De-etiolation is Almost Colour Blind: the Study of Photosynthesis Awakening Under Blue and Red Light.

The synthesis and assembly of functioning photosynthetic complexes in chloroplasts developing from etioplasts during the de-etiolation of angiosperm seedlings are imperative for the plant's autotrophic lifestyle. This study compared de-etiolation process under monochromatic red or blue light of equal photon flux density during a 24-hour illumination period of etiolated Arabidopsis seedlings. The aim was to elucidate the impact of these light wavelength on the etioplast-to-chloroplast transformation and the initiation of light-dependent photosynthetic reactions. Both treatments lead to the formation of functional young chloroplasts; however, the etioplast-to-chloroplast transition and the assembly of photosynthetic complexes occurred unevenly, with individual steps tuned by red or blue light. Ultrastructural analysis suggested faster prolamellar bodies disassembly under blue light, while low temperature fluorescence studies indicated a slower transformation of protochlorophyllide to chlorophyllide, and chlorophyll a, under these conditions. Red light further promoted the synthesis of chlorophyll b and LHCII antenna proteins. However, the efficiency of antennae in dissipating excess absorbed energy was higher for seedlings de-etiolated under blue light; the maximum quantum yield of the photosystem II reached 0.81 after 24-hour de-etiolation, equivalent to mature plants. Blue light seemed to enhance the development of well-functioning photosystems (I and II) and antennae. These findings are important for gaining a deeper understanding of photoreceptor regulation of de-etiolation and for utilizing selected light regimes to improve crop yield.

Do the expected heatwaves pose a threat to lichens?: Linkage between a passive decline in water content in thalli and response to heat stress.

Being poikilohydric, lichens are inherently exposed to alternating desiccation and hydration cycles. They can exhibit extraordinary resistance to extreme temperatures in a dehydrated state but thermal thresholds for hydrated lichens are lower. The ability of the lichen Cetraria aculeata to recovery after high temperature treatment (40°C, 60°C) at different air humidity levels (relative humidity [RH]: <15%, 25%, 50%, 75%, ≅100%) was examined to find a linkage between passive dehydration of the lichen and its physiological resistance to heat stress. The response to heating was determined by measuring parameters related to photosynthesis and respiration after 2- and 24-h recovery. A higher RH level resulted in a slower decline in relative water content (RWC) in hydrated thalli. In turn, the stress resistance of active thalli depended on the ambient humidity and associated RWC reduction. Elevated temperature had a negative impact on bioenergetic processes, but only an unnatural state of permanent full hydration during heat stress resulted in a lethal effect. Hydrated lichen thalli heated at 40°C and 50% relative humidity (RH) tended to be least susceptible to stress-induced damage. Although atypical climatic conditions may lead lichens to lethal thresholds, the actual likelihood of deadly threat to lichens due to heat events per se is debatable.

Cooperativity of ESPT and Aggregation-Induced Emission Effects-An Experimental and Theoretical Analysis of a 1,3,4-Thiadiazole Derivative.

4-[5-(Naphthalen-1-ylmethyl)-1,3,4-thiadiazol-2-yl]benzene-1,3-diol (NTBD) was extensively studied through stationary UV-vis absorption and fluorescence measurements in various solvents and solvent mixtures and by first-principles quantum chemical calculations. It was observed that while in polar solvents (e.g., methanol) only a single emission band emerged; the analyzed 1,3,4-thiadiazole derivative was capable of producing dual fluorescence signals in low polarity solvents (e.g., n-hexane) and certain solvent mixtures (e.g., methanol/water). As clearly follows from the experimental spectroscopic studies and theoretical modeling, the specific emission characteristic of NTBD is triggered by the effect of enol → keto excited-state intramolecular proton transfer (ESIPT) that in the case of solvent mixture is reinforced by aggregation of thiadiazole molecules. Specifically, the restriction of intramolecular rotation (RIR) due to environmental hindrance suppresses the formation of non-emissive twisted intramolecular charge transfer (TICT) excited keto* states. As a result, this particular thiadiazole derivative is capable of simultaneously producing both ESIPT and aggregation-induced emission (AIE).

