Phototriggered Dynamic and Biomimetic Growth of Chlorosomal Self-Aggregates.

Supramolecular polymerizations mimicking native systems, which are step-by-step constructions to form self-aggregates, were recently developed. However, a general system to successively and spontaneously form self-aggregates from monomeric species remains challenging. Here, we report a photoinduced supramolecular polymerization system as a biomimetic formation of chlorophyll aggregates which are the main light-harvesting antennas in photosynthetic green bacteria, called "chlorosomes". In this system, inert chlorophyll derivatives were UV-irradiated to gradually produce active species through deprotection. Such active monomers spontaneously assembled to form fiberlike chlorosomal self-aggregates in a similar manner as a dynamic growth of natural chlorosomal self-aggregates. The study would be useful for elucidation of the formation process of the chlorosomal aggregates and construction of other supramolecular structures in nature.

Ultrafast energy relaxation in single light-harvesting complexes.

Energy relaxation in light-harvesting complexes has been extensively studied by various ultrafast spectroscopic techniques, the fastest processes being in the sub-100-fs range. At the same time, much slower dynamics have been observed in individual complexes by single-molecule fluorescence spectroscopy (SMS). In this work, we use a pump-probe-type SMS technique to observe the ultrafast energy relaxation in single light-harvesting complexes LH2 of purple bacteria. After excitation at 800 nm, the measured relaxation time distribution of multiple complexes has a peak at 95 fs and is asymmetric, with a tail at slower relaxation times. When tuning the excitation wavelength, the distribution changes in both its shape and position. The observed behavior agrees with what is to be expected from the LH2 excited states structure. As we show by a Redfield theory calculation of the relaxation times, the distribution shape corresponds to the expected effect of Gaussian disorder of the pigment transition energies. By repeatedly measuring few individual complexes for minutes, we find that complexes sample the relaxation time distribution on a timescale of seconds. Furthermore, by comparing the distribution from a single long-lived complex with the whole ensemble, we demonstrate that, regarding the relaxation times, the ensemble can be considered ergodic. Our findings thus agree with the commonly used notion of an ensemble of identical LH2 complexes experiencing slow random fluctuations.

Light-induced conformational changes in photosynthetic reaction centers: dielectric relaxation in the vicinity of the dimer.

Conformational changes near the bacteriochlorophyll dimer induced by continuous illumination were identified in the wild type and 11 different mutants of reaction centers from Rhodobacter sphaeroides. The properties of the bacteriochlorophyll dimer, which has a different hydrogen bonding pattern with the surrounding protein in each mutant, were characterized by steady-state and transient optical spectroscopy. After illumination for 1 min, in the absence of the secondary quinone, the recovery of the charge-separated states was nearly 1 order of magnitude slower in one group of mutants including the wild type than in the mutants carrying the Leu to His mutation at the L131 position. The slower recovery was accompanied by a substantial decrease in the electrochromic absorption changes associated with the Q(y) bands of the nearby monomers during the illumination. The other set of mutants containing the Leu L131 to His substitution exhibited slightly altered electrochromic changes that decreased only half as much during the illumination as in the other family of mutants. The correlation between the recovery of the charge-separated states in the light-induced conformation and the electrochromic absorption changes suggests a dielectric relaxation of the protein that stabilizes the charge on the dimer.

Polychromatic solar energy conversion in pigment-protein chimeras that unite the two kingdoms of (bacterio)chlorophyll-based photosynthesis.

Natural photosynthesis can be divided between the chlorophyll-containing plants, algae and cyanobacteria that make up the oxygenic phototrophs and a diversity of bacteriochlorophyll-containing bacteria that make up the anoxygenic phototrophs. Photosynthetic light harvesting and reaction centre proteins from both kingdoms have been exploited for solar energy conversion, solar fuel synthesis and sensing technologies, but the energy harvesting abilities of these devices are limited by each protein's individual palette of pigments. In this work we demonstrate a range of genetically-encoded, self-assembling photosystems in which recombinant plant light harvesting complexes are covalently locked with reaction centres from a purple photosynthetic bacterium, producing macromolecular chimeras that display mechanisms of polychromatic solar energy harvesting and conversion. Our findings illustrate the power of a synthetic biology approach in which bottom-up construction of photosystems using naturally diverse but mechanistically complementary components can be achieved in a predictable fashion through the encoding of adaptable, plug-and-play covalent interfaces.

