EXCITATION ENERGY MIGRATION IN PHYCOBILISOMES: COMPARISON OF EXPERIMENTAL RESULTS AND THEORETICAL PREDICTIONS

Abstract— Quantum yield and fluorescence polarization determinations on phycobilisomes and their constituent phycobiliproteins show that phycobilisomes are energetically effective macromolecular structures. Energy migration within the phycobilisome to allophycocyanin, the longest wavelength absorbing and emitting phycobiliprotein, was indicated by the predominant allophycocyanin fluorescence emission which was independent of the phycobiliprotein being excited. The high efficiency of the energy migration inside the phycobilisome was reflected by the low polarized fluorescence. Excitation of phycobilisomes in the region of major absorption (500–650 nm) resulted in degrees of fluorescence polarization between +0.02 and –0.02, whereas in isolated phycobiliproteins the values were 2 to 12 times greater. Furthermore, 94–98° of the excitation energy of phycoerythrin was transferred to phycocyanin and allophycocyanin as determined from comparisons of fluorescence spectra of intact and dissociated phycobilisomes. The fluorescence quantum yields of phycobilisomes were about 0.60–0.68, very similar to that of pure allophycocyanin in solution (0.68). Phycobilisomes isolated from Fremyella diplosiphon and Nostoc sp. (blue-gree algae) have respective quantum yields of 0.68 and 0. 65, and those isolated from Porphyridium cruentum (red alga), about 0.60. In Fremyella diplosiphon and Nostoc sp., which showed a striking adaptation to different wavelengths, the phycobilisome quantum yields only varied from 0.68 to 0.67 and from 0.65 to 0. 60, respectively. The mean transfer time, calculated on the basis of experimental results, was about 280 ± 40 ps for transfer of excitation from the phycoerythrin to the phycocyanin layer in phycobilisomes. This time corresponds to the mean number of jumps, about 28, of the excitation in the phycoerythrin layer before it is captured by phycocyanin. These values are in reasonable agreement with the values of 250 ± 30 ps and 25 jumps, calculated on the basis of a phycobilisome model (of Porphyridium cruentum) and Pearlstein's theory of energy migration devised for a three-dimensional photosynthetic unit. It was also shown that Paillotin's theory of energy migration predicts similar values for mean transfer time and mean number of jumps, if one assumes that phycocyanin is a perfect sink for phycoerythrin excitation.     

EXCITATION ENERGY TRANSFER IN THE LIGHT HARVESTING ANTENNA SYSTEM OF THE RED ALGA Porphyridium cruentum AND THE BLUE-GREEN ALGA Anacystis nidulans: ANALYSIS OF TIME-RESOLVED FLUORESCENCE SPECTRA

Abstract— Time-resolved fluorescence spectra of intact cells of red and blue-green algae Porphyridium cruentum and Anacystis nidulans were measured by means of a ps laser and a time-correlated photon counting system. Fluorescence spectra were observed successively from various pigments in the light harvesting system in the order of phycoerythrin (PE), phycocyanin (PC), allophycocyanin (APC) and chlorophyll a (Chl a ). The spectrum changes with time in the range of0–400 ps in P. cruentum and of0–1000 ps in A. nidulans . The time-resolved spectra were analyzed into components to obtain the rise and decay curve of each fluorescence component. Overall time behaviors of the sequential fluorescence emissions from various pigments can be interpreted with a decay kinetics ofexp(–2 kt ^½). The rate constants of the energy transfer show that the energy transfer takes place much faster in the red alga P. cruentum than in the blue-green alga A. nidulans , particularly in the step PCAPC. Results also indicated that a special form of APC, far-emitting APC, exists in the pigment system of A. nidulans , but it does not mediate a main energy transfer from phycobilisome to Chl a.     

