去镁叶绿素置换细菌去镁叶绿素对紫细菌反应中心皮秒荧光的影响

选择597 nm作为激发波长,探测范围为600～900 nm的荧光特性,分析了天然反应中心和两种去镁叶绿素置换的紫细菌反应中心的荧光发射光谱.借助细菌叶绿素、细菌去镁叶绿素和植物去镁叶绿素的荧光光谱,对相关组分进行了归类.实验结果表明选择性地置换细菌去镁叶绿素影响了荧光光谱的组成.在天然反应中心、BpheB置换的反应中心和BpheA,B置换的反应中心中可分别解析到4、3和2个荧光发射组分.研究肯定荧光发射组分与去镁叶绿素的结合存在对应关系.实验还分别在686.4、674.1和681.1 nm处测定了不同反应中心内的原初电子供体P的激发态通过荧光衰减的过程,观测到衰减动力学上的差异.说明去镁叶绿素置换影响了细菌反应中心内激发光能传递和原初光化学反应过程.     

Determination of the excitation migration time in Photosystem II consequences for the membrane organization and charge separation parameters

The fluorescence decay kinetics of Photosystem II (PSII) membranes from spinach with open reaction centers (RCs), were compared after exciting at 420 and 484 nm. These wavelengths lead to preferential excitation of chlorophyll (Chl) a and Chl b, respectively, which causes different initial excited-state populations in the inner and outer antenna system. The non-exponential fluorescence decay appears to be 4.3+/-1.8 ps slower upon 484 nm excitation for preparations that contain on average 2.45 LHCII (light-harvesting complex II) trimers per reaction center. Using a recently introduced coarse-grained model it can be concluded that the average migration time of an electronic excitation towards the RC contributes approximately 23% to the overall average trapping time. The migration time appears to be approximately two times faster than expected based on previous ultrafast transient absorption and fluorescence measurements. It is concluded that excitation energy transfer in PSII follows specific energy transfer pathways that require an optimized organization of the antenna complexes with respect to each other. Within the context of the coarse-grained model it can be calculated that the rate of primary charge separation of the RC is (5.5+/-0.4 ps)(-1), the rate of secondary charge separation is (137+/-5 ps)(-1) and the drop in free energy upon primary charge separation is 826+/-30 cm(-1). These parameters are in rather good agreement with recently published results on isolated core complexes [Y. Miloslavina, M. Szczepaniak, M.G. Muller, J. Sander, M. Nowaczyk, M. R?gner, A.R. Holzwarth, Charge separation kinetics in intact Photosystem II core particles is trap-limited. A picosecond fluorescence study, Biochemistry 45 (2006) 2436-2442].     

Chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence transient during freezing and recovery in winter wheat

Chlorophyll (Chl) a fluorescence measurements as evaluators of plant freezing tolerance are frequently insufficiently sensitive to detect the early metabolic changes that are initiated following exposure to freezing temperatures. Using cold-acclimated winter wheat, I analysed the polyphasic transience (from 50 μs to 1 s) of Chl a fluorescence. This enabled detailed studies of the progressive energy flows and efficiencies within the photosystem 2 (PS2) complex that ensue following initial exposure to freezing temperatures right through to the plant recovery stage. The initial consequences of mild frosts that may cause primary damage involve a disturbance to the energy transfer subsequent to Q_A (the primary quinone electron acceptor of PS2). Lower freezing temperatures, on the other hand, may deter energy flow between the PS2 reaction centre (RC), Chl, and Q_A. All primary damage could only be repaired partially. Further freezing-triggered dysfunction of the electron transfer between the PS2 RCs and Q_A was connected with secondary damage that could lead to PS2 deactivation. Both primary and secondary freezing damages were reflected in decreased PI_ABS, the Performance Index based on equal absorption that characterizes all energy bifurcations in PS2. PI_ABS also differentiated cultivars with contrasting freezing-tolerance either subsequent to the onset of freezing or during the recovery stage. In contrast, the potential quantum yield of PS2 (F_v/F_m), which characterizes efficiency of energy trapping in the PS2 RCs, was only different in cultivars with contrasting freezing-tolerance during the recovery stage.     