[Photodynamic therapy: principles and therapeutic indications]. / Principes et applications thérapeutiques de la photothérapie dynamique.

Photodynamic therapy consists in destroying a tumoral or a non tumoral tissue by the effect of both a photosensitizing molecule and a laser light. This simple concept has needed numerous years in order to be used in routine treatments with both photosensitizers and laser light delivered optimally. Researches in chemistry lead to new porphyrin and bacteriochlorophyl derivatives which alleviate the decrease of light absorption by endogenous molecules and in consequence allow a deeper light penetration. Short half-life of these compounds allows an easier treatment monitoring. In parallel, improvements in both laser technology and fibers allow new indications in various pathologies. First applications took place in treatment of respiratory, digestive and urologic cancers. The biggest success to date is recorded in ophthalmology with the treatment of age related macular degeneration. New approaches are explored and clinical studies are ongoing.

Prompt assessment of WST11-VTP outcome using luciferase transfected tumors enables second treatment and increase in overall therapeutic rate.

This study hypothesized that success rate assessment of vascular targeted photodynamic therapy (VTP) of solid tumors 24 h post-treatment may allow prompt administration of a second treatment in case of failure, increasing the overall success rate. Here, we show that treatment of luciferase transfected CT26-luc mouse colon carcinoma tumors in BALB/c mice by VTP with WST11 (a Pd-bacteriochlorophyll-based photosensitizer) allows fast assessment of treatment success 24 h post-treatment, using bioluminescence imaging (BLI). WST11-VTP was found to abolish luciferin bioluminescence in the treated tumors resulting in two types of responses. One, comprising 75% of the mice, signified successful outcome, presenting neither BLI signal nor tumor regrowth (24 h-90 days post-VTP). The second (the remaining 25% of the mice) signified treatment failure, presenting various levels of BLI signal with subsequent tumor regrowth (24 h-90 days). Consequently, the mice that failed the first treatment were treated again. We show that treatment success rate in both VTP sessions was identical and that the cumulative success rate of the treatment increased from 75% to over 90%. These results therefore, present a fast method of assessing VTP outcome and support the feasibility of successive multiple treatments with these sensitizers in the clinical arena. The presented methodology can also be helpful in future preclinical studies, and expedite the development of VTP drugs.

Comparative study of the energy transfer kinetics in artificial BChl e aggregates containing a BChl a acceptor and BChl e-containing chlorosomes of Chlorobium phaeobacteroides.

Chlorosomes are the light-harvesting organelles of green bacteria, containing mainly special bacteriochlorophylls (BChls) carrying a 3(1)-hydroxy side chain. Artificial aggregates of BChl c, d, and e have been shown to resemble the native chlorosomes in many respects. They are therefore seen as good model systems for understanding the spectroscopic properties of these antenna systems. We have investigated the excitation energy transfer in artificial aggregates of BChl e, containing small amounts of BChl a as an energy acceptor, using steady-state and time-resolved fluorescence. Global analysis of the kinetic data yields two lifetimes attributable to energy transfer: a fast one of 12-20 ps and a slower one of approximately 50 ps. For comparison, BChl e-containing native chlorosomes of Chlorobium phaeobacteroides and chlorosomes in which the energy acceptor had been degraded by alkaline treatment were also studied. A similar behavior is seen in both the artificial and the natural systems. The results suggest that the artificial aggregates of BChls have a potential as antenna systems in future artificial photonic devices.

Photostability of bacteriochlorophyll a and derivatives: potential sensitizers for photodynamic tumor therapy.

The photostabilities of bacteriochlorophyll a and several of its derivatives, which are of interest as potential sensitizers in photodynamic tumor therapy, were investigated. The pigments were irradiated with light >630 nm in organic solvents (acetone, tetrahydrofuran, pyridine, methanol, ethanol, n-propanol, 2-propanol and toluene) and in aqueous detergent solutions (cetyl-trimethyl-ammonium bromide [CTAB], lauryldimethyl-aminoxide [LDAO] or sodium dodecyl-sulfate [SDS] and Triton X-100 [TX100]). Their stabilities in these different solvents were determined in the presence and absence of an external sensitizer (pyromethyl-pheophorbide a), oxygen, sodium ascorbate and inert gas (Ar) or vacuum. The photodegradation products of bacteriochlorophyll a in acetone solution were isolated, purified by HPLC and analyzed by their absorption spectra and mass spectroscopy. Besides the well-known dehydrogenation products, such as [3-acetyl]-chlorophyll a, which were obtained as by-products, the major products had low absorption in the visible-near infrared spectral range. The spectral signature of the major component of these products was characteristic of linear open-chain tetrapyrroles, but they lacked the characteristic protonation-deprotonation behavior and reactivity of bilins with Zn(++).

