes involve low-energy shifted chlorophyll spectral forms of the bulk antenna participating in a 30--50-ps process of photochemical trapping of the excitation by P(700). These spectral forms might originate from clustered pigments in the core antenna and coupled chlorophylls of the reaction center. Part of the excitation is trapped on a pool of the longest-wavelength absorbing pigments serving as deep traps at 77 K. Transient hole burning of the ground-state absorption of the PSI with excitation at 710 and 720 nm indicates heterogeneity of the red pigment absorption band with two broad homogeneous transitions at 708 nm and 714 nm (full-width at half-maximum (fwhm) approximately 200--300 cm(-1)). The origin of these two bands is attributed to the presence of two chlorophyll dimers, while the appearance of the early time bleaching bands at 683 nm and 678 nm under excitation into the red side of the absorption spectrum (>690 nm) can be explained by borrowing of the dipole strength by the ground-state absorption of the chlorophyll a monomers from the excited-state absorption of the dimeric red pigments.     

Immunological characterization of chlorophyll a/b-binding proteins of barley thylakoids

Monoclonal antibodies have been raised against the light-harvesting chlorophyll a/b-binding proteins of photosystem I (LHCI) using a photosystem (PS) I preparation (PSI-200) wild-type from barley (Hordeum vulgare L. cv. Svaløf's Bonus) as the antigen. These antibodies cross-reacted with a minor light-harvesting chlorophyll a/b-protein of PSII (Chla/b-P1=CP29), but not with the major one, LHCII (=Chla/b-P2^**). Similarly, a monoclonal antibody to Chla/b-P1, elicited by a PSII preparation as the antigen, cross-reacted with LHCI, but not LHCII. This explains why an antigen consisting of LHCII, free of LHCI, but contaminated with Chla/b-P1, can elicit antibodies which cross-react with LHCI. Immunoblot assays showed that LHCI and Chla/b-P1 have at least two epitopes in common. Immunogold labelling of thin-sectioned wild-type thylakoids confirmed a preferential localisation of Chla/b-P1 in grana partition membranes and LHCI in stroma lamellae. The presence of LHCI was demonstrated in barley mutants lacking the PSI reaction centre (viridis-zb ⁶³) and chlorophyll b (chlorina-f2), and was correlated with the presence of long-wavelength (730 nm) fluorescence emission at 77 K. The mutant viridis-k ²³, which has a 77 K long-wavelength fluorescence peak at 720 nm, was shown by immune-blot assay to lack LHCI, although Chla/b-P1 was present.Abbreviations Chl-P chlorophyll-protein - CM Carlsberg Monoclonal - Da dalton - LHC light-harvesting complex - PAGE polyacrylamide gel electrophoresis - PSI, II photosystem I, II - PSI-200 PSI containing LHCI polypeptides - SDS sodium dodecyl sulphate     

Effects of different excitation and detection spectral regions on room temperature chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence parameters

The effects of different spectral region of excitation and detection of chlorophyll (Chl) a fluorescence at room temperature on the estimation of excitation energy utilization within photosystem (PS) 2 were studied in wild-type barley (Hordeum vulgare L. cv. Bonus) and its Chl b-less mutant chlorina f2 grown under low and high irradiances [100 and 1 000 μmol(photon) m⁻² s⁻¹]. Three measuring spectral regimes were applied using a PAM 101 fluorometer: (1) excitation in the red region (maximum at the wavelength of 649 nm) and detection in the far-red region beyond 710 nm, (2) excitation in the blue region (maximum at the wavelength of 461 nm) and detection beyond 710 nm, and (3) excitation in the blue region and detection in the red region (660– 710 nm). Non-photochemical quenching of maximal (NPQ) and minimal fluorescence (SV₀), determined by detecting Chl a fluorescence beyond 710 nm, were significantly higher for blue excitation as compared to red excitation. We suggest that this results from higher non-radiative dissipation of absorbed excitation energy within light-harvesting complexes of PS2 (LHC2) due to preferential excitation of LHC2 by blue radiation and from the lower contribution of PS1 emission to the detected fluorescence in the case of blue excitation. Detection of Chl a fluorescence originating preferentially from PS2 (i.e. in the range of 660–710 nm) led to pronounced increase of NPQ, SV₀, and the PS2 photochemical efficiencies (F_V/F_M and F_V′/F_M′), indicating considerable underestimation of these parameters using the standard set-up of PAM 101. Hence PS1 contribution to the minimal fluorescence level in the irradiance-adapted state may reach up to about 80 %.     

