Keto Carbonyl Group Research Articles

Cyclization-cycloaddition casade of rhodium carbenoids using different carbonyl groups. Highlighting the position of interaction

A series of 3-diazoalkanediones, when treated with a catalytic quantity of a rhodium(II) carboxylate, were found to afford oxabicyclic dipolar cycloadducts derived by the trapping of a carbonyl ylide intermediate. The reaction involves generation of the 1,3-dipole by intramolecular cyclization of the keto carbenoid onto the oxygen atom of the neighboring keto group. Both five- and six-ring carbonyl ylides are formed with the same efficiency. A study of the tandem cyclization-cycloaddition cascade of several alpha-diazo ketoesters was also carried out, and the cascade sequence proceeded in high yield. When the interacting keto carbonyl group was replaced by an imido group, the rhodium(II)-catalyzed reaction proceeded uneventfully. In contrast, alpha-diazo amidoesters do not undergo nitrogen extrusion on treatment with a Rh(II) catalyst. Instead, the diazo portion of the molecule undergoes 1,3-dipolar cycloaddition with various dipolarophiles to give substituted pyrazoles as the final products.

Cytotoxic activity of cerium complexes with coumarin derivatives. Molecular modeling of the ligands.

Cerium complexes of Umbellipherone, Mendiaxon, Warfarin, Coumachlor, and Niffcoumar have been synthesized by reaction of the ligands with cerium nitrate in a stoichiometric ratio of 1:2. The formation of the complexes has been proved on the basis of elemental analysis, conductivities, IR spectroscopy, and 1H-NMR spectroscopy. The molecules of the ligands were optimized by means of the semiempirical quantum mechanical method PM3 to the energetically most stable conformers. All the ligands were characterized by molecular and submolecular electronic indices and the putative donor centers are proposed. It is concluded that the lactone- and the keto-carbonyl groups of Warfarin, Coumachlor, and Niffcoumar are bonded to the metal ion as bidentate ligands. The other two coumarins are bonded as monodentate ligands. Conductivity measurements show the non-electrolytic nature of the complexes. Cytotoxic screening by MTT assay was carried out. The cerium complexes were found to be more active than the inorganic salts.

Self-assembly of synthetic 8 1-hydroxy-chlorophyll analogues

The aggregation behavior of synthetic zinc-8 1-hydroxy-chlorins in nonpolar organic media and in solid thin film has been compared with that of zinc-3 1-hydroxy-chlorin, a model of bacteriochlorophyll- d in the chlorosomes of green photosynthetic bacteria. Zinc-8 1-hydroxy-13 1-oxo- and 3 1,13 1-dioxo-chlorins 1a and 3a self-aggregate in nonpolar organic solvents and in solid thin films. They show less red-shifted and wider visible absorption bands at the Soret and Q y regions and also more intense circular dichroism (CD) signals at the Soret region than self-aggregates of zinc-3 1-hydroxy-13 1-oxo-chlorin 5a. The 1H-NMR spectroscopic data indicate that the hydroxyl group at the 8 1-position of 1a coordinates with the central zinc of an adjacent molecule in the supramolecule. From the infrared spectra, we can deduce that the interactions of the functional groups of 1a in the aggregates are unspecific or weak. For this reason, the supramolecular structures are partially amorphous. Visible absorption and CD spectra establish that the lack of one of the three interactive moieties (8 1-hydroxy, 13-keto groups and central zinc) suppresses the aggregation ability of chlorins. Furthermore, zinc-3-acetyl-8 1-hydroxy-chlorin 2a does not self-aggregate in nonpolar organic solvents because both the hydroxyl and keto groups take various configurations. However, additional substitution of keto carbonyl group at the 3-position ( 1a→3a) affects the aggregation ability. It is concluded that the fixation of either the OH or CO group to the chlorin moiety possessing an exo-five-membered ring, as well as both the presence of OH, CO and central metal in the molecule and the linear location of the three interactive moieties are necessary for the formation of well-ordered self-aggregates of chlorophyll analogues, which are seen in native chlorosomes.