EXCITATION ENERGY TRANSFER IN THE CHROMATICALLY ADAPTED PHYCOBILIN SYSTEMS OF BLUE-GREEN ALGAE: DIFFERENCE IN THE ENERGY TRANSFER KINETICS AT PHYCOCYANIN LEVEL

Abstract— Excitation energy transfer in chromatically adapted phycobilin system was investigated with the blue-green algae Tolypothrix tenuis and, supplementary, Fremyella diplosiphon with use of time-resolved fluorescence spectrum (Yamazaki et al , 1984). Special attention was paid to the energy migration at the phycocyanin (PC) level in the phycoerythrin (PE)-rich and PE excited system and in the PE-less and PC excited system. The energy transfer from PC to allophycocyanin was far faster in the former than in the latter in both organisms. Such feature was the same as our previous observation for PE-rich system of Porphyridium cruentum and PE-less system of Anacystis nidulans (Yamazaki et al , 1984). Thus, the difference in phycobilisome structure is not a cause for such difference. Based on simulation analysis, we interpreted our observation as that (1) all PC chtomophores do not equally participate to the energy migration within PC compartment but (2) a short transfer path through PC compartment is formed probably by f-type chromophores and (3) the difference in the "length" of this path is a main determinant for kinetic difference between PE-rich and PE-less systems.     

Effects of PAR and UV Radiation on the Structural and Functional Integrity of Phycocyanin,Phycoerythrin and Allophycocyanin Isolated from the Marine Cyanobacterium Lyngbya sp. A09DM

An in vitro analysis of the effects of photosynthetically active and ultraviolet radiations was executed to assess the photostability of biologically relevant pigments phycocyanin (PC), phycoerythrin (PE) and allophycocyanin (APC) isolated from Lyngbya sp. A09DM. Ultraviolet (UV) irradiances significantly affected the integrity of PC, PE and APC; however, PAR showed least effect. UV radiation affected the bilin chromophores covalently attached to phycobiliproteins (PBPs). Almost complete elimination of the chromophore bands associated with α‐ and β‐subunit of PE and APC occurred after 4 h of UV‐B exposure. After 5 h of UV‐B exposure, the content of PC, PE and APC decreased by 51.65%, 96.8% and 96.53%, respectively. Contrary to PAR and UV‐A radiation, a severe decrease in fluorescence of all PBPs was observed under UV‐B irradiation. The fluorescence activity of extracted PBP was gradually inhibited immediately after 15–30 min of UV‐B exposure. In comparison to the PC, the fluorescence properties of PE and APC were severely lost under UV‐B radiation. Moreover, the present study indicates that UV‐B radiation can damage the structural and functional integrity of phycobiliproteins leading to the loss of their ecological and biological functions.     

FLUORESCENCE RELAXATION KINETICS AND QUANTUM YIELD FROM THE PHYCOBILISOMES OF THE BLUE-GREEN ALGA NOSTOC SP. MEASURED AS A FUNCTION OF SINGLE PICOSECOND PULSE INTENSITY*

Abstract— A detailed experimental study of the effect of intensity of a 6 ps excitation pulse on the decay kinetics and yield from phycobilisomes (PBsomes) is presented. The fluorescence from the c-phycoerythrin (PE) emission from PBsomes was found to decay as a single exponential with a time of 31 ± 4ps for an excitation intensity <10¹⁴ photons/cm² per pulse. The risetime of the c-phycocyanin (PC) and allophycocyanin (APC) emission from PBsomes was found to be 34 ± 13 ps. Therefore, at low excitation intensities, the energy transfer time between the constituent phycobiliproteins, PE and PC, is measured to be 34 ± 13ps from the fluorescence decay time of PE and the fluorescence risetime of the PC and APC emission. The fluorescence yield from the PE emission component in PBsomes was found to be intensity dependent for excitation intensities >10¹⁴ photons/cm². The decrease in yield with increased intensity in this case occurred at a higher intensity than in the isolated phycobiliprotein PE. The fluorescence yield of the PC and APC emission component was also found to decrease markedly with increasing excitation intensity. This is in contrast to the case of the isolated phycobiliprotein APC which showed only a slight quenching of the fluorescence. The higher quenching observed for the APC emission in the PBsome evidences the higher effective absorption of APC via energy transfer from PE to PC and APC.     

