Towards elucidating the energy of the first excited singlet state of xanthophyll cycle pigments by X-ray absorption spectroscopy

The first excited singlet state (S(1)) of carotenoids (also termed 2A(g)(-)) plays a key role in photosynthetic excitation energy transfer due to its close proximity to the S(1) (Q(y)) level of chlorophylls. The determination of carotenoid 2A(g)(-) energies by optical techniques is difficult; transitions from the ground state (S(0), 1A(g)(-)) to the 2A(g)(-) state are forbidden ("optically dark") due to parity (g <-- //--> g) as well as pseudo-parity selection rules (- <-- //--> -). Of particular interest are S(1) energies of the so-called xanthophyll-cycle pigments (violaxanthin, antheraxanthin and zeaxanthin) due to their involvement in photoprotection in plants. Previous determinations of S(1) energies of violaxanthin and zeaxanthin by different spectroscopic techniques vary considerably. Here we present an alternative approach towards elucidation of the optically dark states of xanthophylls by near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The indication of at least one pi* energy level (about 0.5 eV below the lowest 1B(u)(+) vibronic sublevel) has been found for zeaxanthin. Present limitations and future improvements of NEXAFS to study optically dark states of carotenoids are discussed. NEXAFS combined with simultaneous optical pumping will further aid the investigation of these otherwise hardly accessible states.     

Towards elucidating the energy of the first excited singlet state of xanthophyll cycle pigments by X-ray absorption spectroscopy

The first excited singlet state (S₁) of carotenoids (also termed 2A_g⁻) plays a key role in photosynthetic excitation energy transfer due to its close proximity to the S₁ (Q_y) level of chlorophylls. The determination of carotenoid 2A_g⁻ energies by optical techniques is difficult; transitions from the ground state (S₀, 1A_g⁻) to the 2A_g⁻ state are forbidden (“optically dark”) due to parity (g ← //→ g) as well as pseudo-parity selection rules (− ← //→ −). Of particular interest are S₁ energies of the so-called xanthophyll-cycle pigments (violaxanthin, antheraxanthin and zeaxanthin) due to their involvement in photoprotection in plants. Previous determinations of S₁ energies of violaxanthin and zeaxanthin by different spectroscopic techniques vary considerably. Here we present an alternative approach towards elucidation of the optically dark states of xanthophylls by near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The indication of at least one π* energy level (about 0.5 eV below the lowest 1B_u⁺ vibronic sublevel) has been found for zeaxanthin. Present limitations and future improvements of NEXAFS to study optically dark states of carotenoids are discussed. NEXAFS combined with simultaneous optical pumping will further aid the investigation of these otherwise hardly accessible states.     

Pigment-pigment interactions in PCP of Amphidinium carterae investigated by nonlinear polarization spectroscopy in the frequency domain

Peridinin-chlorophyll a-protein (PCP) is a unique antenna complex in dinoflagellates that employs peridinin (a carotenoid) as its main light-harvesting pigment. Strong excitonic interactions between peridinins, as well as between peridinins and chlorophylls (Chls) a, can be expected from the short intermolecular distances revealed by the crystal structure. Different experimental approaches of nonlinear polarization spectroscopy in the frequency domain (NLPF) were used to investigate the various interactions between pigments in PCP of Amphidinium carterae at room temperature. Lineshapes of NLPF spectra indicate strong excitonic interactions between the peridinin's optically allowed S(2) (1Bu(+)) states. A comprehensive subband analysis of the distinct NLPF spectral substructure in the peridinin region allows us to assign peridinin subbands to the two Chls a in PCP having different S(1)-state lifetimes. Peridinin subbands at 487, 501, and 535 nm were assigned to the longer-lived Chl, whereas a peridinin subband peaking at 515 nm was detected in both clusters. Certain peridinin(s), obviously corresponding to the subband centered at 487 nm, show(s) specific (possibly Coulombic?) interaction between the optically dark S(1)(2A(g)(-)) and/or intramolecular charge-transfer (ICT) state and S(1) of Chl a. The NLPF spectrum, hence, indicates that this peridinin state is approximately isoenergetic or slightly above S(1) of Chl a. A global subband analysis of absorption and NLPF spectra reveals that the Chl a Q(y)-band consists of two subbands (peaking at 669 and 675 nm and having different lifetimes), confirmed by NLPF spectra recorded at high pump intensities. At the highest applied pump intensities an additional band centered at S(1)/ICT transition of peridinin.     