Chlorophyll fluorescence at 680 and 730 nm and leaf photosynthesis

Chlorophyll fluorescence constitutes a simple, rapid, and non-invasive means to assess light utilization in Photosystem II (PS II). This study examines aspects relating to the accuracy and applicability of fluorescence for measurement of PS II photochemical quantum yield in intact leaves. A known source of error is fluorescence emission at 730 nm that arises from Photosystem I (PS I). We measured this PS I offset using a dual channel detection system that allows measurement of fluorescence yield in the red (660 nm < F < 710 nm) or far red (F > 710 nm) region of the fluorescence emission spectrum. The magnitude of the PS I offset was equivalent to 30% and 48% of the dark level fluorescence F₀ in the far red region for Helianthus annuus and Sorghum bicolor, respectively. The PS I offset was therefore subtracted from fluorescence yields measured in the far red spectral window prior to calculation of PS II quantum yield. Resulting values of PS II quantum yield were consistently higher than corresponding values based on emission in the red region. The basis for this discrepancy lies in the finite optical thickness of the leaf that leads to selective reabsorption by chlorophyll of red fluorescence emission originating in deeper cell layers. Consequently, red fluorescence measurements preferentially sense emission from chloroplasts in the uppermost layer of the leaf where levels of photoprotective nonphotochemical quenching are higher due to increased photon density. It is suggested that far red fluorescence, corrected for the PS I offset, provides the most reliable quantitative basis for calculation of PS II quantum yield because of reduced sensitivity of these measurements to gradients in leaf transmittance and quenching capacity. This revised version was published online in August 2006 with corrections to the Cover Date.     

Red region excitation spectra of protochlorophyllide in dark-grown leaves from plant species with different proportions of its spectral forms

Etiolated leaves of three different species, maize, wheat, and pea, as well as a pea mutant (lip1) were used to compare the excitation spectra of protochlorophyllide (Pchlide) in the red region. The species used have different composition of short-wavelength and long-wavelength Pchlide forms. The relation between different forms was furthermore changed through incubating the leaves in 5-aminolevulinic acid (ALA), which caused an accumulation of short-wavelength Pchlide forms, as shown by changes in absorption and fluorescence spectra. This is the first time a comprehensive comparison is made between excitation spectra from different species covering an emission wavelength range of 675–750 nm using fluorescence equipment with electronic compensation for the variations in excitation irradiance. The different forms of Pchlide having excitations peaks at 628, 632, 637, 650, and 672 nm could be best measured at 675, 700, 710, 725, and 750 nm, respectively. Measuring emission at wavelengths between 675– 710 nm gave an exaggeration of the short-wavelength forms and measuring at longer wavelengths gave for the pea leaves an exaggeration of the 672 nm peak. In general, an energy transfer from short-wavelength Pchlide forms to long-wavelength Pchlide forms occurred, but such an energy transfer sometimes seemed to be limited as a result of a discrete location of the Pchlide spectral forms. The excitation spectra resembling the absorption spectrum most were measured at an emission wavelength of 740 nm. Measuring the excitation at 710 nm gave higher intensity of the spectra but the short-wavelength forms were accentuated.     

