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.     

Radium-224 in natural waters measured by γ-ray spectrometry

A method based on Ge(Li) γ-ray spectrométry is applied to the determination of ²²⁴Ra (t_{$\text{1}\text{2}$}= 3.64 days) in natural waters. The ²²⁴Ra is first removed from several hundred liters of water by preconcentration onto manganese dioxide-impregnated acrylic fibers. The fibers are leached, radium is coprecipitated with barium sulfate, and the γ-ray activity is counted so that activity ratios among ²²⁴Ra, ²²⁵Ra and ²²⁶Ra can be calculated. Concentrations are determined by using the ²²⁶Ra concentration determined on a small separate sample. Results from samples collected from ground water, estuarine, and continental shelf environments are presented.     

Reactions of some organic peroxides with triphenylphosphine and sodium dialkyl phosphites

The reactions of the allylic peroxides ascaridole, cyclohexadiene endoperoxide, and α-phellandrene peroxide with triphenylphosphine follow an S_N ^i'-type mechanism. In contrast, the acyclic allylic compounds, allyl tert-butyl peroxide, α-cumyl cyclohexenyl peroxide, and tert-butyl cyclohexenyl peroxide apparently react with triphenylphosphine by a free-radical mechanism. The saturated cyclic peroxide dihydroascaridole, in which there is no possibility of an allylic rearrangement, gives with triphenylphosphine a mixture of olefinic alcohols. Di-n-butyl peroxide is readily cleaved by sodium dialkyl phosphites, but sterically hindered peroxides do not react under similar conditions. Reaction can, however, take place at the oxygen adjacent to a large group if a smaller substituent is present on the other oxygen atom.     

Synthetic models of the reduced active site of superoxide reductase

We report the synthesis, structural and spectroscopic characterization, and magnetic and electrochemical studies of a series of iron(II) complexes of the pyridyl-appended diazacyclooctane ligand L(8)py(2), including several that model the square-pyramidal [Fe(II)(N(his))(4)(S(cys))] structure of the reduced active site of the non-heme iron enzyme superoxide reductase. Combination of L(8)py(2) with FeCl(2) provides [L(8)py(2)FeCl(2)] (1), which contains a trigonal-prismatic hexacoordinate iron(II) center, whereas a parallel reaction using [Fe(H(2)O)(6)](BF(4))(2) provides [L(8)py(2)Fe(FBF(3))]BF(4) (2), a novel BF(4)(-)-ligated square-pyramidal iron(II) complex. Substitution of the BF(4)(-) ligand in 2 with formate or acetate ions affords distorted pentacoordinate [L(8)py(2)Fe(O(2)CH)]BF(4) (3) and [L(8)py(2)Fe(O(2)CCH(3))]BF(4) (4), respectively. Models of the superoxide reductase active site are prepared upon reaction of 2 with sodium salts of aromatic and aliphatic thiolates. These model complexes include [L(8)py(2)Fe(SC(6)H(4)-p-CH(3))]BF(4) (5), [L(8)py(2)Fe(SC(6)H(4)-m-CH(3))]BF(4) (6), and [L(8)py(2)Fe(SC(6)H(11))]BF(4) (7). X-ray crystallographic studies confirm that the iron(II)-thiolate complexes model the square-pyramidal geometry and N(4)S donor set of the reduced active site of superoxide reductase. The iron(II)-thiolate complexes are high spin (S = 2), and their solutions are yellow in color because of multiple charge-transfer transitions that occur between 300 and 425 nm. The ambient temperature cyclic voltammograms of the iron(II)-thiolate complexes contain irreversible oxidation waves with anodic peak potentials that correlate with the relative electron donating abilities of the thiolate ligands. This electrochemical irreversibility is attributed to the bimolecular generation of disulfides from the electrochemically generated iron(III)-thiolate species.     

Application of NMR SHAPES screening to an RNA target

Several NMR screening techniques have been developed in recent years to aid in the identification of lead drug compounds. These NMR methods have traditionally been used for protein targets, and here we examine their applicability for an RNA target. We used the SHAPES compound library to test three different NMR screening methodologies: the saturation transfer difference (STD), the 2D trNOESY, and the WaterLOGSY experiments. We found that the WaterLOGSY experiment was the most sensitive method for our RNA target, the P4P6 domain of the Tetrahymena thermophila Group I intron. Using the WaterLOGSY experiment, we found that 23 of the 112 SHAPES compounds interact with P4P6. To identify which of these 23 hits bind through nonspecific interactions, we counterscreened with a linear duplex RNA control and identified one of the SHAPES compounds as interacting with P4P6 specifically. We thus demonstrated that the WaterLOGSY experiment in combination with the SHAPES compound library can be used to efficiently find RNA binding lead compounds.     

Biosynthesis of cylindrospermopsin

Studies on the biosynthesis of cylindrospermopsin (1), a potent hepatotoxin associated with the cyanobacterium Cylindrospermopsis raciborskii, indicate that 1 is an acetogenin with guanidinoacetic acid serving as the starter unit of the polyketide chain. Feeding experiments show that C14 and C15 of 1 are derived from C1 and C2 of glycine, respectively, and C4 through C13 arise from five contiguous acetate units attached head to tail. The methyl carbon on C13 originates from the C(1) pool. The starter unit, established by the incorporation of [guanidino-(13)C,alpha-(15)N]-guanidinoacetic acid into N16 and C17 of 1, does not appear to be formed from glycine by known amidination pathways. The origin of the NH-CO-NH segment in the uracil ring is also unknown.     