Preferential binding of equine ferricytochrome c to the bacterial photosynthetic reaction center from Rhodobacter sphaeroides

Redox titration of horse heart cytochrome c (cyt c), in the presence of varying concentrations of detergent-solubilized photosynthetic reaction center (RC) from Rhodobacter sphaeroides, revealed an RC concentration-dependent decrease in the measured cyt c midpoint potential that is indicative of a 3.6 +/- 0.2-fold stronger binding affinity of oxidized cytochrome to a single binding site. This effect was correlated with preferential binding in the functional complex by redox titration of the fraction of RCs exhibiting microsecond, first-order, special pair reduction by cytochrome. A binding affinity ratio of 3.1 +/- 0.4 was determined by this second technique, confirming the result. Redox titration of flash-induced intracomplex electron transfer also showed the association in the electron transfer-active complex to be strong, with a dissociation constant of 0.17 +/- 0.03 microM. The tight binding is associated with a slow off-rate which, in the case of the oxidized form, can influence the kinetics of P(+) reduction. The pitfalls of the common use of xenon flashlamps to photoexcite fast electron-transfer reactions are discussed with relation to the first electron transfer from primary to secondary RC quinone acceptors. The results shed some light on the diversity of kinetic behavior reported for the cytochrome to RC electron-transfer reaction.     

The polarization model in bacterial photosynthesis

The widely accepted model of reaction center /RC/ functioning is proved to come into contradiction with some recent data. In particular, it cannot explain why only a minor part of electronic excitations (approximately 10%) escapes from excited RC special pairs back to antenna BChls. Therefore we believe that the model must be substantially modernized. In 1981 we developed a new model/1,2/. We suggested a femtosecond state to precede primary e-transfer reaction due to reorientation of water molecule dipole in the electric field of excited RC dimer. This mechanism is responsible for energy trapping before the primary e-transport occurs. During last years his mechanism got support from various experimental works. Now this polarization model claims to fit all reliable experimental data at least in bacterial photosynthesis.     

N-苯基-2-萘胺对拟柱胞藻生长、抗氧化酶及光合系统Ⅱ的影响

为探究化感物质对入侵水华蓝藻-拟柱胞藻(Cylindrospermopsis raciborskii)的影响, 研究对不同浓度(0、0.05、0.10、0.50、1.00和2.00 mg/L)N-苯基-2-萘胺处理下拟柱胞藻的生长、抗氧化酶活性及叶绿素荧光诱导动力学进行了测定。结果表明, 藻叶绿素a含量随N-苯基-2-萘胺浓度升高而显著降低, 浓度大于0.50 mg/L时, 拟柱胞藻叶绿素a含量显著低于对照。72h, N-苯基-2-萘胺对拟柱胞藻的EC50为1.02 mg/L。叶绿素荧光诱导动力学参数表明, 浓度小于0.50 mg/L时, 拟柱胞藻的最大光化学效率(P0)、单位面积上有活性的反应中心(RC/CS0)、以吸收光能为基础的性能指数(PIABS)及以单位面积为基础的性能指数(PICS)均显著高于对照, 而反映2ms时有活性的反应中心的开放程度的0则显著低于对照组。当N-苯基-2-萘胺浓度大于0.50 mg/L时, 以上参数均呈现相反趋势。这表明低浓度N-苯基-2-萘胺能提高拟柱胞藻的光合效率, 而在高浓度时, 拟柱胞藻PSⅡ有活性的反应中心减少, 电子传递受到阻碍, 光合效率下降。与对照相比, 在浓度高于0.50 mg/L条件下, 拟柱胞藻的过氧化氢酶(CAT)、过氧化物酶(SOD)及谷胱甘肽过氧化物酶(GPX)的活性显著增加, 表明拟柱胞藻能产生大量的抗氧化酶来减少N-苯基-2-萘胺的损伤。这些结果表明N-苯基-2-萘胺可能通过抑制PSⅡ的电子传递和减少其有活性的反应中心来影响拟柱胞藻的生长, 也暗示了N-苯基-2-萘胺能作为潜在的抑藻物质来控制拟柱胞藻水华。       

Photosystem I complexes associated with fucoxanthin-chlorophyll-binding proteins from a marine centric diatom, Chaetoceros gracilis