Energy expenditure associated with softening and stiffening of echinoderm connective tissue

Catch connective tissue of echinoderms at rest (in the standard state) either stiffens or softens in response to different kinds of stimulation. The energy consumption associated with the changes was estimated by measurement of the oxygen consumption rate (VO(2)) in three types of connective tissues-echinoid catch apparatus (CA), holothuroid body-wall dermis (HD), and asteroid body-wall dermis (AD). Mechanical stimulation by repetitive compression (10%-15% strain), which increased viscosity measured by creep tests, was employed for inducing the stiff state. Noradrenaline (10(-3) mol l(-1)), which decreased viscosity of CA, and static 80% compressive strain, which decreased viscosity of HD, were used to induce the soft state in the respective tissues. The VO(2) (in μl/g/h) values of the standard state were 2.91 (CA), 1.41 (HD), and 0.56 (AD), which were less than 1/4 of the VO(2) of the resting body-wall muscle of the starfish. The VO(2) of the stiff state was about 1.5 times greater than that of the standard state in all types of connective tissues. The VO(2) of the soft state was 3.4 (CA)-9.1 (HD) times greater than that of the standard state. The economical nature of catch connective tissue in posture maintenance is discussed.     

Spontaneous Generation of Chirality in Simple Diaryl Ethers

We studied the spontaneous formation of chiral crystals of four diaryl ethers, 3‐phenoxybenzaldehyde, 1 ; 1,3‐dimethyl‐2‐phenoxybenzene, 2 ; di(4‐aminophenyl) ether, 3 ; and di(p‐tolyl) ether, 4 . Compounds 1 , 3 , and 4 form conformationally chiral molecules in the solid state, while the chirality of 2 arises from the formation of supramolecular helices. Compound 1 is a liquid at ambient temperature, but 2 , 3 , 4 are crystalline, and solid‐state CD‐spectroscopy showed that they could be obtained as optically active bulk samples. It should be noted that the optical activity arise upon crystallization, and no optically active precursors were used. Indeed, even commercial samples of 3 and 4 were found to be optically active, giving evidence for the ease at which total spontaneous resolution may occur in certain systems. Chirality 27:425‐429, 2015. © 2015 Wiley Periodicals, Inc.     

Carotenoid-bacteriochlorophyll energy transfer in LH2 complexes studied with 10-fs time resolution

In this report, we present a study of carotenoid-bacteriochlorophyll energy transfer processes in two peripheral light-harvesting complexes (known as LH2) from purple bacteria. We use transient absorption spectroscopy with approximately 10 fs temporal resolution, which is necessary to observe the very fast energy relaxation processes. By comparing excited-state dynamics of the carotenoids in organic solvents and inside the LH2 complexes, it has been possible to directly evaluate their energy transfer efficiency to the bacteriochlorophylls. In the case of okenone in the LH2 complex from Chromatium purpuratum, we obtained an energy transfer efficiency of etaET2=63+/-2.5% from the optically active excited state (S2) and etaET1=61+/-2% from the optically dark state (S1); for rhodopin glucoside contained in the LH2 complex from Rhodopseudomonas acidophila these values become etaET2=49.5+/-3.5% and etaET1=5.1+/-1%. The measurements also enabled us to observe vibrational energy relaxation in the carotenoids' S1 state and real-time collective vibrational coherence initiated by the ultrashort pump pulses. Our results are important for understanding the dynamics of early events of photosynthesis and relating it to the structural arrangement of the chromophores.     

Kinetically controlled optical resolution of racemic norbornene aldehyde derivatives

A new method for obtaining optically pure 5-norbornene 2-endo-aldehyde derivatives was developed. The reaction of a diastereomeric mixture of the ene acetals 2 and 2', derived from racemic norbornene aldehydes (+/-)-1 and chiral nonracemic (S,S)-hydrobenzoin 7, with NBS (0.5-0.6 eq.) in the presence of H(2)O proceeded in a kinetically controlled manner to give the optically pure hydroxy aldehydes 3 along with the intact ene acetals 2'. Both compounds 3 and 2' were converted into the optically pure norbornene aldehydes 1 and ent-1, respectively. This method opens the way to produce various types of 5-norbornene 2-endo-aldehydes with 3-exo- or 3-endo-substituents in optically pure forms.     