High-order fluorescence and exciton interaction in photosynthetic bacteria.

We have observed fluorescence at visible wavelengths from chromatophores of photosynthetic bacteria excited with infrared radiation which we attribute to bacteriochlorophyll of the antenna system. The fluorescence is prompt (no delay greater than 5 ns). Its spectrum shows peaks at 445, 530 (broad) and 600 nm when excited with either 694 or 868 nm. Quantum yield is of the order of 10(-9). The dependence on intensity indicates generation by mainly third-order processes which could involve triplet state in combination with excited singlets. Second-order single-singlet fusion could also contribute. The high-order fluorescence can also be explained as arising from absorption of a second photon by singlet excited states.

Isolation and spectral characteristics of the photochemical reaction center of Rhodopseudomonas viridis.

A method is described for isolation of the Rhodopseudomonas viridis reaction center complex free of altered, 685 nm absorbing pigment. This improved preparation contains two c-type cytochromes in the ratio P-960: cytochrome c-558: cytochrome c-553 of 1:2:2 to 3. The near infrared spectral forms of the reduced preparation are located at 790, 832, 846, and 987 nm at 77 K; the oxidized complex absorbs at 790, 808, 829 and approx. 1310 nm. The 790 nm band is attributed to bacteriophaeophytin b and the other absorbances to bacteriochlorophyll b, The visible absorption bands may be assigned to these pigments and to the cytochromes present and, probably to a carotenoid. The presence of two bacteriochlorophyll b spectral forms in the P+-830 band suggests that exciton interactions occur among pigments in the oxidized, as well as the reduced, reaction center. Changes in the 790 and 544 nm bands upon illumination of the reaction center preparation at low redox potential may be indicative of a role for bacteriophaeophytin b in primary photochemical events.

Triplet states of bacteriochlorophyll and carotenoids in chromatophores of photosynthetic bacteria.

Chromatophores from photosynthetic bacteria were excited with flashes lasting approx. 15 ns. Transient optical absorbance changes not associated with the photochemical electron-transfer reactions were interpreted as reflecting the conversion of bacteriochlorophyll or carotenoids into triplet states. Triplet states of various carotenoids were detected in five strains of bacteria; triplet states of bacteriochlorophyll, in two strains that lack carotenoids. Triplet states of antenna pigments could be distinguished from those of pigments specifically associated with the photochemical reaction centers. Antenna pigments were converted into their triplet states if the photochemical apparatus was oversaturated with light, if the primary photochemical reaction was blocked by prior chemical oxidation of P-870 or reduction of the primary electron acceptor, or if the bacteria were genetically devoid of reaction centers. Only the reduction of the electron acceptor appeared to lead to the formation of triplet states in the reaction centers. In the antenna bacteriochlorophyll, triplet states probably arise from excited singlet states by intersystem crossing. The antenna carotenoid triplets probably are formed by energy transfer from triplet antenna bacteriochlorophyll. The energy transfer process has a half time of approx. 20 ns, and is about 1 X 10(3) times more rapid than the reaction of the bacteriochlorophyll triplet states with O2. This is consistent with a role of carotenoids in preventing the formation of singlet O2 in vivo. In the absence of carotenoids and O2, they decay half times of the triplet states are 70 mus for the antenna bacteriochlorophyll and 6-10 mus for the reaction center bacteriochlorophyll. The carotenoid triplets decay with half times of 2-8 mus. With eak flashes, the quantum yields of the antenna triplet states are in the order of 0.02. The quantum yields decline severely after approximately one triplet state is formed per photosynthetic unit, so that even extremely strong flashes convert only a very small fraction of the antenna pigments into triplet states. The yield of fluorescence from the antenna bacteriochlorophyll declines similarly. These observations can be explained by the proposal that single-triplet fusion causes rapid quenching of excited single states in the antenna bacteriochlorophyll.