Excited-state electronic asymmetry of the special pair in photosynthetic reaction center mutants: absorption and Stark spectroscopy.

The electronic absorption line shape and Stark spectrum of the lowest energy Q(y)() transition of the special pair in bacterial reaction centers contain a wealth of information on mixing with charge transfer states and electronic asymmetry. Both vary greatly in mutants that perturb the chemical composition of the special pair, such as the heterodimer mutants, and in mutants that alter interactions between the special pair and the surrounding reaction center protein, such as those that add or remove hydrogen bonds. The conventional and higher-order Stark spectra of a series of mutants are presented with the aim of developing a systematic description of the electronic structure of the excited state of the special pair that initiates photosynthetic charge separation. The mutants L168HF, M197FH, L131LH and L131LH/M160LH/M197FH are known to have different hydrogen-bonding patterns to the special pair; however, they exhibit Stark effects that are very similar to wild type. By contrast, the addition of a hydrogen bond to the M-side keto carbonyl group of the special pair in M160LH greatly affects both the absorption and Stark spectra. The heterodimer special pairs, L173HL and M202HL, exhibit much larger Stark effects than wild type, with the greatest effect in the M-side mutant. Double mutants that combine the M-side heterodimer and a hydrogen-bond addition to the L-side of the special pair decrease the magnitude of the Stark effect. These results suggest that the electronic asymmetry of the dimer can be perturbed either by the formation of a heterodimer or by adding or deleting a hydrogen bond to a keto carbonyl group. From the pattern observed, it is concluded that the charge transfer state P(L)(+)P(M)(-) has a larger influence on the excited state of the dimer in wild type than the P(L)(-)P(M)(+)charge transfer state. Furthermore, asymmetry can be varied continuously, from extreme cases in which the heterodimer and hydrogen-bond effects work together, to cases in which hydrogen bonding offsets the effects of the heterodimer, to cases in which the homodimer is perturbed by hydrogen bonds. This leads to a unified model for understanding the effects of perturbations on the electronic symmetry of the special pair, and this can be connected with perturbations on the properties of many other systems such as donor-acceptor-substituted polyenes.

Novel 1,3-diene synthesis from alkyne and ethylene by ruthenium-catalyzed enyne metathesis

A novel 1,3-diene synthetic method from alkyne and ethylene was developed using ruthenium-catalyzed enyne metathesis. The reaction procedure is very simple: A CH 2Cl 2 solution of alkyne was stirred at room temperature under ethylene gas (1 atm.) in the presence of a catalytic amount of ruthenium benzylidene complex. The yield was good and the conversion yield was high. Dienes having functional groups such as a keto-carbonyl group, silyloxy group, ester, and ketal were synthesized from the corresponding alkynes and ethylene. In this reaction, the alkyne having a hetero atom at the propargylic position gave a good result and ether oxygen or amine nitrogen that coordinate strongly to the ruthenium catalyst prevents the reaction.

Enantioselective hydrogenation of α-keto esters over cinchona-Pt/Al 2O 3 catalyst. Molecular modelling of the substrate–modifier interaction

The substrate–modifier interaction involved in the enantioselective hydrogenation of α-keto esters over cinchonidine-Pt/Al 2O 3 catalyst was investigated by molecular modelling. The model is based on our earlier kinetic and nuclear magnetic resonance (NMR) results as well as on analogies found in experimental organic chemistry. The model suggests the formation of a weak complex between the modifier and the substrate in the liquid phase. In the above complex, the modifier provides a specific shielding effect (SE). Due to the particular character of shielding, the α-keto ester can interact with the metal surface only by its unshielded site. If the reactivity of the substrate in the shielded [substrate–modifier] complex is higher than that of the free substrate, pronounced enantio-differentiation (ED) should be observed. In one of the shielded forms, the proper directionality of the quinuclidine nitrogen towards the keto carbonyl group provides the increased reactivity of the keto carbonyl group. The `shielding effect' model can explain both the ED and the rate acceleration (RA) effect observed in the hydrogenation of methyl and ethyl pyruvate, methyl benzoylformate, pantolactone and trifluoroacetophenone (TFAP) over cinchonidine-Pt/Al 2O 3 catalyst. The `shielding effect' model is the first model which can elucidate the substrate specificity of the above enantioselective hydrogenation reaction.