Ionic, not the osmotic component, is responsible for the salinity-induced inhibition of greening in etiolated wheat (Triticum aestivum L. cv. Mv Béres) leaves: a comparative study.

MAIN CONCLUSION: Greening was partially (in 300 mM NaCl, CaCl2, 600 mM KNO3 or KCl) or fully inhibited (in 600 mM NaCl, NaNO3 or NaCl:KCl) by the ionic and not the osmotic component of salinity. Although high soil salinity is an increasing global problem, not much is known about how direct exposure to salinity affects etiolated leaves of seedlings germinating in the soil and then reaching the surface. We investigated the effect of various salt treatments on the greening process of leaves in 8- to 11-day-old etiolated wheat (Triticum aestivum L. Mv. Béres) seedlings. Etiolated leaf segments pre-treated on different salt (600 mM NaCl:KCl 1:1, 600 mM NaCl, 600 mM KCl, 600 mM NaNO3, 600 mM KNO3, 300 mM KCl, 300 mM NaCl or 300 mM CaCl2) or isosmotic polyethylene glycol 6000 (PEG) solutions for 1.5 h in the dark and then greened for 16 h on the same solutions were studied. Leaf segments greened on PEG (osmotic stress) or on 300 mM KCl had similar chloroplasts compared to control samples greened on Hoagland solution. Slightly slower development of chloroplast structure and function (photosynthetic activity) was observed in segments greened on 300 mM NaCl or CaCl2, 600 mM KNO3 or KCl. However, etioplast-to-chloroplast transformation and chlorophyll accumulation were fully inhibited and peculiar prothylakoid swelling occurred in segments greened on 600 mM NaCl, NaNO3 or NaCl:KCl (1:1) solutions. The data indicate that not the high osmolarity of the used salt solution, but its ions, especially Na+, had the strongest negative impact on these processes.

The pattern of photosynthetic response and adaptation to changing light conditions in lichens is linked to their ecological range.

Epiphytic lichens constitute an important component of biodiversity in both deforested and forest ecosystems. Widespread occurrence is the domain of generalist lichens or those that prefer open areas. While, many stenoecious lichens find shelter only in a shaded interior of forests. Light is one of the factors known to be responsible for lichen distribution. Nevertheless, the effect of light intensity on photosynthesis of lichen photobionts remain largely unknown. We investigated photosynthesis in lichens with different ecological properties in relation to light as the only parameter modified during the experiments. The aim was to find links between this parameter and habitat requirements of a given lichen. We applied the methods based on a saturating light pulse and modulated light to perform comprehensive analyses of fast and slow chlorophyll fluorescence transient (OJIP and PSMT) combined with quenching analysis. We also examined the rate of CO2 assimilation. Common or generalist lichens, i.e. Hypogymnia physodes, Flavoparmelia caperata and Parmelia sulcata, are able to adapt to a wide range of light intensity. Moreover, the latter species, which prefers open areas, dissipates the excess energy most efficiently. Conversely, Cetrelia cetrarioides considered an old-growth forest indicator, demonstrates definitely lower range of energy dissipation than other species, although it assimilates CO2 efficiently both at low and high light. We conclude that functional plasticity of the thylakoid membranes of photobionts largely determines the dispersal abilities of lichens and light intensity is one of the most important factors determining the specificity of a species to a given habitat.

Photosynthetic Reactions: From Molecules to Function, and from Simple Models to Complex Systems.

Photosynthesis is the basic process for life on Earth-and the one that has changed life history most drastically [...].

The origin, evolution and diversification of multiple isoforms of light-dependent protochlorophyllide oxidoreductase (LPOR): focus on angiosperms.