EXCITATION ENERGY TRANSFER IN THE CRYPTOPHYTES. FLUORESCENCE EXCITATION SPECTRA AND PICOSECOND TIME-RESOLVED EMISSION SPECTRA OF INTACT ALGAE AT 77 K

Excitation spectra of chlorophyll- a (Chl- a ) fluorescence in intact cells of Cryptomonas ovata, Chroomonas pauciplastida and Chroomonas salina were determined at 77 K. For all species the excitation spectra for emission from Chl- a associated with photosystem II (PSII) showed increased contributions by a carotenoid (493 nm) and phycobiliproteins, and decreased contributions by carotenoid (417 nm, 505 nm) and Chl- a (445 nm) as compared to excitation spectra for emission from Chl- a associated with photosystem I (PSI). Excitation spectra of C. salina and C. ovata showed an increased contribution by Chl- c ² to PSII Chl- a fluorescence emission. In all three species the absorbance band positions of Chl- a , as determined from the excitation spectra, were similar to those previously described in green plants. green algae and phycobilisome-containing organisms. Time-resolved 77 K fluorescence emission spectra of C. ovata and C. salina showed successive emission from both phycoerythrin and Chl- c ², PSII Chl- a , and PSI Chl- a. C. pauciplastida showed successive emission from phycocyanin, PSII Chl- a , and PSI Chl- a. Spectral red-shifts with time were observed for the phycobiliprotein peaks in all three species. The fluorescence decay of phycoerythrin in C. ovata and C. salina was faster than that of phycocyanin in C. pauciplastida. The results are discussed in relation to the organization of the antenna pigments of PSII and PSI in the cryptophyte algae.     

POLARIZED PHOTO ACOUSTIC SPECTRA OF PHYCOBILISOMES IN POLYVINYL ALCOHOL FILMS

Abstract— Phycobilisomes from Porphyridium cruentum , suspended in polyvinyl alcohol were found to be highly stable, and had normal absorption and fluorescence spectra. Intact phycobilisomes had a major emission peak at 680 nm, whereas upon partial dissociating the major emission was at 580 nm. Incorporation of phycobilisomes in thin polyvinyl alcohol films facilitated examination by photoacoustic spectroscopy. The photoacoustic spectra had a broad absorption maximum at 545–565 nm (phycoerythrin), which resolved as two peaks (545 and 563 nm) in absorption spectra. Stretching of films resulted in apparent chromophore reorientation in partially dissociated, but not in intact phycobilisomes. Only in dissociated phycobilisomes was observed a differential chromophore orientation at 685 nm by polarized fluorescence, which is attributed to a change in orientation of the terminal phycobilisome pigment relative to phycoerythrin.     

OCCURRENCE OF PHOTOREVERSIBLE ABSORPTION CHANGES IN DISSOCIATED ALLOPHYCOCYANIN AND PHYCOCYANIN

Abstract— Photochromic response of allophycocyanin (APC) and phycocyanin (PC) was studied. Phycobiliproteins were isolated from the blue-green alga Tolypothrix tennis and supplementary from Anabaena cylindrica. The response was found in both APC and PC when they were dissociated into monomer in the presence of propylene glycol at high concentration. APC in propylene glycol solution showed two photoresponses identical with those of "phycochromes" c and d , respectively. PC showed the photoresponse "phycochrome" a type. After re-association by removal of propylene glycol, photoresponse activities disappeared.
We concluded that (i) some of monomer APC's and PC's became photoreactive when APC and PC were dissociated into monomer thus that (ii) photoresponses of "phycochromes" a, c and d were not attributed to the special pigments independent of APC and PC, but were responsible to special molecules of monomer APC and PC. Spectral patterns of photoresponses were discussed in relation to dissociation-association of protein.     

Antioxidant properties of recombinant allophycocyanin expressed in Escherichia coli

Allophycocyanin (APC) is the main core component of phycobilisome found in blue-green algae. The apo-allophycocyanin and its subunits were expressed in Escherichia coli and their antioxidant properties were evaluated using deoxyribose assay. The result showed that both recombinant allophycocyanin fused with maltose binding protein (MBP) tag and 6 x His-tag and their alpha or beta subunits can scavenge hydroxyl radicals successfully, and the separated alpha or beta subunits had a higher inhibition effect on hydroxyl radicals than that when they combined together. The scavenging effects increased with the increasing concentration. These results clearly suggested that apo-allophycocyanin is involved in the antioxidant and radical scavenging activity of phycocyanin, and the antioxidant activity may be partially responsible to the anti-tumor effect of the recombinant allophycocyanin.     