Synthesis, physicochemical characterisation and cytotoxic screening of new complexes of cerium, lanthanum and neodymium with Warfarin and Coumachlor sodium salts

The complexes of the types Ln(HW) 3• nH 2O and Ln(HC) 3• nH 2O [where Ln = Ce, La, Nd; HW = Warfarin; HC = Coumachlor] have been synthesized by reaction of Warfarin Sodium, resp. Coumachlor Sodium and the appropriate lanthanide nitrates in various stoichiometric ratios. The formation of the complexes have been proved on the basis of elemental analysis, conductivities, IR spectroscopy and 1H-NMR spectroscopy. It is concluded that the lacton- and the keto-carbonyl groups of the ligands are bonded to the metal ion as bidentate ligand. The conductivity measurements show non-electrolytic nature of the complexes. Cytotoxicity determination by MTT-assay shows that the inorganic salts and the complexes with Warfarin did not show any significant activity. The complexes with Coumachlor were found to be more cytotoxic. The most active compound was the complex of Cerium with Coumachlor. All tested complexes showed similar in vitro cytotoxic profiles. This fact is in agreement with the formation of the complexes.

Non‐bonding molecular factors influencing the stretching wavenumbers of the conjugated carbonyl groups of bacteriochlorophyll a

The influence of the macrocycle core size on the keto and acetyl carbonyl stretching wavenumbers of bacteriochlorophyll a and of its Co, Ni, Cu and Zn derivatives was examined. Their sensitivity to the polarity and polarizability of the molecular environment was also studied. It is concluded that the macrocycle core size has a clear, but limited, influence on these wavenumbers. As a consequence, interchange between the five- and six-coordinated states of the central Mg atom of bacteriochlorophyll a should shift the keto and acetyl carbonyl stretching wavenumbers, but by not more than 3 and 1 cm-1, respectively. In line with previous observations on chlorophylls, a marked dependence of the keto and acetyl carbonyl stretching wavenumbers of bacteriophaeophytin a and of bacteriochlorophyll a and its Ni and Zn derivatives on solvent permittivity and refractive index was observed. The sensitivity of the keto carbonyl wavenumber appears to be largely independent of the presence and nature of the central metal and is very close to that reported earlier for chlorophyll a. The wavenumber of the acetyl carbonyl stretching mode of bacteriochlorophyll a derivatives is overall less sensitive to any of these parameters than that of the keto carbonyl group. © 1998 John Wiley & Sons, Ltd.

Non-bonding molecular factors influencing the stretching wavenumbers of the conjugated carbonyl groups of bacteriochlorophylla

The influence of the macrocycle core size on the keto and acetyl carbonyl stretching wavenumbers of bacteriochlorophyll a and of its Co, Ni, Cu and Zn derivatives was examined. Their sensitivity to the polarity and polarizability of the molecular environment was also studied. It is concluded that the macrocycle core size has a clear, but limited, influence on these wavenumbers. As a consequence, interchange between the five- and six-coordinated states of the central Mg atom of bacteriochlorophyll a should shift the keto and acetyl carbonyl stretching wavenumbers, but by not more than 3 and 1 cm-1, respectively. In line with previous observations on chlorophylls, a marked dependence of the keto and acetyl carbonyl stretching wavenumbers of bacteriophaeophytin a and of bacteriochlorophyll a and its Ni and Zn derivatives on solvent permittivity and refractive index was observed. The sensitivity of the keto carbonyl wavenumber appears to be largely independent of the presence and nature of the central metal and is very close to that reported earlier for chlorophyll a. The wavenumber of the acetyl carbonyl stretching mode of bacteriochlorophyll a derivatives is overall less sensitive to any of these parameters than that of the keto carbonyl group. © 1998 John Wiley & Sons, Ltd.