On the spectral properties and excitation dynamics of long-wavelength chlorophylls in higher-plant photosystem I

In higher-plant Photosystem I (PSI), the majority of “red” chlorophylls (absorbing at longer wavelengths than the reaction centre P₇₀₀) are located in the peripheral antenna, but contradicting reports are given about red forms in the core complex. Here we attempt to clarify the spectroscopic characteristics and quantify the red forms in the PSI core complex, which have profound implication on understanding the energy transfer and charge separation dynamics. To this end we compare the steady-state absorption and fluorescence spectra and picosecond time-resolved fluorescence kinetics of isolated PSI core complex and PSI–LHCI supercomplex from Pisum sativum recorded at 77 K. Gaussian decomposition of the absorption spectra revealed a broad band at 705 nm in the core complex with an oscillator strength of three chlorophylls. Additional absorption at 703 nm and 711 nm in PSI–LHCI indicated up to five red chlorophylls in the peripheral antenna. Analysis of fluorescence emission spectra resolved states emitting at 705, 715 and 722 nm in the core and additional states around 705–710 nm and 733 nm in PSI–LHCI. The red states compete with P₇₀₀ in trapping excitations in the bulk antenna, which occurs on a timescale of ~20 ps. The three red forms in the core have distinct decay kinetics, probably in part determined by the rate of quenching by the oxidized P₇₀₀. These results affirm that the red chlorophylls in the core complex must not be neglected when interpreting kinetic experimental results of PSI.     

Characterization of chlorophyll–protein complexes isolated from a Siphonous green alga, Bryopsis corticulans

Six chlorophyll–protein complexes are isolated from thylakoid membranes of Bryopsis corticulans by dodecyl-β-d-maltoside polyacrylamide gel electrophoresis. Unlike that of higher plants, the 77 K fluorescence emission spectrum of the CP1 band, the PSI core complexes of B. corticulans, presents two peaks, one at 675 nm and the other at 715–717 nm. The emission peak at 715–717 nm is slightly higher than that at 675 nm in the CP1 band when excited at 438 or 540 nm. However, the peak at 715 nm is obviously lower than that at 675 nm when excited at 480 nm. The excitation spectra of CP1 demonstrate that the peak at 675 nm is mainly attributed to energy from Chl b while it is the energy from Chl a that plays an important role in exciting the peak at 715–717 nm. Siphonaxanthin is found to contribute to both the 675 nm and 715–717 nm peaks. We propose from the above results that chlorophyll a and siphonaxanthin are mainly responsible for the transfer of energy to the far-red region of PSI while it is Chl b that contributes most of the transfer of energy to the red region of PSI. The analysis of chlorophyll composition and spectral characteristics of LHCP¹ and LHCP³ also indicate that higher content of Chl b and siphonaxanthin, mainly presented in LHCP¹, the trimeric form of LHCII, are evolved by B. corticulans to absorb an appropriate amount of light energy so as to adapt to their natural habitats.     

Energy transfer and charge separation in photosystem I: P700 oxidation upon selective excitation of the long-wavelength antenna chlorophylls of Synechococcus elongatus.

Photosystem I of the cyanobacterium Synechococcus elongatus contains two spectral pools of chlorophylls called C-708 and C-719 that absorb at longer wavelengths than the primary electron donor P700. We investigated the relative quantum yields of photochemical charge separation and fluorescence as a function of excitation wavelength and temperature in trimeric and monomeric photosystem I complexes of this cyanobacterium. The monomeric complexes are characterized by a reduced content of the C-719 spectral form. At room temperature, an analysis of the wavelength dependence of P700 oxidation indicated that all absorbed light, even of wavelengths of up to 750 nm, has the same probability of resulting in a stable P700 photooxidation. Upon cooling from 295 K to 5 K, the nonselectively excited steady-state emission increased by 11- and 16-fold in the trimeric and monomeric complexes, respectively, whereas the quantum yield of P700 oxidation decreased 2.2- and 1.7-fold. Fluorescence excitation spectra at 5 K indicate that the fluorescence quantum yield further increases upon scanning of the excitation wavelength from 690 nm to 710 nm, whereas the quantum yield of P700 oxidation decreases significantly upon excitation at wavelengths longer than 700 nm. Based on these findings, we conclude that at 5 K the excited state is not equilibrated over the antenna before charge separation occurs, and that approximately 50% of the excitations reach P700 before they become irreversibly trapped on one of the long-wavelength antenna pigments. Possible spatial organizations of the long-wavelength antenna pigments in the three-dimensional structure of photosystem I are discussed.     