Determination of phycobiliproteins by capillary electrophoresis with laser-induced fluorescence detection

Phycobiliproteins are derived from the photosynthetic apparatus of cyanobacteria and eukaryotic algae. They are composed of a protein backbone to which linear tetrapyrrole chromophores are covalently bound. Furthermore, they are water-soluble highly fluorescent, and relatively stable at room temperature and neutral pH. For this reason, capillary electrophoresis-laser induced fluorescence (CE-LIF) seems the idea method for determination of these important proteins. The effects of buffer additives such as sodium dodecyl sulfate (SDS)and putrescine on the separation of the three major phycobiliprotein types, namely allophycocyanin, phycocyanin, and phycoerythrin, with excitation and emission maxima at 652/660, 615/647, and 565(494)/575 nm, respectively, are considered. Detection limits for these proteins by CE-LIF are some 60-500 times better than by absorbance detection. The development of a fast and sensitive CE-LIF assay such as this is of potential significance to our understand ing of chemical and biological oceanographic processes.     

PHOTOPHYSICAL PROPERTIES OF PHYCOBILIPROTEINS FROM PHYCOBILISOMES: FLUORESCENCE LIFETIMES, QUANTUM YIELDS, AND POLARIZATION SPECTRA

Abstract— Absorption and fluorescence polarization spectra, as well as absolute fluorescence quantum yields, and lifetimes of phycobiliproteins separated from intact phycobilisomes of Porphyridium cruentum, Nostoc sp. and Fremyella diplosiphon were measured. Two different types of phycoerythrin, in addition to phycocyanin and allophycocyanin, were separated from both Porphyridium cruentum and Nostoc sp. phycobilisomes. They were distinguishable by the shape of their absorption spectra, values of fluorescence quantum yields and their limiting polarization. Phycobilisomes of Fremyella diplosiphon had a type of phycoerythrin that was different from the above kinds. By the use of fluorescence quantum yields and lifetime data, the values of natural lifetimes, the decadic molar extinction coefficients, as well as Förster's critical distances R ₀ for excitation energy transfer, between phycobiliproteins in phycobilisomes, were estimated. The values obtained of Förster's critical distances indicate that for most efficient energy transfer from phycoerythrin to allophycocyanin, the outer layers of Porphyridium cruentum and Nostoc sp. phycobilisomes should be composed of bangiophycean, phycoerythrin and cyanophytan phycoerythrin-II respectively.     

STRUCTURAL DIFFERENCES BETWEEN PHYCOCYANIN and ALLOPHYCOCYANIN FROM THEIR RESONANCE RAMAN SPECTRA*

Abstract— Resonance Raman spectra of the chromophores of the cyanobacterial light-harvesting proteins phycocyanin (CPC) and allophycocyanin (APC) were recorded using 364 nm excitation. The1500–1700 cm^-1 regions of these spectra were analyzed for the pH-induced structural changes accompanying the disruption of the native trimers into monomers as well as the progressive denaturation of these monomers. Computer-assisted decomposition of the 1642 cm^-1 marker bands of these spectra yielded up to four components (named I-IV), the frequencies of which were constant within 5 cm^-1 (CPC) and 10 cm^-1 (APC). The relative intensities of two of these components, namely I and III, were sensitive to chromophore conformations. The previously reported downshift of the 1642 cm^-1 band upon folding of the chromophores was shown to result from a weakening of component I and a concomitant enhancement of component III. Components I-IV had different relative intensity patterns in CPC and APC spectra. In particular, the higher relative intensity of component I at 1646 cm^-1 indicated more extended average conformations of the chromophores in trimeric APC than in trimeric CPC. This difference likely resulted from the extra β-155 chromophore present in CPC. Component III was sizably active in RR spectra of monomelic APC but was not observed either in those of monomelic CPC or in those of trimeric APC and CPC. This indicated that, in APC monomers, chromophore(s) did not assume the native conformations found in the trimer, while monomer formation did not sizably alter the structures of the CPC chromophores.     