Light-dependent protochlorophyllide oxidoreductase (LPOR) catalyzes the reduction of protochlorophyllide to chlorophyllide, which is a key reaction for angiosperm development. Dark operative light-independent protochlorophyllide oxidoreductase (DPOR) is the other enzyme able to catalyze this reaction, however, it is not present in angiosperms. LPOR, which evolved later than DPOR, requires light to trigger the reaction. The ancestors of angiosperms lost DPOR genes and duplicated the LPORs, however, the LPOR evolution in angiosperms has not been yet investigated. In the present study, we built a phylogenetic tree using 557 nucleotide sequences of LPORs from both bacteria and plants to uncover the evolution of LPOR. The tree revealed that all modern sequences of LPOR diverged from a single sequence ∼1.36 billion years ago. The LPOR gene was then duplicated at least 10 times in angiosperms, leading to the formation of two or even more LPOR isoforms in multiple species. In the case of Arabidopsis thaliana, AtPORA and AtPORB originated in one duplication event, in contrary to the isoform AtPORC, which diverged first. We performed biochemical characterization of these isoforms in vitro, revealing differences in the lipid-driven properties. The results prone us to hypothesize that duplication events of LPOR gave rise to the isoforms having different lipid-driven activity, which may predispose them for functioning in different locations in plastids. Moreover, we showed that LPOR from Synechocystis operated in the lipid-independent manner, revealing differences between bacterial and plant LPORs. Based on the presented results, we propose a novel classification of LPOR enzymes based on their biochemical properties and phylogenetic relationships.

Non-Typical Fluorescence Effects and Biological Activity in Selected 1,3,4-thiadiazole Derivatives: Spectroscopic and Theoretical Studies on Substituent, Molecular Aggregation, and pH Effects.

The below article presents the results of spectroscopic research, theoretical (time-dependent density functional theory (TD-DFT)), microbiological, and antioxidative calculations for three compounds from the group of 1,3,4-thiadiazoles: 2-amino-5-phenyl-1,3,4-thiadiazole (TB), 2-amino-5-(2-hydroxyphenyl)-1,3,4-thiadiazole (TS), 2-amino-5-(2-hydroxy-5-sulfobenzoyl)-1,3,4-thiadiazole (TSF). In the fluorescence emission spectra (TS) of solutions with varying concentrations of hydrogen ions, a particularly interesting effect of dual fluorescence was observed. The aforementioned effect was observed even more clearly in the environment of butan-1-ol, relative to the compound's concentration. Depending on the modification of the resorcylic substituent (TS and TSF), we observed the emergence of two separate, partially overlapping, fluorescence emission spectra or a single emission spectrum. Interpretation of the obtained spectra using stationary and time-resolved spectroscopy allowed the correlation of the effect's emergence with the phenomenon of molecular aggregation (of a particular type) as well as, above all, the structure of the substituent system. The overlap of said effects most likely induces the processes related to the phenomenon of charge transfer (in TS) and is responsible for the observed fluorescence effects. Also, the position of the -OH group (in the resorcylic ring) is significant and can facilitate the charge transfer (CT). The determinations of the changes in the dipole moment and TD-DFT calculations further corroborate the above assumption. The following paper presents the analysis (the first for this particular group of analogues) of the fluorescence effects relative to the changes in the structure of the resorcylic group combined with pH effects. The results of biological studies also indicate the highest pharmacological potential of the analogue in the case where the effects of dual fluorescence emission are observed, which predisposes this particular group of fluorophores as effective fluorescence probes or potential pharmaceuticals with antimycotic properties.

MGDG, PG and SQDG regulate the activity of light-dependent protochlorophyllide oxidoreductase.