Diatoms occupy a key position as a primary producer in the global aquatic ecosystem. We developed methods to isolate highly intact thylakoid membranes and the photosystem I (PS I) complex from a marine centric diatom, Chaetoceros gracilis. The PS I reaction center (RC) was purified as a super complex with light-harvesting fucoxanthin-chlorophyll (Chl)-binding proteins (FCP). The super complex contained 224 Chl a, 22 Chl c, and 55 fucoxanthin molecules per RC. The apparent molecular mass of the purified FCP-PS I super complex (approximately 1000 kDa) indicated that the super complex was composed of a monomer of the PS I RC complex and about 25 copies of FCP. The complex contained menaquinone-4 as the secondary electron acceptor A1 instead of phylloquinone. Time-resolved fluorescence emission spectra at 77 K indicated that fast (16 ps) energy transfer from a Chl a band at 685 nm on FCP to Chls on the PS I RC complex occurs. The ratio of fucoxanthin to Chl a on the PS I-bound FCP was lower than that of weakly bound FCP, suggesting that PS I-bound FCP specifically functions as the mediator of energy transfer between weakly bound FCPs and the PS I RC.     

The interaction of quinone and detergent with reaction centers of purple bacteria. I. Slow quinone exchange between reaction center micelles and pure detergent micelles.

The kinetics of light-induced electron transfer in reaction centers (RCs) from the purple photosynthetic bacterium Rhodobacter sphaeroides were studied in the presence of the detergent lauryldimethylamine-N-oxide (LDAO). After the light-induced electron transfer from the primary donor (P) to the acceptor quinone complex, the dark re-reduction of P+ reflects recombination from the reduced acceptor quinones, QA- or QB-. The secondary quinone, QB, which is loosely bound to the RC, determines the rate of this process. Electron transfer to QB slows down the return of the electron to P+, giving rise to a slow phase of the recovery kinetics with time tau P approximately 1 s, whereas charge recombination in RCs lacking QB generates a fast phase with time tau AP approximately 0.1 s. The amount of quinone bound to RC micelles can be reduced by increasing the detergent concentration. The characteristic time of the slow component of P+ dark relaxation, observed at low quinone content per RC micelle (at high detergent concentration), is about 1.2-1.5 s, in sharp contrast to expectations from previous models, according to which the time of the slow component should approach the time of the fast component (about 0.1 s) when the quinone concentration approaches zero. To account for this large discrepancy, a new quantitative approach has been developed to analyze the kinetics of electron transfer in isolated RCs with the following key features: 1) The exchange of quinone between different micelles (RC and detergent micelles) occurs more slowly than electron transfer from QB- to P+; 2) The exchange of quinone between the detergent "phase" and the QB binding site within the same RC micelle is much faster than electron transfer between QA- and P+; 3) The time of the slow component of P+ dark relaxation is determined by (n) > or = 1, the average number of quinones in RC micelles, calculated only for those RC micelles that have at least one quinone per RC (in excess of QA). An analytical function is derived that relates the time of the slow component of P+ relaxation, tau P, and the relative amplitude of the slow phase. This provides a useful means of determining the true equilibrium constant of electron transfer between QA and QB (LAB), and the association equilibrium constant of quinone binding at the QB site (KQ+). We found that LAB = 22 +/- 3 and KQ = 0.6 +/- 0.2 at pH 7.5. The analysis shows that saturation of the QB binding site in detergent-solubilized RCs is difficult to achieve with hydrophobic quinones. This has important implications for the interpretation of apparent dependencies of QB function on environmental parameters (e.g. pH) and on mutational alterations. The model accounts for the effects of detergent and quinone concentration on electron transfer in the acceptor quinone complex, and the conclusions are of general significance for the study of quinone-binding membrane proteins in detergent solutions.     

Nonlinear effect of dynamic self-organization in macromolecular systems caused by photocontrolled electron flux

We use the electron-conformational interaction approach to develop a physical model which self-consistently describes the photomobilized electron transfer kinetics and structure conformational transitions in reaction centers (RCs) of purple bacteria. We consider the kinetics of electron transition from pigment onto primary acceptor and the subsequent charge recombination accounting for the change of distance between the above-mentioned cofactors. It is shown that, given natural values of RC parameters, the kinetic constant's dependence on the acting light intensity is monotone. As opposed to the previous case, similar dependencies for the chain of electron transfer between primary and secondary quinone acceptors revealed anS-like relationship. This can lead to bistability of the RC optical transmission coefficient and a fundamental dependence of charge recombination kinetics upon the prehistory of the RC's interaction with exciting radiation.     