Phycobiliproteins encapsulated in sol-gel glass

Light transducing phycobiliproteins are encapsulated in optically transparent sol-gel matrices. Absorption and fluorescence spectroscopies are used to characterize the effect of the sol-gel encapsulation on the conformation and aggregation states of the three major phycobiliproteins present in phycobilisomes: phycoerythrin, phycocyanin, and allophycocyanin. It is found that the effects of sol-gel entrapment on the spectroscopic properties are significantly different for the three phycobiliproteins. The results indicate that phycoerythrin undergoes only minor change in its native structure when entrapped in sol-gel. However, significant changes in conformation and aggregation state occur when phycocyanin and allophycocyanin are entrapped in sol-gel matrices. A thin film of sol-gel encapsulated phycoerythrin is also coated on an optical fiber surface and strong fluorescence from the evanescent wave excitation is detected. The potential applications of sol-gel encapsulated phycobiliproteins in biosensors are discussed.     

ALLOPHYCOCYANIN FORMS ISOLATED FROM NOSTOC SP. PHYCOBILISOMES*

Abstract— Allophycocyanin from dissociated phycobilisomes of Nostoc sp. occurs in three spectrally identifiable forms that fractionate on calcium phosphate adsorption chromatography as: allophycocyanin (APC) I (15–20%), APC II (4&50%), and APC III (30–40%). APC I has a single absorption maximum at 654 nm, and a fluorescence emission peak at 678 nm. The absorption peaks of APC II and III are both at 650 nm, but the relative absorbance at 620/650 nm of APC III is less than that of APC II. The emission of both is maximum at 660 nm. On zone sedimentation in sucrose, their S_20,w values of 6.0 ± 0.1 (APC I), 5.0 ± 0.1 (APC II), and 5.3 ± 0.2 (APC III) were comparable to the order of their elution from Sephadex G-200. On SDS acrylamide gel electrophoresis two subunits were resolved with apparent molecular weights of 16,900 and 18,400 daltons. When stained by Coomassie blue, they were present in a ratio of 1α:1β in APC II and III, and a probable ratio of 2a:3β in APC I. The larger size of APC I may be accounted for by additional β subunits, by the presence of an additional polypeptide of 35,000 daltons, or both. Over several days, bleaching as noted by a decrease in absorbance at 650 nm, occurred in all three forms; in addition, the more pronounced bleaching at 650 nm, relative to 620 nm, results in APC III becoming spectrally identical to APC II. A trace of a fourth pigment, probably comparable to allophycocyanin-B, was occasionally detected. The results suggest that several in vitro APC forms (sharing similar subunits) arise upon phycobilisome dissociation, and that APC I is the form most closely related to the final fluorescence emitter of intact phycobilisomes. In this form it probably serves as the bridging pigment in energy transfer from the phycobilisomes to chlorophyll.     

Computer simulation on the energy transfer processes in allophycocyanins

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Spectroscopic study of the light-harvesting protein C-phycocyanin associated with colorless linker peptides

C-Phycocyanin (PC) trimers associated with linker polypeptides were isolated from the phycobilisome (PBS) rods of Synechococcus sp. PCC 7002. LXY refers to a linker polypeptide (L) having an apparent mass of Y kDa, located at position X in the phycobilisome where X can be R (rod), C (core) or RC (rod-core junction). Measurements of the absorption, fluorescence and excitation anisotropy of PC trimer, PC.LR32.3 and PC.LRC28.5 complexes document the spectroscopic modulation of each linker polypeptide on the PC chromophores. The difference spectra between the PC trimer and the PC-linker complexes show that although the effect induced by the linker polypeptides is qualitatively similar in behavior, the extent of the modulation is greater in PC.LRC28.5. Measurements taken at 77 K show that a red-wavelength component of the PC trimer absorption-fluorescence spectra is the target of the linker's influence and that this component is altered to a greater extent by LRC28.5. In addition the 77 K absorbance of the PC trimer resolves band features that are consistent with an excitonic coupling interaction between neighboring alpha 84 and beta 84 chromophores. These band features are also evident in the absorbance of PC.LR32.3 but are absent in PC.LRC28.5 indicating that LRC28.5 may be perturbing the coupling interaction established in the PC trimer alpha 84-beta 84 chromophore pairs. Structurally, the linker polypeptide should disrupt the C3 symmetry in the central cavity of the associated phycobiliprotein and this asymmetric interaction should serve to guide the transfer of excitation energy along PBS rods toward the core elements.     