Light-dependent protochlorophyllide oxidoreductase (POR) is a plant enzyme involved in the chlorophyll biosynthesis pathway. POR reduces one of the double bonds of the protochlorophyllide (Pchlide) using NADPH and light. In the present study, we found out that phosphatidylglycerol and sulfoquinovosyl diacylglycerol are allosteric regulators of the nucleotide binding, which increase the affinity towards NADPH a 100-fold. Moreover, we showed for the first time that NADH can, like NADPH, form active complexes with Pchlide and POR, however, at much higher concentrations. Additionally, monogalactosyldiacylglycerol (MGDG) was shown to be the main factor responsible for the red shift of the fluorescence emission maximum of Pchlide:POR:NADPH complexes. Importantly, the emission maximum at 654 nm was obtained only for the reaction mixtures supplemented with MGDG and at least one of the negatively charged plant lipids. Moreover, the site-directed mutagenesis allowed us to identify amino acid residues that may be responsible for lipid binding and Pchlide coordination. Our experiments allowed us to identify six different Pchlide:POR complexes that differ in the fluorescence emission maxima of the pigment. The results presented here reveal the contribution of thylakoid lipids in the regulation of the chlorophyll biosynthesis pathway; however, the molecular mechanisms of this process are to be clarified.

Insight into the oligomeric structure of PORA from A. thaliana.

Light-dependent protochlorophyllide oxidoreductase (POR, E.C. 1.3.1.33) is a plant enzyme that directly needs light to conduct a biochemical reaction. In the present paper we confirmed that POR forms large oligomers in solution before binding of substrates. We carried out the research using different techniques: cross-linking, native gel electrophoresis and FRET measurements. Mass spectrometry analysis of the cross-link products provided the first structural data about the organisation of the oligomer of POR. The results indicated that the catalytic motifs of the adjacent subunits become close to each other upon binding of substrates. Moreover, we identified two mutations of POR that disturbed its oligomerisation properties: Δ85-88 and Δ240-270. Additionally, a complete loss of the catalytic activity was observed for the following mutations: Δ189-194, Δ240-270, Δ318-331 and Δ392-393.

Light-Dependent Protochlorophyllide Oxidoreductase: Phylogeny, Regulation, and Catalytic Properties.

This Current Topic focuses on light-dependent protochlorophyllide oxidoreductase (POR, EC 1.3.1.33). POR catalyzes the penultimate reaction of chlorophyll biosynthesis, i.e., the light-triggered reduction of protochlorophyllide to chlorophyllide. In this reaction, the chlorin ring of the chlorophyll molecule is formed, which is crucial for photosynthesis. POR is one of very few enzymes that are driven by light; however, it is unique in the need for its substrate to absorb photons to induce the conformational changes in the enzyme, which are required for its catalytic activation. Moreover, the enzyme is also involved in the negative feedback of the chlorophyll biosynthesis pathway and controls chlorophyll content via its light-dependent activity. Even though it has been almost 70 years since the first isolation of active POR complexes, our knowledge of them has markedly advanced in recent years. In this review, we summarize the current state of knowledge of POR, including the phylogenetic roots of POR, the mechanisms of the regulation of POR genes expression, the regulation of POR activity, the import of POR into plastids, the role of POR in PLB formation, and the molecular mechanism of protochlorophyllide reduction by POR. To the best of our knowledge, no previous review has compiled such a broad set of recent findings about POR.

Influence of Solvent Polarizability on the Keto-Enol Equilibrium in 4-[5-(naphthalen-1-ylmethyl)-1,3,4-thiadiazol-2-yl]benzene-1,3-diol.

