Light-induced reversible reorganizations in closed Type II reaction centre complexes: physiological roles and physical mechanisms.

The purpose of this review is to outline our understanding of the nature, mechanism and physiological significance of light-induced reversible reorganizations in closed Type II reaction centre (RC) complexes. In the so-called 'closed' state, purple bacterial RC (bRC) and photosystem II (PSII) RC complexes are incapable of generating additional stable charge separation. Yet, upon continued excitation they display well-discernible changes in their photophysical and photochemical parameters. Substantial stabilization of their charge-separated states has been thoroughly documented-uncovering light-induced reorganizations in closed RCs and revealing their physiological importance in gradually optimizing the operation of the photosynthetic machinery during the dark-to-light transition. A range of subtle light-induced conformational changes has indeed been detected experimentally in different laboratories using different bRC and PSII-containing preparations. In general, the presently available data strongly suggest similar structural dynamics of closed bRC and PSII RC complexes, and similar physical mechanisms, in which dielectric relaxation processes and structural memory effects of proteins are proposed to play important roles.

Heat- and light-induced detachment of the light-harvesting antenna complexes of photosystem I in isolated stroma thylakoid membranes.

The multisubunit pigment-protein complex of photosystem I (PSI) consists of a core and peripheral light-harvesting antenna (LHCI). PSI is thought to be a rather rigid system and very little is known about its structural and functional flexibility. Recent data, however, suggest LHCI detachment from the PSI supercomplex upon heat and light treatments. Furthermore, it was suggested that the splitting off of LHCI acts as a safety valve for PSI core upon photoinhibition (Alboresi et al., 2009). In this work we analyzed the heat- and light-induced reorganizations in isolated PSI vesicles (stroma membrane vesicles enriched in PSI). Using differential scanning calorimetry we revealed a stepwise disassembly of PSI supercomplex above 50°C. Circular dichroism, sucrose gradient centrifugation and 77K fluorescence experiments identified the sequence of events of PSI destabilization: 3min heating at 60°C or 40min white light illumination at 25°C resulted in pronounced Lhca1/4 detachment from the PSI supercomplex, which is then followed by the degradation of Lhca2/3. The similarity of the main structural effects due to heat and light treatments supports the notion that thermo-optic mechanism, structural changes induced by ultrafast local thermal transients, which has earlier been shown to be responsible for structural changes in the antenna system of photosystem II, can also regulate the assembly and functioning of PSI antenna.

The ultrastructure and flexibility of thylakoid membranes in leaves and isolated chloroplasts as revealed by small-angle neutron scattering.

We studied the periodicity of the multilamellar membrane system of granal chloroplasts in different isolated plant thylakoid membranes, using different suspension media, as well as on different detached leaves and isolated protoplasts-using small-angle neutron scattering. Freshly isolated thylakoid membranes suspended in isotonic or hypertonic media, containing sorbitol supplemented with cations, displayed Bragg peaks typically between 0.019 and 0.023Å(-1), corresponding to spatially and statistically averaged repeat distance values of about 275-330 Å⁻¹. Similar data obtained earlier led us in previous work to propose an origin from the periodicity of stroma thylakoid membranes. However, detached leaves, of eleven different species, infiltrated with or soaked in D2O in dim laboratory light or transpired with D2O prior to measurements, exhibited considerably smaller repeat distances, typically between 210 and 230 Å⁻¹, ruling out a stromal membrane origin. Similar values were obtained on isolated tobacco and spinach protoplasts. When NaCl was used as osmoticum, the Bragg peaks of isolated thylakoid membranes almost coincided with those in the same batch of leaves and the repeat distances were very close to the electron microscopically determined values in the grana. Although neutron scattering and electron microscopy yield somewhat different values, which is not fully understood, we can conclude that small-angle neutron scattering is a suitable technique to study the periodic organization of granal thylakoid membranes in intact leaves under physiological conditions and with a time resolution of minutes or shorter. We also show here, for the first time on leaves, that the periodicity of thylakoid membranes in situ responds dynamically to moderately strong illumination. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.

Mapping microscopic order in plant and mammalian cells and tissues: novel differential polarization attachment for new generation confocal microscopes (DP-LSM).