Two-photon excitation of a phytofluor protein

Phytofluors are highly fluorescent proteins in which the chromophore in a phytochrome is replaced with phycoerythrobilin (PEB), the pigment precursor of the cyanobacterial light harvesting protein phycoerythrin. We examined the fluorescence spectra of the N-terminal region of the cyanobacterial phytochrome 1 from cyanobacterium Synechocystis sp. Pcc 6803 bound to PEB. This protein, Cph1(N514)-PEB, displayed a good two-photon cross-section of 20–30 GM for excitation at 792 nm. This phytofluor also exhibits a high fundamental anisotropy at most practical two-photon excitation (2PE) wavelengths from 700 to 900 nm. Identical lifetimes and correlation times with one and 2PE indicates that the phytofluor is not adversely affected by the intensities needed for 2PE. The one-photon absorption extends well beyond the absorption spectrum and even beyond the emission spectrum to 700 nm. The phytofluor thus appears to be a suitable probe for 2PE and/or cellular imaging.     

RESONANCE-ENHANCED CARS SPECTROSCOPY OF BILIPROTEINS. INFLUENCE OF AGGREGATION and LINKER PROTEINS ON CHROMOPHORE STRUCTURE IN ALLOPHYCOCYANIN (Mastigocladus laminosus)

Abstract— Resonance-enhanced coherent anti-Stokes Raman spectra (CARS) are reported for monomers and for trimers with and without linker proteins of allophycocyanin isolated from Mastigocladus laminosus. The CARS spectrum of the monomer is independent of the presence of linker proteins and is very similar to that of phycocyanin monomers indicating that the equivalent chromophores exhibit like structures in both biliproteins. Large differences are, however, observed between the spectra of phycocyanin trimers and those of allophycocyanin trimers with or without linker proteins (L_c^8,9). The observed differences between monomer and trimer spectra are consistent with a change of the α-chromophore-protein arrangement upon aggregation without linker. If linker proteins are present in the trimer, then additional geometry changes of the β-chromophores are induced; these could relate to a transition from the 15Z- anti to 15Z- syn conformation.     

富硒螺旋藻光谱特性及其含硒蛋白质组分析

以富硒螺旋藻( Selenium enriched spirulina platensis, Se-SP)为材料,采用红外、紫外-可见吸收光谱分析Se-SP的光谱特征,并利用二维电泳技术(Two-dimensional electrophoresis,2DE)分离、飞行时间质谱(Matrix assisted laser desorption ionization time of flight, MALDI-TOF)鉴定和电感耦合等离子体质谱( Inductively coupled plasma mass spectrometry, ICP-MS)检测硒含量,对Se-SP蛋白质组及其含硒蛋白质分布进行初步鉴定和分析。结果显示,硒对螺旋藻蛋白质组中多种蛋白质表达有一定影响,观察到对捕光色素蛋白如藻蓝蛋白( Phycocya-nin, PC)、别藻蓝蛋白( Allophycocyanin , APC)和 Robisco大亚基及超氧化物歧化酶( Superoxide Dismutase, SOD)等蛋白质具有明显上调作用。硒在Se-SP蛋白质组中存在较为普遍,但分布相对集中在pH 4~7、分子质量14.4~43 kDa的蛋白质中。硒在藻体特定蛋白质中的存在形式还有待深入研究和鉴定。     

藻胆体核心复合物结构与功能的研究(Ⅱ)──4种变藻蓝蛋白复合物发色团相互作用机制

从螺旋藻藻胆体中分离出4种不同结构和光谱形式的变藻蓝蛋白复合物APⅠ、APⅡ、APⅢ和APB, 利用吸收光谱、荧光光谱比较了三聚体和单体的光谱特性, 通过对吸收光谱的光谱解曾以及各组分的归属, 研究了变藻蓝蛋白复合物内各色团间相互作用的性质和在能量传递中的功能.结果表明, 复合物内色团间的作用关系可以用Forster偶极-隅极作用机制来解释, 由于连接蛋白和同源亚基的存在影响其结构的对称性, 进而影响各色团间相互作用的形式和性质.