Effects of hydrogen bonds on the redox potential and electronic structure of the bacterial primary electron donor.

The primary donor, P, of photosynthetic bacterial reaction centers (RCs) is a dimer of excitonically interacting bacteriochlorophyll (BChl) molecules. The two constituents are named PL and PM to designate their close association with the L- and M-subunits, respectively, of the RC protein. A series of site-directed mutants of RCs from Rhodobacter sphaeroides has been constructed in order to model the effects of hydrogen bonding on the redox midpoint potential and electronic structure of P. The leucine residue at position M160 was genetically replaced with eight other amino acid residues capable of donating a hydrogen bond to the C9 keto carbonyl group of the PM BChl a molecule of P. Fourier transform (FT) (pre)resonance Raman spectroscopy with 1064 nm excitation was used to (i) determine the formation and strengths of hydrogen bonds on this latter keto carbonyl group in the reduced, neutral state (PO), and (ii) determine the degree of localization of the positive charge on one of the two constituent BChl molecules of P in its oxidized, radical cation state (P*+). A correlation was observed between the strength of the hydrogen bond and the increase in PO/P*+ redox midpoint potential. This correlation is less pronounced than that observed for another series of RC mutants where hydrogen bonds to the four pi-conjugated carbonyl groups of P were broken or formed uniquely involving histidinyl residues [Mattioli, T. A., Lin, X., Allen, J. P. and Williams, J. C. (1995) Biochemistry 34, 6142-6152], indicating that histidinyl residues are more effective in raising the PO/P*+ redox midpoint potential via hydrogen bond formation than are other hydrogen bond-forming residues. In addition, an increase in positive charge localization is correlated with the strength of the hydrogen bond and with the PO/P*+ redox midpoint potential. This latter correlation was analyzed using an asymmetric bacteriochlorophyll dimer model based on Hückel-type molecular orbitals in order to obtain estimates of certain energetic parameters of the primary donor. Based on this model, the correlation is extrapolated to the case of complete localization of the positive charge on PL and gives a predicted value for the P/P+ redox midpoint potential similar to that experimentally determined for the Rb. sphaeroides HL(M202) heterodimer. The model yields parameters for the highest occupied molecular orbital energies of the two BChl a constituents of P which are typical for the oxidation potential of isolated BChl a in vitro, suggesting that the protein, as compared to many solvents, does not impart atypical redox properties to the BChl a constituents of P.

Resonance Raman spectroscopy of a light-harvesting protein from the brown alga Laminaria saccharina.

Resonance Raman spectroscopy of an antenna protein from the brown alga Laminaria saccharina has been used to investigate the molecular structure of this light-harvesting complex (LHC) at the level of its bound pigments, chlorophylls (chl) a and c and the xanthophyll fucoxanthin. Evidence has been obtained for the conservation of pigment structure during the isolation procedure used. Six chl a and two chl c molecules are indicated from the positions and relative contributions of stretching modes of their keto-carbonyl groups. Of special interest is the presence of a population of chls a having a protein-binding conformation highly similar to that seen in antenna proteins from higher plants, possibly indicating a common structural motif within this extended gene family. The eight fucoxanthin molecules evidenced are all in the all-trans conformation; however, one or two have a highly twisted configuration. The results are discussed in terms of common and varying structural features of LHCs in higher plants and algae.