Novel water-soluble bacteriochlorophyll derivatives for vascular-targeted photodynamic therapy: synthesis, solubility, phototoxicity and the effect of serum proteins.

New negatively charged water-soluble bacteriochlorophyll (Bchl) derivatives were developed in our laboratory for vascular-targeted photodynamic therapy (VTP). Here we focused on the synthesis, characterization and interaction of the new candidates with serum proteins and particularly on the effect of serum albumin on the photocytotoxicity of WST11, a representative compound of the new derivatives. Using several approaches, we found that aminolysis of the isocyclic ring with negatively charged residues markedly increases the hydrophilicity of the Bchl sensitizers, decreases their self-association constant and selectively increases their affinity to serum albumin, compared with other serum proteins. The photocytotoxicity of the new candidates in endothelial cell culture largely depends on the concentration of the serum albumin. Importantly, after incubation with physiological concentrations of serum albumin (500-600 microM), WST11 was found to be poorly photocytotoxic (>80% endothelial cell survival in cell cultures). However, in a recent publication (Mazor, O. et al. [2005] Photochem. Photobiol. 81, 342-351) we showed that VTP of M2R melanoma xenografts with a similar WST11 concentration resulted in approximately 100% tumor flattening and >70% cure rate. We therefore propose that the two studies collectively suggest that the antitumor activity of WST11 and probably of other similar candidates does not depend on direct photointoxication of individual endothelial cells but on the vascular tissue response to the VTP insult.

Light control over the size of an antenna unit building block as an efficient strategy for light harvesting in photosynthesis.

It was shown that an increase in the bacteriochlorophyll (BChl) c antenna size observed upon lowering growth light intensities led to enhancement of the hyperchromism of the BChl c Q(y) absorption band of the green photosynthetic bacterium Chloroflexus aurantiacus. With femtosecond difference absorption spectroscopy, it was shown that the amplitude of bleaching of the oligomeric BChl c Q(y) band (as compared to that for monomeric BChl a) increased with increasing BChl c content in chlorosomes. This BChl c bleaching amplitude was about doubled as the chlorosomal antenna size was about trebled. Both sets of findings clearly show that a unit BChl c aggregate in the chlorosomal antenna is variable in size and governed by the grow light intensity, thus ensuring the high efficiency of energy transfer within the BChl c antenna regardless of its size.

Delayed fluorescence from bacteriochlorophyll in Chromatium vinosum chromatophores: characteristics in the presence of o-phenanthroline.

Delayed fluorescence from bacteriochlorophyll in the chromatophores of Chromatium vinosum, a photosynthetic purple sulfur bacterium, was studied in the presence of o-phenanthroline (o-phen) under intermittent illumination. Re-reduction of the photooxidized reaction center bacteriochlorophyll (P+) in the dark interval was accelerated by o-phen. This effect was attributed to the return of electrons trapped in the primary electron acceptor (A) to P+. In the presence of o-phen, the time course of the decay of delayed fluorescence was not coincident with that of the re-reduction of P+. The delayed fluorescence was somewhat intensified at the early stage (within 30 ms) of relaxation in the dark period. Prolonged illumination (longer than 20 ms) or uncouplers such as carbonylcyanide m-chlorophenylhydrazone (CCCP) or valinomycin plus nigericin decreased the intensity of delayed fluorescence and suppressed the stimulation of delayed fluorescence at the early stage. Delayed fluorescence from reaction center-rich subchromatophore particles decayed with a time course identical to that of the reduction of P+ and was not affected by CCCP, in the presence of o-phen. The intensification at the early stage in the chromatophores can be interpreted in terms of charge separation between pairs of P and A, primary electron donor and acceptor molecules, oriented perpendicular to the intact chromatophore membrane, the effect decreasing in parallel with the recombination of P+ and A-.

Time-resolved spectroscopy of energy transfer and trapping upon selective excitation in membranes of Heliobacillus mobilis at low temperature.