Elucidation of the molecular architecture of complex, highly organized molecular macro-assemblies is an important, basic task for biology. Differential polarization (DP) measurements, such as linear (LD) and circular dichroism (CD) or the anisotropy of the fluorescence emission (r), which can be carried out in a dichrograph or spectrofluorimeter, respectively, carry unique, spatially averaged information about the molecular organization of the sample. For inhomogeneous samples-e.g. cells and tissues-measurements on macroscopic scale are not satisfactory, and in some cases not feasible, thus microscopic techniques must be applied. The microscopic DP-imaging technique, when based on confocal laser scanning microscope (LSM), allows the pixel by pixel mapping of anisotropy of a sample in 2D and 3D. The first DP-LSM configuration, which, in fluorescence mode, allowed confocal imaging of different DP quantities in real-time, without interfering with the 'conventional' imaging, was built on a Zeiss LSM410. It was demonstrated to be capable of determining non-confocally the linear birefringence (LB) or LD of a sample and, confocally, its FDLD (fluorescence detected LD), the degree of polarization (P) and the anisotropy of the fluorescence emission (r), following polarized and non-polarized excitation, respectively (Steinbach et al 2009 Acta Histochem.111 316-25). This DP-LSM configuration, however, cannot simply be adopted to new generation microscopes with considerably more compact structures. As shown here, for an Olympus FV500, we designed an easy-to-install DP attachment to determine LB, LD, FDLD and r, in new-generation confocal microscopes, which, in principle, can be complemented with a P-imaging unit, but specifically to the brand and type of LSM.

Measurement of the optical parameters of purple membrane and plant light-harvesting complex films with optical waveguide lightmode spectroscopy.

Purple membrane (bacteriorhodopsin) and plant light-harvesting complexes (LHCII) were dried on the optical waveguide sensor with varying thicknesses in a wide range (from 20 to several hundreds of nanometers) and the optical parameters were studied with optical waveguide lightmode spectroscopy. It was found that applying the approximate 4-layer mode equations for the measured effective refractive indices resulted in unacceptable results for the optical parameters: with increasing thickness the refractive index decreased monotonously from 1.5 to 1.1. Therefore an inverse waveguide numerical method was developed and used to obtain reliable results from the experiments. The inverse method yielded an approximately constant (1.53) refractive index independently of the thickness for the purple membrane and LHCII films. Light-induced changes in the optical parameters of the purple membrane and LHCII films were also studied. For purple membrane films the most significant effect is the change in refractive index and absorption. For LHCII films prolonged illumination induced irreversible structural changes, most probably of thermo-optic origin.

Effects of polyethylene glycol on stability of alpha-chymotrypsin in aqueous ethanol solvent.

The effects of polyethylene glycol (PEG) of different molecular weights (400, 2000, 6000, 12,000, 20,000, and 35,000) on the conformational stability and catalytic activity of alpha-chymotrypsin in 60% ethanol were studied. The inactivation caused by the organic solvent was not influenced by PEG 400. However, the PEGs with higher molecular weights up to 35,000 increased the stability of the enzyme, but this alpha-chymotrypsin stabilizing effect was molecular weight-independent. With increase of the molecular weight of PEG, a more stable tertiary structure of the enzyme was observed.

Functional significance of the lipid-protein interface in photosynthetic membranes.

The functional significance of the lipid-protein interface in photosynthetic membranes, mainly in thylakoids, is reviewed with emphasis on membrane structure and dynamics. The lipid-protein interface is identified primarily by the restricted molecular dynamics of its lipids as compared with the dynamics in the bulk lipid phase of the membrane. In a broad sense, lipid-protein interfaces comprise solvation shell lipids that are weakly associated with the hydrophobic surface of transmembrane proteins but also include lipids that are strongly and specifically bound to membrane proteins or protein assemblies. The relation between protein-associated lipids and the overall fluidity of the thylakoid membrane is discussed. Spin label electron paramagnetic resonance spectroscopy has been identified as the technique of choice to characterize the protein solvation shell in its highly dynamic nature; biochemical and direct structural methods have revealed an increasing number of protein-bound lipids. The structural and functional roles of these protein-bound lipids are mustered, but in most cases they remain to be determined. As suggested by recent data, the interaction of the non-bilayer-forming lipid, monogalactosyldyacilglycerol (MGDG), with the main light-harvesting chlorophyll a/b-binding protein complexes of photosystem-II (LHCII), the most abundant lipid and membrane protein components on earth, play multiple structural and functional roles in developing and mature thylakoid membranes. A brief outlook to future directions concludes this review.

Sodium dependency of the photosynthetic electron transport in the alkaliphilic cyanobacterium Arthrospira platensis.