Novel 1,3-Diene Synthesis from Alkyne and Ethylene by Ruthenium-Catalyzed Enyne Metathesis

Abstract A novel 1,3-diene synthetic method from alkyne and ethylene was developed using ruthenium-catalyzed enyne metathesis. The reaction procedure is very simple: A CH 2 Cl 2 solution of alkyne was stirred at room temperature under ethylene gas (1 atm.) in the presence of a catalytic amount of ruthenium benzylidene complex. The yield was good and the conversion yield was high. Dienes having functional groups such as a keto-carbonyl group, silyloxy group, ester, and ketal were synthesized from the corresponding alkynes and ethylene. In this reaction, the alkyne having a hetero atom at the propargylic position gave a good result and ether oxygen or amine nitrogen that coordinate strongly to the ruthenium catalyst prevents the reaction.

Control of Molecular Arrangement in Langmuir−Blodgett Films of Chlorophyll a Prepared in Various Gas Phases Studied by Ultraviolet−Visible and Infrared Spectroscopies and Atomic Force Microscopy,

Attenuated total reflection (ATR)/Fourier-transform infrared (FT-IR) spectra were measured for one-monolayer Langmuir−Blodgett (LB) films of chlorophyll a (Chl-a) fabricated in an argon (Ar) atmosphere. The frequencies of CO stretching bands and marker bands for the coordination number of the central Mg atom suggest that Chl-a takes a five-coordinated dimer in the films prepared in the Ar atmosphere. Ultraviolet−visible (UV−vis) as well as IR spectra were obtained for multilayer LB films of Chl-a prepared in air, Ar, nitrogen (N2), and oxygen (O2) atmospheres. In the cases of the multilayer LB films, spectral features in the CO stretching band region suggest that Chl-a exists as a monomer in the film prepared in air while it assumes a dimer in the films prepared in the Ar, N2, and O2 atmospheres. The marker bands for the coordination number of the Mg atom in the IR spectra indicate that Chl-a is in a five-coordinated state. These results imply that one can control the molecular arrangement in the LB films of Chl-a by changing the atmosphere in which they are prepared. Both the UV−vis and IR spectra of the LB films fabricated in the Ar, N2, and O2 atmospheres are almost identical to each other. This means that O2 does not affect the stability and structure of Chl-a in the monolayer on the aqueous subphase. On the basis of the postulate that CO2, which exists only in air, caused a change in pH of the aqueous subphase, we investigated pH-dependent IR spectral changes for the LB films. The results indicate that the dimer of Chl-a changes into pheophytine a (Phe-a) below pH 6.0 and that the monomer species do not exist through pH 6.0−4.0; therefore, it is unlikely that CO2 changes the pH. Probably CO2 coordinates to the central Mg atom as a fifth ligand in the LB films prepared in air, preventing the keto carbonyl group of another Chl-a molecule from coordinating to it.

Fourier transform Raman investigation of the electronic structure and charge localization in a bacteriochlorophyll-bacteriopheophytin dimer of reaction centers from Rhodobacter sphaeroides

The Raman spectra of a bacteriochlorophyll (BChl)-bacteriopheophytin (BPhe) heterodimeric primary electron donor from mutant Rhodobacter sphaeroides reaction centers, where histidine M202 has been replaced by leucine, have been obtained in (pre)resonance with its lowest electronic Q y state using 1064 nm excitation. For reaction centers where the heterodimer is in its reduced, neutral state, the dominant Raman scattering is from the BPhe constituent of the ground state heterodimer, and indicates that the 1064 nm-excitation wavelength is interacting with an electronic state or transition which is largely BPhe in character, such as a charge transfer state with a dominant (BChl +BPhe −) configuration. Previous electron paramagnetic studies have established that the unpaired electron spin density on the oxidized, cation radical heterodimer resides almost totally on the BChl constituent. Near infrared electronic absorption spectra of the oxidized heterodimer exhibit a weak band at 900 nm, characteristic of a BChl a ·+ species. The (pre)resonance Raman spectrum of this cation radical, excited at 1064 nm, exhibits a 1723 cm −1 band attributable to the C 9 keto carbonyl group of the BChl constituent whose corresponding band in wild type is observed at 1715–1717 cm −1. This 1723 cm −1 frequency for BChl in the oxidized heterodimer, compared to 1691 cm −1 for the reduced state, represents an oxidation-induced increase in frequency of +32 cm −1, similar to what is observed for the one-electron oxidation of chlorophylls in non-protic solvents. The results presented here indicate that the oxidation-induced change in vibrational frequency of the C 9 keto carbonyl group of the BChl in the reaction center is not significantly perturbed by the protein.