The long-wavelength chlorophyll states of plant LHCI at room temperature: a comparison with PSI-LHCI

The red antenna states of the external antenna complexes of higher plant photosystem I, known as LHCI, have been analyzed by measurement of their preequilibrium fluorescence upon direct excitation at 280 K. In addition to the previously detected F735 state, a hitherto undetected low-energy state with emission maximum around 713 nm was observed. The 280 K bandwidths (FWHM) are 55 nm for the F735 state and approximately 27 nm for the F713-nm state, much greater than for non-red-shifted antenna chlorophylls. The origin absorption band for the F735-nm state was directly detected by determination of its excitation (action) spectrum and lies at 709-710 nm. The absorption spectrum for F735, calculated using the Stepanov expression, closely overlaps the excitation spectrum, indicating that the very large Stokes shift (25 nm) is due to vibrational relaxation within the excited-state manifold and solvent effects can be excluded. Fluorescence anisotropy measurements, with direct excitation of F735, indicate that the transition dipoles of the two red states are parallel. Similar experiments performed in the long-wavelength absorbing tail of PSI-LHCI indicate the presence of emission state(s) that are red-shifted with respect to F735 of isolated LHCI. It is suggested that these are brought about by interactions between the complexes in PSI-LHCI, which occur in some yet undefined way, and which are broken upon solubilization of the component parts.     

Electric field effects on red chlorophylls, β-carotenes and P700 in cyanobacterial Photosystem I complexes

We have probed the absorption changes due to an externally applied electric field (Stark effect) of Photosystem I (PSI) core complexes from the cyanobacteria Synechocystis sp. PCC 6803, Synechococcus elongatus and Spirulina platensis. The results reveal that the so-called C719 chlorophylls in S. elongatus and S. platensis are characterized by very large polarizability differences between the ground and electronically excited states (with Tr(Δα) values up to about 1000 ų f⁻²) and by moderately high change in permanent dipole moments (with average Δμ values between 2 and 3 D f⁻¹). The C740 chlorophylls in S. platensis and, in particular, the C708 chlorophylls in all three species give rise to smaller Stark shifts, which are, however, still significantly larger than those found before for monomeric chlorophyll. The results confirm the hypothesis that these states originate from strongly coupled chlorophyll a molecules. The absorption and Stark spectra of the β-carotene molecules are almost identical in all complexes and suggest similar or slightly higher values for Tr(Δα) and Δμ than for those of β-carotene in solution. Oxidation of P700 did not significantly change the Stark response of the carotenes and the red antenna states C719 and C740, but revealed in all PSI complexes changes around 700-705 and 690-693 nm, which we attribute to the change in permanent dipole moments of reduced P700 and the chlorophylls responsible for the strong absorption band at 690 nm with oxidized P700, respectively.     

Fluorescence lifetimes of dimers and higher oligomers of bacteriochlorophyll c from Chlorobium limicola

Fluorescence lifetimes have been measured for bacteriochlorophyll (BChl) c isolated from Chlorobium limicola in different states of aggregation in non-polar solvents. Two different homologs of BChl c were used, one with an isobutyl group at the 4 position, the other with n-propyl. Species previously identified as dimers (Olson and Pedersen 1990, Photosynth Res, this issue) decayed with lifetimes of 0.64 ns for the isobutyl homolog, 0.71 ns for n-propyl. Decay-associated spectra indicate that the absorption spectrum of the isobutyl dimer is slightly red-shifted from that of the n-propyl dimer. Aggregates absorbing maximally at 710 nm fluoresced with a principal lifetime of 3.1 ns, independent of the homolog used. In CCl₄, only the isobutyl homolog forms a 747-nm absorbing oligomer spectrally similar to BChl c in vivo. This oligomer shows non-exponential fluorescence decay with lifetimes of 67 and 19 ps. Because the two components show different excitation spectra, the higher oligomer is probably a mixture of more than one species, both of which absorb at 747 nm.Abbreviations BChl bacteriochlorophyll - Chl chlorophyll - % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGGipm0dc9vqaqpepu0xbbG8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeq4Xdm2aaW% baaSqabeaacaaIYaaaaaaa!3777!\[\chi ^2 \] chi-square - FWHM full-width at half-maximum     