Primary light-energy conversion in tetrameric chlorophyll structure of photosystem II and bacterial reaction centers: II. Femto- and picosecond charge separation in PSII D1/D2/Cyt b559 complex

In Part I of the article, a review of recent data on electron-transfer reactions in photosystem II (PSII) and bacterial reaction center (RC) has been presented. In Part II, transient absorption difference spectroscopy with 20-fs resolution was applied to study the primary charge separation in PSII RC (DI/DII/Cyt b 559 complex) excited at 700 nm at 278 K. It was shown that the initial electron-transfer reaction occurs within 0.9 ps with the formation of the charge-separated state P680(+)Chl(D1)(-), which relaxed within 14 ps as indicated by reversible bleaching of 670-nm band that was tentatively assigned to the Chl(D1) absorption. The subsequent electron transfer from Chl(D1)(-) within 14 ps was accompanied by a development of the radical anion band of Pheo(D1) at 445 nm, attributable to the formation of the secondary radical pair P680(+)Pheo(D1)(-). The key point of this model is that the most blue Q(y) transition of Chl(D1) in RC is allowing an effective stabilization of separated charges. Although an alternative mechanism of charge separation with Chl(D1)* as a primary electron donor and Pheo(D1) as a primary acceptor can not be ruled out, it is less consistent with the kinetics and spectra of absorbance changes induced in the PSII RC preparation by femtosecond excitation at 700 nm.     

Effect of deuteration and cryosolvents on the energy transduction in primary processes of photosynthesis.

Effects of cryosolvents and D2O/H2O substitution on the reaction centres (RCs) isolated from photosynthetic bacteria were studied with respect to the role of intra-protein hydrogen bonds in the primary photosynthetic electron transfer. As a result of such treatment of RCs, the charge separation rate between the photoactive bacteriochlorophyll (P2 dimer) and bacteriopheophytin and the rate of electron transfer to the primary quinone slowed down. The energy migration rate from bacteriopheophytin (BPheM), inactive in electron transport, to P2 decreased as well. Although cryosolvents can shift the redox potential of the photoactive pigment, there is no direct correlation between the P2 potential and the effects of these modifying agents on the photosynthetic process in RCs occurring with participation of P2. The removal of H subunit from the pigment-protein complex results in the pronounced weakening of the dimethyl sulfoxide modifying effects on the RC hydrogen bonds. The role of structural and dynamic state in the functioning of the photosynthetic bacterial RCs is analyzed. Relaxation processes in purple bacteria RCs accompanying the primary picosecond steps of energy transformation proceed with the participation of small proton-containing molecular groups in the immediate surroundings of electron transfer carriers. In this paper, we present results concerning mechanisms of primary photosynthetic steps, which were initiated by A. A. Krasnovsky and have been studied for several years at the Department of Biophysics. This paper is dedicated to the memory of our teacher Prof. A. A. Krasnovsky.     

Substitution of Isoleucine L177 by Histidine Affects the Pigment Composition and Properties of the Reaction Center of the Purple Bacterium <Emphasis Type="Italic">Rhodobacter sphaeroides</Emphasis>

Using site-directed mutagenesis, we obtained the mutant of the purple bacterium Rhodobacter sphaeroides with Ile to His substitution at position 177 in the L-subunit of the photosynthetic reaction center (RC). The mutant strain forms stable and photochemically active RC complexes. Relative to the wild type RCs, the spectral and photochemical properties of the mutant RC differ significantly in the absorption regions corresponding to the primary donor P and the monomer bacteriochlorophyll (BChl) absorption. It is shown that the RC I(L177)H contains only three BChl molecules compared to four BChl molecules in the wild type RC. Considering the fact that the properties of both isolated and membrane-associated mutant RCs are similar, we conclude that the loss of a BChl molecule from the mutant RC is caused by the introduced mutation but not by the protein purification procedure. The new mutant missing one BChl molecule but still able to perform light-induced reactions forming the charge-separated state P+QA- appears to be an interesting object to study the mechanisms of the first steps of the primary electron transfer in photosynthesis.     