Modeling the bonding changes in chlorophyll cation radicals: resonance Raman spectroscopy of nickel(II) methyl pyropheophorbide a

Electrochemistry, UV-Vis absorption and resonance Raman (RR) spectra are reported for nickel methyl pyropheophorbide a (NiMPPh) and its cation radical, in order to help understand the structural changes upon oxidation in chlorophyll cation radicals. RR band shifts, and also the oxidation potential, are consistent with the highest occupied molecular orbital (HOMO) of NiMPPh being an A 1u-like orbital, which interacts with the keto-carbonyl group on the isocyclic ring. Of particular interest is the upshift of the keto-carbonyl stretching mode in the NiMPPh cation radical. The strengthening of the carbonyl bond upon oxidation is consistent with the expected anti-bonding character of the A 1u-like orbital with respect to the keto CO bond.

Pigment‐Protein Interactions in the Antenna‐Reaction Center Complex of Heliobacillus mobilis

AbstractMembrane fragments of Heliobacillus (Hc.) mobilis were characterized using resonance Raman (RR) spectroscopy in order to determine the configuration of the neurosporene carotenoid, the pigment‐protein interactions of the bacteriochlorophyll (BChl) g molecules, and the Chl a‐like chlorin pigments present in the antenna‐reaction center complex constituting the photosynthetic apparatus. Using 363.8 nm excitation, the Raman contributions of the BChl g molecules were selectively resonantly enhanced over those of the carotenoid and the Chl a‐like chlorin pigments. The RR spectrum of BChl g in these membranes excited at 363.8 nm exhibits bands at 1614 and 1688 cm−1, which correspond to a CaCm methine bridge stretching mode and a keto carbonyl group stretching mode, respectively. Both of these bands are 16 cm−1 wide (full width at half maximum, FWHM), indicating that a sole population of BChl g molecules is being enhanced at this excitation wavelength. The observed frequency of the CaCm stretching mode (1614 cm−1) indicates that the bulk of BChl g molecules is pentacoordinated with only one axial ligand to the central Mg atom while that of the keto carbonyl stretching mode (1668 cm−1) indicates that these groups are engaged in a hydrogen bond. This homogeneous population of BChl g molecules bound to the heliobacterial core polypeptides is in contrast to the heterogeneous population of Chl a molecules bound to the core polypeptides of the reaction center of photosystem I of Synechocystis 6803 as observed by the inhomogeneously broadened C9 keto carbonyl band in its RR spectrum. The RR spectrum of the Chl a‐like chlorin pigments in Hc. mobilis excited at 441.6 nm exhibits a broad keto carbonyl band (43 cm−1 FWHM) with components at 1665, 1683 and 1695 cm−1, indicating several populations of these pigments differing in their protein interactions at the level of the keto carbonyl group. Fourier transform (FT) pre‐RR spectroscopic measurements of intact whole cells and membrane fragments at room temperature using 1064 nm excitation indicate that high quality vibrational spectra of the BChl g molecules can be obtained with no photodegradation. Low‐temperature FT Raman spectra excited at 1064 nm reveals an inhomogeneously broadened 1665 cm−1 band corresponding to the C9 keto carbonyl stretching mode. Spectral deconvolution and second derivative analysis of this band reveal that it is comprised of components at 1665, 1682 and 1695 cm−1, the latter two most likely arising from BChl g photoconversion products. Excitation using 885 nm to enhance the preresonance effect of the BChl g molecules yields an FT Raman spectrum where the keto carbonyl band at 1665 cm−1 is narrow, as is the case in the Soret RR spectra, reflecting a sole population of BChl g molecules, which are engaged in an H bond. The RR spectrum of the neurosporene molecule in Hc. mobilis membranes excited at 496.5 nm is compared to that of 1,2‐dihydroneurosporene bound in a cis configuration in reaction centers of Rhodopseudomona viridis and to that of the same carotenoid in its all‐trans configuration extracted from these reaction centers in the presence of light. The similarity of this latter RR spectrum with that of neurosporene in the Hc. mobilis membranes indicates that it is bound in an all‐trans configuration.