Arthrospira (Spirulina) platensis (A. platensis) is a model organism for investigation of adaptation of photosynthetic organisms to extreme environmental conditions: the cell functions in this cyanobacterium are optimized to high pH and high concentration (150-250 mM) of Na+. However, the mechanism of the possible fine-tuning of the photosynthetic functions to these extreme conditions and/or the regulation of the cellular environment to optimize the photosynthetic functions is poorly understood. In this work we investigated the effect of Na-ions on different photosynthetic activities: linear electron transport reactions (measured by means of polarography and spectrophotometry), the activity of photosystem II (PS II) (thermoluminescence and chlorophyll a fluorescence induction), and redox turnover of the cytochrome b6f complex (flash photolysis); and measured the changes of the intracellular pH (9-aminoacridine fluorescence). It was found that sodium deprivation of cells in the dark at pH 10 inhibited, within 40 min, all measured photosynthetic reactions, and led to an alkalinization of the intracellular pH, which rose from the physiological value of about 8.3-9.6. These were partially and totally restored by readdition of Na-ions at 2.5-25 mM and about 200 mM, respectively. The intracellular pH and the photosynthetic functions were also sensitive to monensin, an exogenous Na+/H+ exchanger, which collapses both proton and sodium gradients across the cytoplasmic membrane. These observations explain the strict Na+-dependency of the photosynthetic electron transport at high extracellular pH, provide experimental evidence on the alkalization of the intracellular environment, and support the hypothesized role of an Na+/H+ antiport through the plasma membrane in pH homeostasis (Schlesinger et al. (1996). J. Phycol. 32, 608-613). Further, we show that (i) the specific site of inactivation of the photosynthetic electron transport at alkaline pH is to be found at the water splitting enzyme; (ii) in contrast to earlier reports, the inactivation occurs in the dark and, for short periods, without detectable damage in the photosynthetic apparatus; and (iii) in contrast to high pH, Na+ dependency in the neutral pH range is shown not to originate from PSII, but from the acceptor side of PSI. These data permit us to conclude that the intracellular environment rather than the machinery of the photosynthetic electron transport is adjusted to the extreme conditions of high pH and high Na+ concentration.

Effects of polyhydroxy compounds on the structure and activity of alpha-chymotrypsin.

The effects of glycerol, polyethylene glycol, fructose, glucose, sorbitol, and saccharose on the conformation and catalytic activity of alpha-chymotrypsin were studied in 0.1 M sodium phosphate buffer and buffered aqueous 60% ethanol (pH 8.0). The enzyme activity was practically completely lost within 10 min in 60% ethanol, but in the presence of stabilizers the activity was retained. With the exception of polyethylene glycol, the stabilizing effect decreased with increase of the incubation time. The preservation of the catalytic activity was accompanied by changes in the secondary and tertiary structures of alpha-chymotrypsin.

Labeling phosphorylated LHCII with microspheres for tracking studies and force measurements.

We report a method to selectively label phosphorylated, membrane proteins with microscopic particles. This technology is particularly useful in single particle studies. In such studies, the particles may serve to visualize protein diffusion and/or as 'handles' to study the force of interaction between the labeled protein and the membrane matrix. In the latter kind of experiments, forces can be applied and measured by calibrated optical tweezers. Optical tweezers were used in this work to test the strength of the particle labeling. Labeling a single protein with a particle produces a long-lived, distinct tag and is particularly useful for proteins in photosynthetic membranes, which contain endogenous fluorophores that would render single fluorescent proteins difficult to detect.

Structure and activity of alpha-chymotrypsin and trypsin in aqueous organic media.

The effects of different concentrations (20-95%) of organic solvents (ethanol, 1,4-dioxane and acetonitrile) were studied on alpha-chymotrypsin and trypsin from bovine pancreas. The changes in secondary structure were followed by CD measurements, and the apparent Michaelis constants (KMapp) and the stabilities of the enzymes were determined. Significant alterations in the CD spectra were found for both enzymes at the different organic solvent concentrations. The apparent KM values of trypsin and alpha-chymotrypsin decreased as the low solvent concentrations were elevated, but then increased in the presence of higher organic solvent concentrations. The stabilities of the enzymes changed on increase of the organic solvent concentration; trypsin exhibited a higher stability than that of alpha-chymotrypsin in all organic solvents. These results show that at an organic solvent content of 95% the manifestation of an enzyme activity similar to that measured in water can be attributed to the similar compositions of the secondary structural elements.