Transient absorption difference spectra in the Qy absorption band of bacteriochlorophyll (BChl) g and in the 670 nm absorption band of the primary acceptor A0 in membranes of Heliobacillus mobilis (Hc. mobilis) were measured at 20 K upon selective excitation at 668, 793, 810, and 815 nm with a 5 nm spectral bandwidth. When excited at 793 nm, the spectral equilibration of excitations from shorter to longer wavelength-absorbing pigments occurred within 3 ps and mostly localized at the band centered around 808 nm. When excited at 668 nm, the excitation energy transfer from the 670 nm absorbing pigment to the Qy band of BChl g took less than 0.5 ps, and the energy redistribution occurred and localized at 808 nm as in the case of the 793 nm excitation. All of the excitations were localized at the long wavelength pigment pool centered around 810 or 813 nm when excited at 810 or 815 nm. A slower energy transfer process with a time constant of 15 ps was also observed within the pool of long wavelength-absorbing pigments upon selective excitation at different wavelengths as has been observed by Lin et al. (Biophys. J. 1994, 67, 2479) when excited at 590 nm. Energy transfer from long wavelength antenna molecules to the primary electron donor P798 followed by the formation of P+ took place with a time constant of 55-70 ps for all excitations. Direct excitation of the primary electron acceptor A0, which absorbed at 670 nm, showed the same kinetic behavior as in the case when different forms of antenna pigments were excited in the Qy region. This observation generally supports the trapping-limited case of energy transfer in which the excitations have high escape probability from the reaction center (RC) until the charge separation takes place. Possible mechanisms to account for the apparent "uphill" energy transfer from the long wavelength antenna pigments to P798 are discussed.

Photophysical consequences of coupling bacteriochlorophyll a with serine and its resulting solubility in water.

We investigated the dependence on solvents of optical absorption and emission of the bacteriochlorophyll a-serine (BChl-ser), a water soluble bacteriochlorophyll (BChl) derivative. Comparison between the experimental data and those collected for BChl in nonaqueous solvents shows that only a minor interaction takes place between serine and the macrocycle's pi-electron system. Nevertheless, the coupling with serine results in a small enhancement of the nonradiative relaxation rate from the first excited singlet state S1. In buffered aqueous solution (pH = 7.4), the Stokes shift of the BChl-ser fluorescence and its nonradiative relaxation rate are enhanced compared with those in nonaqueous solutions (Scherz, A., S. Katz, Y. Vakrat, V. Brumfeld, E. Gabelmann, D. Leupold, J. R. Norris, H. Scheer and Y. Salomon (1998) Photosynthesis: Mechanisms and Effects, Vol. V (Edited by G. Garab), pp. 4207-4212. Kluwer Academic, Dordrecht.), probably as a result of a hydrogen bonding between the BChl macrocycle and the water molecules. In aprotic solvents, without hydrogen bonds, the permanent dipole moment of the first excited singlet state in both BChl and BChl-ser is increased compared with the ground state by at least 2.5 Debye.

Diastereoselective self-aggregation of synthetic 3-(1-hydroxyethyl)-bacteriopyrochlophyll-a as a novel photosynthetic antenna model absorbing near the infrared regions.

3-Deacetyl-3-(1-hydroxyethyl)bacteriopyrochlorophyll-a (1), 7,8-dihydrobacteriochlorophyll-d possessing 8-ethyl, 12-methyl and 17(4)-phytyl groups, was prepared by modifying naturally occurring bacteriochlorophyll-a. The synthetic 3(1)-epimers were separated by high-performance liquid chromotagraphy, and the absolute configuration at the 3(1)-position was determined by derivatization of 1 to a structurally determined chlorin. A dichloromethane solution of 3(1)R-1 or 3(1)S-1 was diluted with a 1000-fold volume of cyclohexane to give self-aggregation species absorbing light at a near-infrared (NIR) region (<910 nm). The resulting Qy maximum in 3(1)R-1 was 860 nm and redshifted by 2170 cm(-1) from the monomeric one, whereas epimeric 3(1)S-1 showed a less redshifted peak at ca 800 nm, with a small dimeric band around 740 nm. Such visible spectra indicated that 3(1)R/S-1 formed different supramolecular structures in the self-aggregates. In contrast, self-aggregation of the 7,8-dehydro-compound 2, bacteriochlorophyll-dP, found in natural antennas of photosynthetic green bacteria showed much smaller diastereomeric control. The self-aggregates of 3(1)R-1 absorbing light in the NIR region would be models of intrinsic membraneous light-harvesting complexes 1 in photosynthetic purple bacteria as well as extramembranous antennas in green bacteria.