Structure and protein binding interactions of the primary donor of the Chloroflexus aurantiacus reaction center.

Soret resonance, QX resonance, and QY near-infrared Fourier transform (FT) (pre)resonance Raman spectroscopies were used to determine pigment-protein interactions of specific bacteriochlorin molecules in the reaction center from Chloroflexus aurantiacus. FT Raman spectroscopy, using 1064 nm excitation, was used to selectively obtain preresonance and resonance vibrational Raman spectra of the primary donor (P) of reaction centers (RCs) from Chloroflexus aurantiacus in the Po and P.+ states, respectively. The FT Raman spectrum of RCs in their neutral P (Po) state exhibits bands at 1605, 1632, 1648, and 1696 cm-1 which are attributable to P in its resting neutral state. Specifically, the latter three Raman bands can be assigned to the conjugated C2 acetyl and C9 keto carbonyl groups of the bacteriochlorophyll (BChl) molecules constituting P. The observation of at least three such bands is indicative of a non-monomeric nature of P, consistent with the proposal that it is a dimer of BChl molecules. The 1632 cm-1 band is consistent only with a hydrogen bonded BChl acetyl carbonyl, while the 1648 cm-1 band is assigned to a non-hydrogen bonded acetyl carbonyl. The 1696 cm-1 band is consistent only with a non-hydrogen bonded keto carbonyl group; from the unusually high intensity of this latter band compared to the others, we propose that the 1696 cm-1 band contains contributions from two keto carbonyl groups, both free of hydrogen bonds. From published protein sequence alignments of the L and M subunits of Rhodobacter (Rb.) sphaeroides and Chloroflexus aurantiacus we assign the 1632 cm-1 band as arising from the C2 acetyl carbonyl of the analogous PM constituent of P, which is hydrogen bonded to tyrosine M187 in the Chloroflexus RC, and propose a pigment-protein structural model for the primary donor of Chloroflexus aurantiacus. The FT Raman spectrum of RCs in the P degrees+ state indicates that one component of the 1696 cm-1 band has upshifted 21 cm-1 to 1717 cm-1. Compared to Rb. sphaeroides which showed a 26 cm-1 upshift for the corresponding band, the 21 cm-1 upshift indicates that the + charge is more delocalized over the P.+ species of Chloroflexus; we estimate that ca. 65% of the + charge is localized on one of the two BChl molecules of the Chloroflexus primary donor as compared to ca. 80% for Rb. sphaeroides. The consequences of the proposed structure of the Chloroflexus primary donor in terms of its Po/P.+ redox midpoint potential are discussed.

Reactions of carbonyl compounds in basic solutions. Part 21. The mechanisms of the alkaline hydrolysis of substituted methyl 2-(2-oxopropyl)- and 2-(2-oxo-2-phenylethyl)-benzoates and 2-(2-acetylphenyl)- and 2-(2-benzoylphenyl)-acetates.