Enantioselective photoreaction of achiral tropolone ether in inclusion crystals with optically active host compounds

Achiral 4-isopropyltropolone methyl ether (3) included in an optically active host compound (-)-1 yielded optically active photocyclization products (-)-5 and (-)-6 in 96 and 90% ee, respectively, upon photoirradiation in the solid state.     

Optically active cantharidin analogues possessing selective inhibitory activity on Ser/Thr protein phosphatase 2B (calcineurin): implications for the binding mode

Cantharidin is well known as a potent serine/threonine protein phosphatase 1 and 2A (PP1 and PP2A) inhibitor, with less potent inhibitory activity for PP2B, which regulates T-cell proliferation. We synthesized and evaluated four optically pure stereoisomers of 1-substituted norcantharidin analogues. The absolute stereochemistry of each stereoisomer was determined based on X-ray crystal structure analysis. Remarkably, optically active cantharidin analogues having (1S)-configuration showed selective inhibition of PP2B, without inhibiting PP1 or PP2A.     

Microbial reduction of ethyl acetoacetate using Sclerotium rolfsii

Summary A method for the quantitative enantioselective bioreduction of ethyl acetoacetate [1] to optically pure (+)-S-ethyl-3 hydroxybutyrate [II] usingSclerotium rolfsii mycelium is described. In a synthetic medium 1 g mycelium (dry weight) could convert 1 g of I to II within 2–3 days of fermentation (pH 5.8, 30°C). This is the first report demonstrating use ofS. rolfsii biomass for asymmetric reduction to get chiral building blocks.     

Evolution in bioids: hypercompetitivity as a source of bistability and a possible role of metal complexes as prenucleoprotic mediators of molecular asymmetry.

Spontaneous production of optically active compounds can occur through kinetic instability of an asymmetric steady state in open systems, in which two enantiomeric autocatalysts compete for a common prochiral substrate in a stereoselective reaction of order n greater than 2. For the case of n=3, a proof of instability of a symmetric reacting state in the general case, and functions of reaction parameters ('Chemical Reynolds Numbers') governing the existence and stability of 7 different steady states are derived. The 'extinct state' (without autocatalyst) is stable; a finite amount of products is required to shift it into one of the reacting steady states. A mutation from one state into another in such system ('bioids') involves an amplification of different 'kinds of information', as 'stochastic' (noise into dissipative structures), 'molecular' (autocatalysts), and 'stoichimetric' information. Stereospecific third order kinetics are believed to be realizable on octahedral metal complexes with two-dentated ligands and to have played a role in the prebiological evolution of optically active compounds.     

A scalable synthesis of (+)-coronafacic acid

A facile, efficient, and scalable synthesis of optically pure coronafacic acid by resolution of racemic coronafacic acid obtained using an improved version of Watson's method has been developed. By optimizing the boron-mediated aldol reaction of Watson, we were able to prepare 2.1 g of racemic coronafacic acid. This was coupled with (S)-4-isopropyl-2-oxazolidinone to give a mixture of diastereomeric coronafacyl oxazolidinones, which were readily separable by silica-gel column chromatography to give 630 mg of optically pure (+)-coronafacic acid.     

Light-harvesting function of carotenoids in photo-synthesis: the roles of the newly found 1(1)Bu- state

This minireview article highlights the energetics and the dynamics of the 1(1)B(u)(-) and 3(1)A(g)(-) states of carotenoids discovered very recently. Those "hidden" covalent states have been revealed by measurements of resonance-Raman excitation profiles of crystalline carotenoids. The dependence of the energies of the low-lying singlet states, including the 1(1)B(u)(+), 3(1)A(g)(-), 1(1)B(u)(-), and 2(1)A(g)(-) states, on the number of conjugated double bonds (n) is in agreement with the extrapolation of those state energies calculated by Tavan and Schulten for shorter polyenes (P. Tavan and K. Schulten, Journal of Chemical Physics, 1986, vol. 85, pp. 6602-6609). It has also been shown that the internal-conversion processes among those singlet states take place in accord with the state ordering, i.e., 1(1)B(u)(+) --> 1(1)B(u)(-) --> 2(1)A(g)(-) --> 1(1)A(g)(-) (the ground state) for carotenoids having n = 9 and 10, whereas 1(1)B(u)(+) --> 3(1)A(g)(-) --> 1(1)B(u) (-) --> 2(1)A(g)(-) --> 1(1)A(g)(-) for carotenoids having n = 11-13. Radiative transitions of 1(1)B(u)(+) --> 2(1)A(g)(-) and 1(1)B(u)(-) --> 2(1)A(g)(-) as well as a branching into the triplet manifold of 1(1)B(u)(-) --> 1(3)A(g) --> 1(3)B(u) have also been found. Those low-lying singlet states of all-trans carotenoids can facilitate multiple channels of singlet-energy transfer to bacteriochlorophyll in the LH2 antenna complexes of purple photosynthetic bacteria. Thus, the newly found 1(1)B(u)(-) and 3(1)A(g)(-) states of carotenoids need to be incorporated into the picture of carotenoid-to-bacteriochlorophyll singlet-energy transfer.     