This work presents spectroscopic studies of the keto-enol equilibrium induced by solvent polarizability in 4-[5-(naphthalen-1-ylmethyl)-1,3,4-thiadiazol-2-yl]benzene-1,3-diol a strong antiproliferative and anticancer thiadiazol derivative. Electronic absorption, steady state and time resolved fluorescence, and infrared spectroscopies were applied to investigate the keto and enol forms of this compound in a series of polar and non-polar solvents. The enol form dominates in polar solvents while, surprisingly, the keto form dominates in non-polar solvents with high average electric dipole polarizability e.g. n-alkenes. The electronic absorption spectrum of this derivative is more dependent on spatially averaged electric dipole polarizability of the solvent than on Kirkwood's correlation or on Lorenz-Lorenz electric polarizability. By analogy of n-alkanes to the alkyl parts of lipids, one can expect that the transformation of 1,3,4-thiadiazoles to the keto form may be facilitated in the hydrophobic core of the lipid membrane. Such a transition may be of great practical importance for the design of biologically active pharmaceutics, which are able to interact with the hydrophobic regions of cell membranes in a specific manner.

Protochlorophyllide and protochlorophyll in model membranes - an influence of hydrophobic side chain moiety.

In the present work, a comparative study of protochlorophyllide- and protochlorophyll-lipid interaction was performed on liposomes prepared from phospholipids and galactolipids, which had a pigment content varying from 0.1 to 4mol%. The incorporation of pigment molecules into the lipid bilayer and pigment-pigment interactions were investigated. Protochlorophyllide entered the lipid bilayer spontaneously and showed fluorescence spectra characteristic of its monomers. Similar spectra were observed for protochlorophyll where its concentration was low. However, the fluorescence maxima of protochlorophyll monomers were blue-shifted compared to those of protochlorophyllide by about 5nm. Protochlorophyll at high concentrations formed transient aggregates that showed an additional fluorescence band with a maximum at around 685nm, especially in liposomes prepared from phospholipids. For both compounds, the Stern-Volmer constant for KI quenching was much lower in liposomes than in solution, which confirmed the incorporation of these compounds into the lipid bilayer. Two populations of protochlorophyll that differed in their accessibility to quenching by KI were determined, and the proportions between them for different lipids are discussed. Protochlorophyllide showed such heterogeneity only in DPPC membranes. Quenching with 5- and 16-SASL revealed a localization of the porphyrin ring of both Pchl and Pchlide in the polar headgroup area of the lipid bilayer. The side chain of protochlorophyll forced these molecules to localize deeper in the bilayer in the case of DPPC in gel phase.

Cadmium inhibitory action leads to changes in structure of ferredoxin:NADP(+) oxidoreductase.

This study deals with the influence of cadmium on the structure and function of ferredoxin:NADP(+) oxidoreductase (FNR), one of the key photosynthetic enzymes. We describe changes in the secondary and tertiary structure of the enzyme upon the action of metal ions using circular dichroism measurements, Fourier transform infrared spectroscopy and fluorometry, both steady-state and time resolved. The decrease in FNR activity corresponds to a gentle unfolding of the protein, caused mostly by a nonspecific binding of metal ions to multiple sites all over the enzyme molecule. The final inhibition event is most probably related to a bond created between cadmium and cysteine in close proximity to the FNR active center. As a result, the flavin cofactor is released. The cadmium effect is compared to changes related to ionic strength and other ions known to interact with cysteine. The complete molecular mechanism of FNR inhibition by heavy metals is discussed.Electronic supplementary material The online version of this article (doi:10.1007/s10867-012-9262-z) contains supplementary material, which is available to authorized users.

Impact of electromagnetic stimulation on the mechanical and photophysical properties of alfalfa leaves.