Rate coefficients have been measured for the alkaline hydrolysis of methyl 2-[2-oxa-2-(3- or 4-substituted phenyl)ethyl]benzoates, 2-[2-(3- or 4-substituted benzoyl)phenyl]acetates, 2-(2-oxopropyl) and 2-(1,1-dimethyl-2-oxopropyl)benzoates, 2-(2-acetylphenyl)acetate and 2-(2-acetylphenyl)-2,2-dimethylacetates in 70%(v/v) dioxane–water at 30.0 °C. Those for the six parent esters were also measured at 45.0 and 60.0 °C and the enthalpies and entropies of activation have been evaluated. The relative rates of hydrolysis, activation parameters and substituent effects have been used to demonstrate neighbouring participation by the keto-carbonyl groups in the alkaline hydrolysis of the esters under study. For comparable systems, participation by six-membered ring intermediates appears somewhat less advantageous than five-membered.

The reaction of some endo-5-acetyl norborn-2-enes with benzene thiol: a thermally induced non-allylic [1,3]-acetyl shift, as anticipated by a photochemical precedent

The thermal reaction of benzene thiol with rac-1-(4aS*,5R*,8S*,8aR*-5,8-methano-1,2,3,4,4a,5,8,8a-octahydro-4a-naphthyl)ethanone (10b), an endo-5-acetylnorborn-2-ene with a particularly close distance between one olefinic carbon and the keto carbon, and with its 8aS* epimer 10a, has been investigated. Besides the conventional unrearranged adducts 16a, b and 17a, b of the thiol to the C=C bonds of 10a, b, an adduct 18 was formed from 10b that involved a [1,3]-acetyl shift; no other products were formed. The dependence of adduct ratios on thiol concentration is consistent with two competing reactions of an intermediate radical 14b formed by addition of thiyl radical to the C=C bond of 10b, namely, abstraction of hydrogen from thiol (which is the conventional reaction path) vs. intramolecular attack of the radical on the keto group inducing the [1,3] shift of the latter. This shift constitutes an intramolecular variety of the known alkyl transposition reaction of ketones initiated by a reversible attack of an alkyl radical on a keto carbonyl group to generate an intermediate tert-alkoxyl radical; however, it is very much faster than the intermolecular reaction and corresponds to an effective molarity between 107 and 1010. These findings have a bearing on the mechanism of photorearrangements homologous to the ODPM (oxa-di-π-methane) photorearrangement, such as ODPE (oxa-di-π-ethane) and higher homologues, in that they support a two-step mechanism involving this type of acyl shift. Keywords: oxadi-π-methane photorearrangement; oxadi-π-ethane photorearrangement; acetyl shift, non-allylic [1,3]; thiol, addition to C=C bonds; norbornenes.

Symmetric structural features and binding site of the primary electron donor in the reaction center of Chlorobium.

The protein binding interactions of the constituent bacteriochlorophyll a molecules of the primary electron donor, P840, in isolated reaction centers from Chlorobium limicola f thiosulphatophilum and the electronic symmetry of the radical cation P840+. were determined using near-infrared Fourier transform (FT) Raman spectroscopy excited at 1064 nm. The FT Raman vibrational spectrum of P840 indicates that it is constituted of a single population of BChl a molecules which are spectrally indistinguishable. The BChl a molecules of P840 are pentacoordinated with only one axial ligand on the central Mg atom, and the pi-conjugated C2 acetyl and C9 keto carbonyls are free of hydrogen-bonding interactions. The FT Raman spectrum of P840+. exhibits a 1707 cm-1 band attributable to a BChl a C9 keto carbonyl group vibrational frequency that has upshifted 16 cm-1 upon oxidation of P840; this upshift is exactly one-half of that expected for the one-electron oxidation of monomeric BChl a in vitro. The 16 cm-1 upshift, thus, indicates that the resulting +1 charge is equally shared between two BChl a molecules. This situation is markedly different from that of the oxidized primary donor of the purple bacterial reaction center of Rhodobacter sphaeroides, (i) which exhibits a 1717 cm-1 band that has upshifted 26 cm-1, indicating an asymmetric distribution of the resulting +1 charge over the two constituent BChl a molecules, and (ii) whose H-bonding pattern with respect to the pi-conjugated carbonyl groups is asymmetric.(ABSTRACT TRUNCATED AT 250 WORDS)