Photodynamics of the bacteriochlorophyll-carotenoid system. 1. Bacteriochlorophyll-photosensitized oxygenation of beta-carotene in acetone.

Carotenoids are well-known physical quenchers of chlorophyll excited states and reactive oxygen species both in vivo and in vitro. They may also be involved in chemical quenching undergoing, e.g. isomerizations or oxidations. We have found that beta-carotene (Car) in aerobic acetone is rapidly oxygenated under strong illumination with red light (lambda exc > or = 630 nm) in the presence of bacteriopheophytin a. At the same time the photosensitizer undergoes only slight (< 10%) photodegradation. By preparative high-performance liquid chromatography as many as seven major products of oxygen attachment to Car have been isolated. Their molecular masses show that Car sequentially accumulates up to six oxygen atoms while its C40-skeleton remains intact.

Nanosecond laser photolysis studies of chlorosomes and artificial aggregates containing bacteriochlorophyll e: evidence for the proximity of carotenoids and bacteriochlorophyll a in chlorosomes from Chlorobium phaeobacteroides strain CL1401.

Time-resolved, laser-induced changes in absorbance, delta A(lambda; t), have been recorded with a view to probing pigment-pigment interactions in chlorosomes (control as well as carotenoid-depleted) and artificial aggregates of bacteriochlorophyll e (BChle). Control chlorosomes were isolated from Chlorobium phaeobacteroides strain CL1401, whose chromophores comprise BChle, bacteriochlorophyll a (BChla) and several carotenoid (Car) pigments; Car-depleted chlorosomes, from cells grown in cultures containing 2-hydroxybiphenyl. Artificial aggregates were prepared by dispersing BChle in aqueous phase in the presence of monogalactosyl diglyceride. In chlorosomes delta A(lambda; t) shows, besides a signal attributable to triplet Car (with a half-life of about 4 microseconds), signals in the Qy regions of both BChl. The BChla signal decays at the same rate as the Car signal, which is explained by postulating that some Car are in intimate contact with some baseplate BChla pigments, and that when a ground-state Car changes into a triplet Car, the absorption spectrum of its BChla neighbors undergoes a concomitant change (termed transient environment-induced perturbation). The signal in the Qy-region of BChle behaves differently: its amplitude falls, under reducing conditions, by more than a factor of two during the first 0.5 microsecond (a period during which the Car signal suffers negligible diminution), and is much smaller under nonreducing conditions. The BChle signal is also attributed to transient environment-induced perturbation, but in this case the perturber is a BChle photoproduct (probably a triplet or a radical ion). The absence of long-lived BChle triplets in all three systems, and of long-lived BChla triplets in chlorosomes, indicates that BChle in densely packed assemblies is less vulnerable to photodamage than monomeric BChle and that, in chlorosome, BChla rather than BChle needs, and receives, photoprotection from an adjacent Car.

The photophysics of monomeric bacteriochlorophylls c and d and their derivatives: properties of the triplet state and singlet oxygen photogeneration and quenching.

Measurements of pigment triplet-triplet absorption, pigment phosphorescence and photosensitized singlet oxygen luminescence were carried out on solutions containing monomeric bacteriochlorophylls (Bchl) c and d, isolated from green photosynthetic bacteria, and their magnesium-free and farnesyl-free analogs. The energies of the pigment triplet states fell in the range 1.29-1.34 eV. The triplet lifetimes in aerobic solutions were 200-250 ns; they increased to 280 +/- 70 microseconds after nitrogen purging in liquid solutions and to 0.7-2.1 ms in a solid matrix at ambient or liquid nitrogen temperatures. Rate constants for quenching of the pigment triplet state by oxygen were (2.0-2.5) x 10(9) M-1 s-1, which is close to 1/9 of the rate constant for diffusion-controlled reactions. This quenching was accompanied by singlet oxygen formation. The quantum yields for the triplet state formation and singlet oxygen production were 55-75% in air-saturated solutions. Singlet oxygen quenching by ground-state pigment molecules was observed. Quenching was the most efficient for magnesium-containing pigments, kq = (0.31-1.2) x 10(9) M-1 s-1. It is caused mainly by a physical process of singlet oxygen (1O2) deactivation. Thus, Bchl c and d and their derivatives, as well as chlorophyll and Bchl a, combine a high efficiency of singlet oxygen production with the ability to protect photochemical and photobiological systems against damage by singlet oxygen.