On the reactions of DL-1,3-dimethylthymine epoxide with amines and L-amino acid derivatives

Reaction of DL-1,3-dimethylthymine epoxide (1) with aniline gave (2A) and (3A). Isomerization of (2A) provided (3A), (4) and (5). Reaction of (1) with L-amino acid ethyl ester afforded four optically active diastereomers.     

Polarization-Dependent Evanescent Optical Field Interaction with Atom and Its Effect on Atomic Emission Spectra

In this report, we present experimental results on evanescent optical field (EOF) interaction with ⁸⁵Rb vapor atoms. A circularly polarized EOF, 780 nm in wavelength, optically pumps the ⁸⁵Rb atoms from its upper ground state to its D2 excited state. A probe beam connecting the lower ground state of D2 transition stimulates emission of radiation from the excited atoms, which indicates additional atomic population excited by the circularly polarized EOF. Theoretical analysis and experimental results indicate that the circularly polarized EOF excites the D2 transition from ⁸⁵Rb(5²S_1/2, F?=?3, m _F ?=?±1) ground state to excited state (5²P_3/2, F’?=?3, m _F ?=?0) that creates an additional population to this state, which can be detected with the help of a probe laser beam.     

Ultrafast carotenoid-to-chlorophyll singlet energy transfer in the cytochrome b6f complex from Bryopsis corticulans

Ultrafast carotenoid-to-chlorophyll (Car-to-Chl) singlet excitation energy transfer in the cytochrome b(6)f (Cyt b(6)f) complex from Bryopsis corticulans is investigated by the use of femtosecond time-resolved absorption spectroscopy. For all-trans-alpha-carotene free in n-hexane, the lifetimes of the two low-lying singlet excited states, S(1)(2A(g)(-)) and S(2)(1B(u)(+)), are determined to be 14.3 +/- 0.4 ps and 230 +/- 10 fs, respectively. For the Cyt b(6)f complex, to which 9-cis-alpha-carotene is bound, the lifetime of the S(1)(2A(g)(-)) state remains unchanged, whereas that of the S(2)(1B(u)(+)) state is significantly reduced. In addition, a decay-to-rise correlation between the excited-state dynamics of alpha-carotene and Chl a is clearly observed. This spectroscopic evidence proves that the S(2)(1B(u)(+)) state is able to transfer electronic excitations to the Q(x) state of Chl a, whereas the S(1)(2A(g)(-)) state remains inactive. The time constant and the partial efficiency of the energy transfer are determined to be 240 +/- 40 fs and (49 +/- 4)%, respectively, which supports the overall efficiency of 24% determined with steady-state fluorescence spectroscopy. A scheme of the alpha-carotene-to-Chl a singlet energy transfer is proposed based on the excited-state dynamics of the pigments.     

Production of D-Alanine by Corynebacterium fascians

A strain identified as Corynebacterium fascians was found to accumulate extracellular D-alanine from glycerol. Cultural conditions for the accumulation of D-alanine were investigated and, as a result, a yield of 7 g of D-alanine per liter was obtained after a 96-h incubation in a medium containing 5% glycerol, 4% (NH(4))(2)HPO(4), and 0.3% corn steep liquor. Optical purity of D-alanine was dependent upon the concentration of corn steep liquor. At the optimal condition, almost optically pure D-alanine was formed and readily isolated (5 g/liter) from the fermentation broth. The product was not contaminated with any detectable amount of other amino acids, except for glycine which was present at a concentration of less than 1 percent.     