PMS: Photosystem I electron donor or fluorescence quencher

Light energy harvested by the pigments in Photosystem I (PSI) is used for charge separation in the reaction center (RC), after which the positive charge resides on a special chlorophyll dimer called P700. In studies on the PSI trapping kinetics, P700(+) is usually chemically reduced to re-open the RCs. So far, the information available about the reduction rate and possible chlorophyll fluorescence quenching effects of these reducing agents is limited. This information is indispensible to estimate the fraction of open RCs under known experimental conditions. Moreover, it would be important to understand if these reagents have a chlorophyll fluorescence quenching effects to avoid the introduction of exogenous singlet excitation quenching in the measurements. In this study, we investigated the effect of the commonly used reducing agent phenazine methosulfate (PMS) on the RC and fluorescence emission of higher plant PSI-LHCI. We measured the P700(+) reduction rate for different PMS concentrations, and show that we can give a reliable estimation on the fraction of closed RCs based on these rates. The data show that PMS is quenching chlorophyll fluorescence emission. Finally, we determined that the fluorescence quantum yield of PSI with closed RCs is 4% higher than if the RCs are open.     

Determination of the excitation migration time in Photosystem II: Consequences for the membrane organization and charge separation parameters

The fluorescence decay kinetics of Photosystem II (PSII) membranes from spinach with open reaction centers (RCs), were compared after exciting at 420 and 484 nm. These wavelengths lead to preferential excitation of chlorophyll (Chl) a and Chl b, respectively, which causes different initial excited-state populations in the inner and outer antenna system. The non-exponential fluorescence decay appears to be 4.3 ± 1.8 ps slower upon 484 nm excitation for preparations that contain on average 2.45 LHCII (light-harvesting complex II) trimers per reaction center. Using a recently introduced coarse-grained model it can be concluded that the average migration time of an electronic excitation towards the RC contributes ~ 23% to the overall average trapping time. The migration time appears to be approximately two times faster than expected based on previous ultrafast transient absorption and fluorescence measurements. It is concluded that excitation energy transfer in PSII follows specific energy transfer pathways that require an optimized organization of the antenna complexes with respect to each other. Within the context of the coarse-grained model it can be calculated that the rate of primary charge separation of the RC is (5.5 ± 0.4 ps)^− 1, the rate of secondary charge separation is (137 ± 5 ps)^− 1 and the drop in free energy upon primary charge separation is 826 ± 30 cm^− 1. These parameters are in rather good agreement with recently published results on isolated core complexes [Y. Miloslavina, M. Szczepaniak, M.G. Muller, J. Sander, M. Nowaczyk, M. Rögner, A.R. Holzwarth, Charge separation kinetics in intact Photosystem II core particles is trap-limited. A picosecond fluorescence study, Biochemistry 45 (2006) 2436-2442].     

Pigment exchange of Photosystem II reaction center by chlorophyll d

Pigment exchanges among photosystem reaction centers (RCs) are useful for the identification and functional analysis of chromophores in photosynthetic organisms. Pigment replacement within the spinach Photosystem II RC was performed with Chl d derived from the oxygenic alga Acaryochloris marina, using a protocol similar to that reported previously [Gall et al. (1998) FEBS Lett 434: 88–92] based on the incubation of reaction centers with an excess of other pigments. In this study, we analyzed Chl d-modified monomeric RC which was separated from Chl d-modified dimeric RC by size-exclusion chromatography. Based on the assumption of a constant ratio of two Pheo a molecules per RC, the number of Chl a molecules in Chl d-modified monomeric RCs was found to decrease from six to four. The absorption spectrum of the Chl d-modified monomeric RC at room temperature showed a large peak at 699.5 nm originating from Chl d and a small peak at 672.5 nm orignating from Chl a. Photoaccumulation of the Pheo a⁻ in Chl d-modified monomeric RC, in the presence of sodium dithionate and methyl viologen, did not differ significantly from that in control RC, showing that the Chl d-modified monomeric RC retains its charge separation activity and photochemically active Pheo a.     

Structural basis of bacterial photosynthetic reaction centers

The photosynthetic reaction center (RC) is the first membrane protein whose three-dimensional structure was revealed at the atomic level by X-ray crystallograph more than fifteen years ago. Structural information about RC made a great contribution to the understanding of the reaction mechanism of the complicated membrane protein complex. High-resolution structures of RCs from three photosynthetic bacteria are now available, namely, those from two mesophilic purple non-sulfur bacteria, Blastochloris viridis and Rhodobacter sphaeroides, and that from a thermophilic purple sulfur bacterium, Thermochromatium tepidum. In addition, a variety of structural studies, mainly by X-ray crystallography, are still being performed to give more detailed insight into the reaction mechanism of this membrane protein. This review deals with structural studies of bacterial RC complexes, and a discussion about the electron transfer reaction between RCs and electron donors is the main focus out of several topics addressed by these structural studies. The structural data from three RCs and their electron donors provided reliable models for molecular recognition in the primary step of bacterial photosynthesis.     