Non-photochemical chlorophyll fluorescence quenching and structural rearrangements induced by low pH in intact cells of Chlorella fusca (Chlorophyceae) and Mantoniella squamata (Prasinophyceae).

We have used circular dichroism (CD) spectroscopy and chlorophyll fluorescence induction measurements in order to examine low-pH-induced changes in the chiral macro-organization of the chromophores and in the efficiency of non-photochemical quenching of the chlorophyll a fluorescence (NPQ) in intact, dark-adapted cells of Chlorella fusca (Chlorophyceae) and Mantoniella squamata (Prasinophyceae). We found that: (i) high proton concentrations enhanced the formation of chiral macrodomains of the complexes, i.e. the formation of large aggregates with long-range chiral order of pigment dipoles; this was largely independent of the low-pH-induced accumulation of de-epoxidized xanthophylls; (ii) lowering the pH led to NPQ; however, efficient energy dissipation, in the absence of excess light, could only be achieved if a substantial part of violaxanthin was converted to zeaxanthin and antheraxanthin in Chlorella and Mantoniella, respectively; (iii) the low-pH-induced changes in the chiral macro-organization of pigments were fully reversed by titrating the cells to neutral pH; (iv) at neutral pH, the presence of antheraxanthin or zeaxanthin did not bring about a sizeable NPQ. Hence, low-pH-induced NPQ in dark adapted algal cells appears to be associated both with the presence of de-epoxidized xanthophylls and structural changes in the chiral macrodomains. It is proposed that the macrodomains, by providing a suitable structure for long-distance migration of the excitation energy, in the presence of quenchers associated with de-epoxidized xanthophylls, facilitate significantly the dissipation of unused excitation energy.

Thermooptic effect in chloroplast thylakoid membranes. Thermal and light stability of pigment arrays with different levels of structural complexity.

In chloroplast thylakoid membranes, chiral macrodomains, i.e., large arrays of pigment molecules with long-range chiral order, have earlier been shown to undergo light-induced reversible and irreversible structural changes; such reorganizations did not affect the short-range, excitonic pigment-pigment interactions. These structural changes and similar changes in lamellar aggregates of the main chlorophyll a/b light-harvesting complexes exhibited a linear dependence on the intensity of light that was not utilized in photosynthesis. It has been hypothesized that the light-induced rearrangements are driven by a thermooptic effect, i.e., thermal fluctuations due to the dissipation of excess excitation energies [Barzda, V., et al. (1996) Biochemistry 35, 8981-8985]. To test this hypothesis, we have utilized circular dichroism (CD) spectroscopy to investigate the structural stability of the chiral macrodomains and the constituent bulk pigment-protein complexes of granal thylakoid membranes against heat and prolonged, intense illumination. (i) In intact thylakoid membranes, the chiral macrodomains displayed high stability below 40 degrees C, but they were gradually disassembled between 50 and 60 degrees C; the thermal stability of the chiral macrodomains could be decreased substantially by suspending the membranes in reaction media that were hypotonic or had low ionic strength. (ii) The chiral macrodomains were also susceptible to high light: prolonged illumination with intense white light (25 min, 2500 microE m(-)(2) s(-)(1), 25 degrees C) induced similar, irreversible disassembly to that observed at high temperatures; in different preparations, lower thermal stability was coupled to lower light stability. (iii) The light stability depended significantly on the temperature: between about 5 and 15 degrees C, the macrodomains in the intact thylakoids were virtually not susceptible to high light; in contrast, the same preillumination at 35-40 degrees C almost completely destroyed the chiral macrodomains. (iv) As testified by the excitonic CD bands, the molecular organization of the pigment-protein complexes in all samples exhibited very high thermal stability between about 15 and 65 degrees C, and virtually total immunity against intense illumination. These data are fully consistent with the hypothesis of a thermooptic effect, and are interpreted within the frame of a simple model.

Self-regulation of the lipid content of membranes by non-bilayer lipids: a hypothesis.

Many biological membranes contain lipids that do not form a lamellar phase but the roles of these lipids are not well understood. An artificial membrane assembled from the main non-bilayer lipid and the major integral protein of pea thylakoids revealed that the protein spatially inhibits the formation of non-bilayer structures in the lamellae. Without this inhibition, excess lipids are secreted, creating lipid reservoirs for metabolism and/or later uptake. This determines the protein:lipid ratio in the membrane and hence the balance between structural flexibility and the stability of the key constituents that participate in cooperative interactions.