The aim of the study was to measure the tensile strength of 4-year-old alfalfa leaves cultivated from seeds exposed to pre-sowing stimulation with He-Ne laser light for 1 or 5 min (designated respectively as F1 and F5) and alternating magnetic field with the exposure time of 1 or 5 min (respectively, L1 and L5). The leaves were measured in terms of blade length and width as well as petiole thickness prior to the tensile test. Measurements were also conducted to determine the chlorophyll fluorescence lifetime and content of photosynthetic pigments (chlorophyll a, b, a + b and carotenoids). The observed tensile strength was between 1.59 and 2.45 N. In the test group, the observed strength was lower in leaves collected from the top and central section of the stem but higher in the bottom part of the stem as compared to the control. The maximum increase of the tearing tensile force relative to the control (C) was observed for the L1 and F1 stimulation samples in leaves collected from the top and bottom parts of the stem, while the maximum decrease for that force was recorded for L5 leaves from the middle and top part of the stem. Chlorophyll fluorescence lifetimes and the overall content of photosynthetic pigments (chlorophyll a and b and carotenoids) were noticeably decreased in the leaves subjected to the stressors/stimulants applied. The results obtained for F1, L5 and, L1 stimulation revealed a decrease in fluorescence lifetimes. The content of photosynthetic pigments was also decreased under the influence of laser light stimulation (L1). This was a clear indication of plant ageing.

Spectroscopic characterization and assessment of microbiological potential of 1,3,4-thiadiazole derivative showing ESIPT dual fluorescence enhanced by aggregation effects.

In the presented study, advanced experimental techniques, including electronic absorption and fluorescence spectroscopies [with Resonance Light Scattering (RLS)], measurements of fluorescence lifetimes in the frequency domain, calculations of dipole moment fluctuations, quantum yields, and radiative and non-radiative transfer constants, were used to characterize a selected analogue from the group of 1,3,4-thiadiazole, namely: 4-[5-(naphthalen-1-ylmethyl)-1,3,4-thiadiazol-2-yl]benzene-1,3-diol (NTBD), intrinsically capable to demonstrate enol → keto excited-states intramolecular proton transfer (ESIPT) effects. The results of spectroscopic analyses conducted in solvent media as well as selected mixtures were complemented by considering biological properties of the derivative in question, particularly in terms of its potential microbiological activity. The compound demonstrated a dual fluorescence effect in non-polar solvents, e.g. chloroform and DMSO/H2O mixtures, while in polar solvents only a single emission maximum was detected. In the studied systems, ESIPT effects were indeed observed, as was the associated phenomenon of dual fluorescence, and, as demonstrated for the DMSO: H2O mixtures, the same could be relatively easily induced by aggregation effects related to aggregation-induced emission (AIE). Subsequently conducted quantum-chemical (TD-)DFT calculations supported further possibility of ESIPT effects. The following article provides a comprehensive description of the spectroscopic and biological properties of the analyzed 1,3,4-thiadiazole derivatives, highlighting its potential applicability as a very good fluorescence probes as well as a compound capable of high microbiological activity.

The Role of Membranes and Lipid-Protein Interactions in the Mg-Branch of Tetrapyrrole Biosynthesis.

Chlorophyll (Chl) is essential for photosynthesis and needs to be produced throughout the whole plant life, especially under changing light intensity and stress conditions which may result in the destruction and elimination of these pigments. All steps of the Mg-branch of tetrapyrrole biosynthesis leading to Chl formation are carried out by enzymes associated with plastid membranes. Still the significance of these protein-membrane and protein-lipid interactions in Chl synthesis and chloroplast differentiation are not very well-understood. In this review, we provide an overview on Chl biosynthesis in angiosperms with emphasis on its association with membranes and lipids. Moreover, the last steps of the pathway including the reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide), the biosynthesis of the isoprenoid phytyl moiety and the esterification of Chlide are also summarized. The unique biochemical and photophysical properties of the light-dependent NADPH:protochlorophyllide oxidoreductase (LPOR) enzyme catalyzing Pchlide photoreduction and located to peculiar tubuloreticular prolamellar body (PLB) membranes of light-deprived tissues of angiosperms and to envelope membranes, as well as to thylakoids (especially grana margins) are also reviewed. Data about the factors influencing tubuloreticular membrane formation within cells, the spectroscopic properties and the in vitro reconstitution of the native LPOR enzyme complexes are also critically discussed.

Dynamics of Etiolation Monitored by Seedling Morphology, Carotenoid Composition, Antioxidant Level, and Photoactivity of Protochlorophyllide in Arabidopsis thaliana.