Synthesis of optically active 2-benzyldihydrobenzopyrans for the hypoglycemic agent englitazone

Two syntheses of some optically active 2-benzyl-2,3-dihydro-4H-benzopyrans and benzopyran-4-ones are presented. An asymmetric synthesis starting from D- and L-phenylalanine was used to provide both enantiomers of 2-benzyl-6-(methoxycarbonyl)-2,3-dihydro-4H-benzopyran-4-one 19. Phenylalanine was diazotized in aqueous sulfuric acid to 2-hydroxy-3-phenylpropionic acid 6 which was converted in four steps to 1-bromo-2-(4-methoxycarbonylphenoxy)-3-phenylpropane 11. (4R,S)-Benzamido-2-benzyl-2,3-dihydro-6-(methoxycarbonyl)-4H-1-benzopyran-4-carboxylic acid 16 was prepared from 11 by amidoalkylation with α-hydroxyhippuric acid in methanesulfonic acid solution followed by spiroalkylation to (4R,S)-2-benzyl-2,3-dihydro-6-(methoxycarbonyl)spiro[4H-benzopyran-4,4′-2′-phenyloxazolidin]-5′-one 15. After the phenyloxazolidin-5-one 15 was hydrolyzed to the spirobenzamido carboxylic acid 16 , oxidative decarboxylation with sodium hypochlorite yielded optically active 2-benzyl-6-(methoxycarbonyl)-2,3-dihydro-4H-benzopyran-4-one 19. The ketone in 19 was reduced by hydrogenation over palladium on carbon to a methylene group and the ester was converted to the aldehyde to give both isomers of the desired intermediate 2-benzyl-6-(formyl)-2,3-dihydro-4H-benzopyran 25. The second synthesis relied on an enzymatic hydrolysis of ethyl 2,3-dihydrobenzopyran-2-carboxylate 27 with the lipase from P. fluorescens to provide the desired 2R-ester. The ester group in (R)- 27 was converted to the triflate (R)- 29. Displacement of the triflate group with phenylmagnesium bromide and cuprous bromide as catalyst gave 2R-benzyl-2,3-dihydro-4H-benzopyran (R)- 30. Formylation of (R)- 30 provided 2R-benzyl-6-(formyl)-2,3-dihydro-4H-benzopyran (R)- 25 identical with that from the first synthesis. These optically active intermediates are used in the preparation of the hypoglycemic agent englitazone.     

Photochemistry of artificial photosynthetic reaction centers in liquid crystals probed by multifrequency EPR (9.5 and 95 GHz)

Photoinduced charge separation and recombination in a carotenoid-porphyrin-fullerene triad C-P-C(60)(1) have been followed by multifrequency time-resolved electron paramagnetic resonance (TREPR) at intermediate magnetic field and microwave frequency (X-band) and high field and frequency (W-band). The electron-transfer process has been characterized in the different phases of two uniaxial liquid crystals (E-7 and ZLI-1167). The triad undergoes photoinduced electron transfer, with the generation of a long-lived charge-separated state, and charge recombination to the triplet state, localized in the carotene moiety, mimicking different aspects of the photosynthetic electron-transfer process. Both the photoinduced spin-correlated radical pair and the spin-polarized recombination triplet are observed starting from the crystalline up to the isotropic phase of the liquid crystals. The W-band TREPR radical pair spectrum has allowed unambiguous assignment of the spin-correlated radical pair spectrum to the charge-separated state C(.+)-P-C(60)(.-). The magnetic interaction parameters have been evaluated by simulation of the spin-polarized radical pair spectrum and the spin-selective recombination rates have been derived from the time dependence of the spectrum. The weak exchange interaction parameter (J = +0.5 +/- 0.2 G) provides a direct measure of the dominant electronic coupling matrix element V between the C(.+)-P-C(60)(.-) radical pair state and the recombination triplet state (3)C-P-C(60). The kinetic parameters have been analyzed in terms of the effect of the liquid crystal medium on the electron-transfer process. Effects of orientation of the molecular triad in the liquid crystal are evidenced by simulations of the carotenoid triplet state EPR spectra at different orientations of the external magnetic field with respect to the director of the mesophase. The order parameter (S = 0.5 +/- 0.05) has been evaluated.     

Voltammetric determination of iron(II) and iron(III) in standard rocks and other materials

Voltammetry at a rotating platinum electrode in concentrated electrolyte solutions containing HCl and LiCl is described for simultaneous determinations of iron(II) and iron(III) in a variety of materials. Applications of the method to some U.S. Geological Survey standard rocks, a Georgian clay, and an iron-containing multiple vitamin tablet are described. For materials which can be dissolved without altering the original iron(II)/ iron(III) ratio, this method enables the two valence states to be determined rapidly and simultaneously. The technique is precise (r.s.d. ≈ 2%) and relatively free from interference, and requires only a recording d.c. polarograph.