NaCl胁迫下黄连木叶片光合特性及快速叶绿素荧光诱导动力学曲线的变化

以1年生黄连木为试材,设置NaCl浓度分别为0(CK)、0.15%、0.3%、0.45%、0.6%5个处理,利用快速叶绿素荧光诱导动力学曲线分析技术(JIP-test),研究了NaCl胁迫对黄连木叶片光合特性和快速叶绿素荧光诱导动力学参数的影响.结果表明: 随着NaCl浓度的升高, 黄连木叶片中的叶绿素a、叶绿素b和总叶绿素含量逐渐降低,叶绿素a/b比值先升高后下降,类胡萝卜素含量逐渐增加; 叶片的净光合速率(P_n)、气孔导度(g_s)逐渐降低,其中NaCl浓度＜0.3%时,叶片P_n下降主要受气孔限制;当NaCl浓度＞0.3%时, P_n下降主要由非气孔因素限制;单位面积捕获的光能(TR_o/CS_o)、电子传递的量子产额(ET_o/CS_o)、单位面积的反应中心数量(RC/CS_o)、量子产额或能量分配比率(ψ_o和φ_Eo)逐渐降低,而单位面积吸收的光能(ABS/CS_o)、荧光诱导曲线中K点(W_k)和J点(V_j)明显增加,说明盐胁迫对黄连木叶片放氧复合体(OEC)、受体侧和PSⅡ反应中心造成了伤害.当NaCl浓度为0.3%时,PSⅡ最大光化学效率(F_v/F_m)和光化学性能指数(PI_ABS)分别比对照下降 17.7%和36.6%.     

An overview on chlorophylls and quinones in the photosystem I-type reaction centers

Minor but key chlorophylls (Chls) and quinones in photosystem (PS) I-type reaction centers (RCs) are overviewed in regard to their molecular structures. In the PS I-type RCs, the prime-type chlorophylls, namely, bacteriochlorophyll (BChl) a′ in green sulfur bacteria, BChl g′ in heliobacteria, Chl a′ in Chl a-type PS I, and Chl d′ in Chl d-type PS I, function as the special pairs, either as homodimers, (BChl a′)₂ and (BChl g′)₂ in anoxygenic organisms, or heterodimers, Chl a/a′ and Chl d/d′ in oxygenic photosynthesis. Conversions of BChl g to Chl a and Chl a to Chl d take place spontaneously under mild condition in vitro. The primary electron acceptors, A ₀, are Chl a-derivatives even in anoxygenic PS I-type RCs. The secondary electron acceptors are naphthoquinones, whereas the side chains may have been modified after the birth of cyanobacteria, leading to succession from menaquinone to phylloquinone in oxygenic PS I.     

Spectral resolution of low-energy chlorophylls in Photosystem I of Synechocystis sp. PCC 6803 through direct excitation

We have measured fluorescence spectra from Photosystem I (PS I) on a PS II-less mutant of the cyanobacterium Synechocystis sp. PCC 6803 at room temperature as a function of excitation wavelength. Our data show a gradual enhancement of long-wavelength fluorescence at 710 nm as the excitation wavelength is increased from 695 to 720 nm. This verifies the presence of low-energy chlorophylls (LE Chls), antenna Chls with energy levels below that of the primary electron donor, P700. The change in fluorescence with excitation wavelength is attributed to the finite time it takes for equilibration of excitations between the bulk and LE Chls. The spectra were deconvoluted into the sum of two basis spectra, one an estimate for fluorescence from the majority or bulk Chls and the other, the LE Chls. The bulk Chl spectrum has a major peak at 688 nm and a lower amplitude vibrational band around 745 nm and is assumed independent of excitation wavelength. The LE Chl spectrum has a major peak at 710 nm, with shoulders at 725 and 760 nm. The relative amplitude of emission at the vibrational side bands increases slightly as the excitation wavelength increases. The ratio of the fluorescence yields from LE Chls to that from bulk Chls ranges from 0.3 to 1.3 for excitation wavelengths of 695 to 720 nm, respectively. These values are consistent with a model where the LE Chls are structurally close to P700 allowing for direct transfer of excitations from both the bulk and LE Chls to P700.