57Fe and 1H electron-nuclear double resonance of three doubly reduced states Escherichia coli sulfite reductase

We have employed electron-nuclear double resonance (ENDOR) spectroscopy to study the 57Fe hyperfine interactions in the bridged-siroheme [4Fe-4S] cluster that forms the catalytically active center of the two-electron-reduced hemoprotein subunit of Escherichia coli NADPH-sulfite reductase (SiR2-). Previous electron paramagnetic resonance (EPR) and M?ssbauer studies have shown that this enzyme oxidation state can exist in three distinct spectroscopic forms: (1) a "g = 2.29" EPR species that predominates in unligated SiR2-, in which the siroheme Fe2+ is believed to be in an S = 1 state; (2) a "g = 4.88" type of EPR species that predominates in SiR2- in the presence of small amounts of guanidinium sulfate, in which the siroheme Fe2+ is in an S = 2 state; and (3) a classical "g = 1.94" type of EPR species that is seen in SiR2- ligated with CO, in which the siroheme Fe2+ is in an S = 0 state. In all three species, the cluster is in the [4Fe-4S]1+ state, and two distinct types of Fe site are seen in M?ssbauer spectroscopy. ENDOR studies confirm the M?ssbauer assignments for the cluster 57Fe in the g = 1.94 state, with A values of 37, 37, and 32 MHz for site I and ca. 19 MHz for site II. The hyperfine interactions are not too different on the g = 2.29 state, with site I Fe showing more anisotropic A values of 32, 24, and 20 MHz (site II was not detected).(ABSTRACT TRUNCATED AT 250 WORDS)     

Structure-activity relationships of a novel class of endothelin receptor selective antagonists; 6-carboxy-2-isopropylamino-5,7-diarylcyclopenteno[1,2-b]pyridines

The synthesis and structure-activity relationships of 6-carboxy-2-isopropylamino-5,7-diarylcyclopenteno[1,2-b]pyridine class of ET(A) receptor selective antagonists were described. These derivatives were prepared from the optically active key intermediates (3, 4, 10, and 13). Optimization of the substituent at the 2-position of the bottom 4-methoxyphenyl ring of the lead compound 1 led to identification of 2-hydroxy-1-methylethoxy (2g and h), hydroxyalkyl (2i, m, and p), 3-methoxy-2-methylpropyl (2t and u), N-acetyl-N-methylaminomethyl (2v), and 2-(dimethylcarbamoyl)propyl (2w) derivatives that showed greater than 1000-fold selectivity for the ET(A) receptor over the ET(B) receptor with excellent binding affinity (IC(50)<0.10 nM). Further screening of these compounds by assessing the plasma exposures at 1 h, 4 h, and 8 h after oral administration (3 or 10 mg/kg) in rats led to identification of the hydroxymethyl (2i) and 3-methoxy-2-methylpropyl (2u) derivatives exhibiting good oral bioavailability in rats.     

Purification, characterization, and preliminary crystallographic study of copper-containing nitrous oxide reductase from Pseudomonas nautica 617

The aerobic purification of Pseudomonas nautica 617 nitrous oxide reductase yielded two forms of the enzyme exhibiting different chromatographic behaviors. The protein contains six copper atoms per monomer, arranged in two centers named Cu(A) and Cu(Z). Cu(Z) could be neither oxidized nor further reduced under our experimental conditions, and exhibits a 4-line EPR spectrum (g(x)=2.015, A(x)=1.5 mT, g(y)=2.071, A(y)=2 mT, g(z)=2.138, A(z)=7 mT) and a strong absorption at approximately 640 nm. Cu(A) can be stabilized in a reduced EPR-silent state and in an oxidized state with a typical 7-line EPR spectrum (g(x)=g(y)= 2.021, A(x) = A(y)=0 mT, g(z) = 2.178, A(z)= 4 mT) and absorption bands at 480, 540, and approximately 800 nm. The difference between the two purified forms of nitrous oxide reductase is interpreted as a difference in the oxidation state of the Cu(A) center. In form A, Cu(A) is predominantly oxidized (S = (1)/(2), Cu(1.5+)-Cu(1.5+)), while in form B it is mostly in the one-electron reduced state (S = 0, Cu(1+)-Cu(1+)). In both forms, Cu(Z) remains reduced (S = 1/2). Complete crystallographic data at 2.4 A indicate that Cu(A) is a binuclear site (similar to the site found in cytochrome c oxidase) and Cu(Z) is a novel tetracopper cluster [Brown, K., et al. (2000) Nat. Struct. Biol. (in press)]. The complete amino acid sequence of the enzyme was determined and comparisons made with sequences of other nitrous oxide reductases, emphasizing the coordination of the centers. A 10.3 kDa peptide copurified with both forms of nitrous oxide reductase shows strong homology with proteins of the heat-shock GroES chaperonin family.