雨生红球藻(Haematococcus pluvialis)的光谱特性研究

实验测定了雨生红球藻不同生长阶段的色素组成,吸收光谱,荧光光谱,并对其进行了分析。结果表明,用490nm波长激发时,雨生红球藻在绿色细胞阶段存在710nm和730nm附近的长波长荧光发射峰,而在红色细胞阶段仅存在730nm的长波长荧光发射峰,预示着雨生红球藻不同生长阶段在PSⅠ结构,组成,及其色素蛋白的排布等方面有很大差异。     

Structurally flexible macro-organization of the pigment–protein complexes of the diatom Phaeodactylum tricornutum

By means of circular dichroism (CD) spectroscopy, we have characterized the organization of the photosynthetic complexes of the diatom Phaeodactylum tricornutum at different levels of structural complexity: in intact cells, isolated thylakoid membranes and purified fucoxanthin chlorophyll protein (FCP) complexes. We found that the CD spectrum of whole cells was dominated by a large band at (+)698 nm, accompanied by a long tail from differential scattering, features typical for psi-type (polymerization or salt-induced) CD. The CD spectrum additionally contained intense (−)679 nm, (+)445 nm and (−)470 nm bands, which were also present in isolated thylakoid membranes and FCPs. While the latter two bands were evidently produced by excitonic interactions, the nature of the (−)679 nm band remained unclear. Electrochromic absorbance changes also revealed the existence of a CD-silent long-wavelength (∼545 nm) absorbing fucoxanthin molecule with very high sensitivity to the transmembrane electrical field. In intact cells the main CD band at (+)698 nm appeared to be associated with the multilamellar organization of the thylakoid membranes. It was sensitive to the osmotic pressure and was selectively diminished at elevated temperatures and was capable of undergoing light-induced reversible changes. In isolated thylakoid membranes, the psi-type CD band, which was lost during the isolation procedure, could be partially restored by addition of Mg-ions, along with the maximum quantum yield and the non-photochemical quenching of singlet excited chlorophyll a, measured by fluorescence transients.     

The photochemical trapping rate from red spectral states in PSI–LHCI is determined by thermal activation of energy transfer to bulk chlorophylls

The average fluorescence decay lifetimes, due to reaction centre photochemical trapping, were calculated for wavelengths in the 690- to 770-nm interval from the published fluorescence decay-associated emission spectra for Photosystem I (PSI)–light-harvesting complex of Photosystem I (LHCI) [Biochemistry 39 (2000) 6341] at 280 and 170 K. For 280 K, the overall trapping time at 690 nm is 81 ps and increases with wavelength to reach 103 ps at 770 nm. For 170 K, the 690-nm value is 115 ps, increasing to 458 ps at 770 nm. This underlines the presence of kinetically limiting processes in the PSI antenna (diffusion limited). The explanation of these nonconstant values for the overall trapping time band is sought in terms of thermally activated transfer from the red absorbing states to the “bulk” acceptor chlorophyll (chl) states in the framework of the Arrhenius–Eyring theory. It is shown that the wavelength-dependent “activation energies” come out in the range between 1.35 and 2.7 kcal mol⁻¹, increasing with the emission wavelength within the interval 710–770 nm. These values are in good agreement with the Arrhenius activation energy determined for the steady-state fluorescence yield over the range 130–280 K for PSI–LHCI. We conclude that the variable trapping time in PSI–LHCI can be accounted for entirely by thermally activated transfer from the low-energy chl states to the bulk acceptor states and therefore that the position of the various red states in the PSI antenna seems not to be of significant importance. The analysis shows that the bulk antenna acceptor states are on the low-energy side of the bulk antenna absorption band.