Although etiolated Arabidopsis thaliana seedlings are widely used as a model to study the de-etiolation process, the etiolation itself at the molecular level still needs elucidation. Here, we monitored the etiolation dynamics for wild type A. thaliana seedlings and lutein-deficient (lut2) mutant between 2 and 12 days of their growth in the absence of light. We analyzed the shape of the apex, the growth rate, the carotenoids and protochlorophyllide (Pchlide) accumulation, and the light-dependent protochlorophyllide oxidoreductase (LPOR) transcripts. Differences concerning the apical hook curvature and cotyledon opening among seedlings of the same age were observed, mostly after day 6 of the culture. We categorized the observed apex shapes and presented quantitatively how distribution among the categories changed during 12 days of seedling growth. The Pchlide654/Pchlide633 ratio, corresponding to the amount of the photoactive Pchlide, was the highest in the youngest seedlings, and decreased with their age. LPORA, LPORB, and LPORC transcripts were detected in etiolated seedlings, and their content decreased during seedling growth. Expression of SAG12 or SAG13 senescence markers, depletion in antioxidants, and excess ion leakage were not observed during the etiolation. Lack of lutein in the lut2 mutant resulted in slow Pchlide accumulation and affected other xanthophyll composition.

[Light-induced reduction of protochlorophyllide in angiosperms and chloroplast development]. / Indukowana swiatlem redukcja protochlorofilidu u okrytonasiennych a rozwój chloroplastów.

One of the final reactions of chlorophyll (Chl) biosynthesis, e.g: photoreduction of protochlorophyllide (Pchlid) to chlorophyllide (Chlid) is a light-induced process in Angiosperm plants and it is catalyzed by light-dependent NADPH-Pchlid oxidoreductase (1.3.1.33; LPOR). In darkness, Chl biosynthesis is stopped at the stage of Pchlid formation. Seedlings and plastids develop according to a different pattern than that observed in the light. Moreover, synthesis of some proteins of the photosynthetic apparatus is inhibited. Light triggers the Pchlid photoreduction to Chlid, which induces the cascade of biochemical reactions and structural changes leading to the assembly of thylakoid membranes. In the present paper, the current knowledge on LPOR protein, mechanism of Pchlid to Chlid photoreduction, the role of lipid structure in etioplasts as well as spectral properties of Pchlid in etiolated seedlings and model systems is summarized.

Understanding chlorophylls: central magnesium ion and phytyl as structural determinants.

Phytol, a C20 alcohol esterifying the C-17(3) propionate, and Mg2+ ion chelated in the central cavity, are conservative structural constituents of chlorophylls. To evaluate their intramolecular structural effects we prepared a series of metal- and phytyl-free derivatives of bacteriochlorophyll a and applied them as model chlorophylls. A detailed spectroscopic study on the model pigments reveals meaningful differences in the spectral characteristics of the phytylated and non-phytylated pigments. Their analysis in terms of solvatochromism and axial coordination shows how the central Mg and phytyl residue shape the properties of the pigment. Surprisingly, the presence/absence of the central Mg has no effect on the solvatochromism of (bacterio)chlorophyll pi-electron system and the hydrophobicity of phytyl does not interfere with the first solvation shell of the chromophore. However, both residues significantly influence the conformation of the pigment macrocycle and the removal of either residue increases the macrocycle flexibility. The chelation of Mg has a flattening effect on the macrocycle whereas bulky phytyl residue seems to control the conformation of the chromophore via steric interactions with ring V and its substituents. The analysis of spectroscopic properties of bacteriochlorophyllide (free acid) shows that esterification of the C-17(3) propionate is necessary in chlorophylls because the carboxyl group may act as a strong chelator of the central Mg. These observations imply that the truncated chlorophylls used in theoretical studies are not adequate as models of native chromophores, especially when fine effects are to be modeled.