Organization of the pigment molecules in the chlorophyll a/b/c containing alga Mantoniella squamata (Prasinophyceae) studied by means of absorption, circular and linear dichroism spectroscopy.

In order to obtain information on the organization of the pigment molecules in chlorophyll (Chl) a/b/c-containing organisms, we have carried out circular dichroism (CD), linear dichroism (LD) and absorption spectroscopic measurements on intact cells, isolated thylakoids and purified light-harvesting complexes (LHCs) of the prasinophycean alga Mantoniella squamata. The CD spectra of the intact cells and isolated thylakoids were predominated by the excitonic bands of the Chl a/b/c LHC. However, some anomalous bands indicated the existence of chiral macrodomains, which could be correlated with the multilayered membrane system in the intact cells. In the red, the thylakoid membranes and the LHC exhibited a well-discernible CD band originating from Chl c, but otherwise the CD spectra were similar to that of non-aggregated LHC II, the main Chl a/b LHC in higher plants. In the Soret region, however, an unusually intense (+) 441 nm band was observed, which was accompanied by negative bands between 465 and 510 nm. It is proposed that these bands originate from intense excitonic interactions between Chl a and carotenoid molecules. LD measurements revealed that the Q(Y) dipoles of Chl a in Mantoniella thylakoids are preferentially oriented in the plane of the membrane, with orientation angles tilting out more at shorter than at longer wavelengths (9 degrees at 677 nm, 20 degrees at 670 nm and 26 degrees at 662 nm); the Q(Y) dipole of Chl c was found to be oriented at 29 degrees with respect to the membrane plane. These data and the LD spectrum of the LHC, apart from the presence of Chl c, suggest an orientation pattern of dipoles similar to those of higher plant thylakoids and LHC II. However, the tendency of the Q(Y) dipoles of Chl b to lie preferentially in the plane of the membrane (23 degrees at 653 nm and 30 degrees at 646 nm) is markedly different from the orientation pattern in higher plant membranes and LHC II. Hence, our CD and LD data show that the molecular organization of the Chl a/b/c LHC, despite evident similarities, differs significantly from that of LHC II.

Self-assembly of large, ordered lamellae from non-bilayer lipids and integral membrane proteins in vitro.

In many biological membranes, the major lipids are "non-bilayer lipids," which in purified form cannot be arranged in a lamellar structure. The structural and functional roles of these lipids are poorly understood. This work demonstrates that the in vitro association of the two main components of a membrane, the non-bilayer lipid monogalactosyldiacylglycerol (MGDG) and the chlorophyll-a/b light-harvesting antenna protein of photosystem II (LHCII) of pea thylakoids, leads to the formation of large, ordered lamellar structures: (i) thin-section electron microscopy and circular dichroism spectroscopy reveal that the addition of MGDG induces the transformation of isolated, disordered macroaggregates of LHCII into stacked lamellar aggregates with a long-range chiral order of the complexes; (ii) small-angle x-ray scattering discloses that LHCII perturbs the structure of the pure lipid and destroys the inverted hexagonal phase; and (iii) an analysis of electron micrographs of negatively stained 2D crystals indicates that in MGDG-LHCII the complexes are found in an ordered macroarray. It is proposed that, by limiting the space available for MGDG in the macroaggregate, LHCII inhibits formation of the inverted hexagonal phase of lipids; in thylakoids, a spatial limitation is likely to be imposed by the high concentration of membrane-associated proteins.

Reinvestigation of the triplet-minus-singlet spectrum of chloroplasts.

A comparison of the triplet-minus-singlet (TmS) absorption spectrum of spinach chloroplasts, recorded some thirty years ago, with the more recently published TmS spectrum of isolated Chla/b LHCII (light-harvesting complexes associated with photosystem II of higher plants) shows that the two spectra are very similar, which is to be expected, since only the carotenoid pigments contribute to each spectrum. Be that as it may, the comparison also reveals a dissimilarity: photoexcitation of the sample does, or does not, affect the absorbance in the Qy region (650-700 nm), depending on whether the sample is a suspension of chloroplasts or of isolated LHCII. The Qy-signal in the TmS spectrum of LHCII decays, it should be noted, at the same rate as the rest of the difference spectrum, and its most prominent feature is a negative peak. As the carotenoids do not absorb in the Qy region, the presence of a signal in this region calls for an explanation: van der Vos, Carbonera and Hoff, the first to find as well as fathom the phenomenon, attributed the Qy-signal to a change, in the absorption spectrum of a chlorophyll a (Chla) molecule, brought about by the presence of triplet excitation on a neighbouring carotenoid (Car). The difference in the behaviours of chloroplasts and LHCII, if reproducible, would imply that the Car triplets which give rise to the TmS spectrum of chloroplasts do not influence the absorption spectra of their Chla neighbours. With a view to reaching a firm conclusion about this vexed issue, spinach chloroplasts and thylakoids have been examined with the aid of the same kinetic spectrometer as that used for investigating LHCII; the TmS spectra of both chloroplasts and thylakoids contain prominent bleaching signals centred at 680 nm, and the triplet decay time in each case is comparable to that of the Chla/b LHCII triplets. Results pertaining to other closely related systems are recalled, and it is concluded that, so far as the overall appearance of the TmS spectrum is concerned, spinach chloroplasts are by no means abnormal.

Induction of polyunsaturated fatty-acid synthesis enhances tolerance of a cyanobacterium, Cylindrospermopsis raciborskii, to low-temperature photoinhibition.

Acyl-lipid desaturation introduces double bonds (unsaturated bonds) at specifically defined positions of fatty acids that are esterified to the glycerol backbone of membrane glycerolipids. Desaturation pattern of the glycerolipids of Cylindrospermopsis raciborskii (C. raciborskii), a filamentous cyanobacterial strain, was determined in cells grown at 35 degrees C and 25 degrees C. The lowering of the growth temperature from 35 degrees C to 25 degrees C resulted in a considerable accumulation of polyunsaturated octadecanoic fatty acids in all lipid classes. Lipid unsaturation of C. raciborskii was also compared to Synechocystis PCC6803. In C. raciborskii cells, a shift in growth temperature induced a much more pronounced alteration in the desaturation pattern of all lipid classes than in Synechocystis PCC6803. The tolerance to low-temperature photoinhibition of the C. raciborskii cells grown at 25 degrees C and 35 degrees C was also compared to the tolerance of Synechocystis cells grown at the same temperatures. Lower growth temperature increased the tolerance of C. raciborskii cells but not that of Synechocystis cells. These results strengthen the importance of polyunsaturated glycerolipids in the tolerance to environmental stresses and may give a physiological explanation for the determinative role of C. raciborskii strain in algal blooming in the Lake Balaton (Hungary).

Low dose UV-B induced modification of chromophore conformation and it's interaction with microenvironment in cyanobacterial phycobilisomes.

Phycobilisomes (Pbsomes) are the supra macromolecular pigment protein complexes of cyanobacteria. Synechococcus Pbsomes are comprised of phycocyanins (PC) and allophycocyanins (APC). Pbsomes are major light harvesting antennae and also absorb ultraviolet-B (UV-B) radiation (280-320 nm). Synechococcus Pbsomes, upon exposure to low dose of UV-B (0.28 mW cm-2) for different time intervals showed profound alteration in their steady state absorption, fluorescence excitation and emission characteristics (Sah et. al. Biochem. Mol. Biol.Int., Vol. 44, No. 2, 245-247). In the present study, we investigated the effect of low dose of UV-B on isolated Pbsome of Synechococcus. Our results demonstrate the following alterations. Absorbance at 623 nm initially showed a sharp decrease with increasing exposure time to UV-B radiation. The changes in the visible to near ultraviolet absorption and excitation ratio indicated a change in chromophore conformation, upon prolonged exposure of Pbsomes to UV-B radiation. This modification of chromophore conformation appeared to be associated with the loss of energy transfer from PC to APC. Circular dichroism spectra in the amide region showed a significant loss of the alpha helical content of Pbsomes when exposed for longer duration to UV-B. CD spectra in the visible region revealed a marked decrease in the rotational strength at 620 nm. Close monitoring of CD signals emanating in the 500 to 700 nm range further revealed that the decrease in the rotational strength was closely associated with an initial red shift in the positive CD band of Pbsomes when exposed to UV-B for short duration. However, the peak became constant over prolonged exposure to UV-B radiation and accompanied a prominent blue shoulder in the positive CD band which suggests the modification and uncoupling of the various phycocyanobilin (PCB) chromophores of the Synechococcus Pbsomes.