Spectroscopy and quantum chemical modeling reveal a predominant contribution of excitonic interactions to the bathochromic shift in alpha-crustacyanin, the blue carotenoprotein in the carapace of the lobster Homarus gammarus

To resolve the molecular basis of the coloration mechanism of alpha-crustacyanin, we used (13)C-labeled astaxanthins as chromophores for solid-state (13)C NMR and resonance Raman spectroscopy of [6,6',7,7']-(13)C(4) alpha-crustacyanin and [8,8',9,9',10,10',11,11',20,20']-(13)C(10) alpha-crustacyanin. We complement the experimental data with time-dependent density functional theory calculations on several models based on the structural information available for beta-crustacyanin. The data rule out major changes and strong polarization effects in the ground-state electron density of astaxanthin upon binding to the protein. Conformational changes in the chromophore and hydrogen-bond interactions between the astaxanthin and the protein can account only for about one-third of the total bathochromic shift in alpha-crustacyanin. The exciton coupling due to the proximity of two astaxanthin chromophores is found to be large, suggesting that aggregation effects in the protein represent the primary source of the color change.     

Recrystallization of 223Ra with barium sulfate

In this work, the kinetics of barium sulfate recrystallization has been studied in acidic 0.01 mol dm^?3 sodium sulfate solution using ²²³Ra and ¹³³Ba tracers at very low total radium concentration, i.e. less than 10^?13 mol dm^?3. It was found that the system follows the homogeneous recrystallization model and that recrystallization rates, inferred by the decrease of ²²³Ra and ¹³³Ba in the aqueous solution, are fast. Therefore, even at very low concentrations, below the solubility limit, radium will be retained by barium sulfate—a mineral present in the deep underground repository.     

Ambidentate coordination of dimethyl sulfoxide in rhodium(III) complexes

The two dimethyl sulfoxide solvated rhodium(III) compounds, [Rh(dmso-κO)(5)(dmso-κS)](CF(3)SO(3))(3) (1 & 1* at 298 K and 100 K, respectively) and [Rh(dmso-κO)(3)(dmso-κS)(2)Cl](CF(3)SO(3))(2) (2), crystallize with orthorhombic unit cells in the space group Pna2(1) (No. 33), Z = 4. In the [Rh(dmso)(6)](3+) complex with slightly distorted octahedral coordination geometry, the Rh-O bond distance is significantly longer with O trans to S, 2.143(6) ? (1) and 2.100(6) ? (1*), than the mean Rh-O bond distance with O trans to O, 2.019 ? (1) and 2.043 ? (1*). In the [RhCl(dmso)(5)](3+) complex, the mean Rh-O bond distance with O trans to S, 2.083 ?, is slightly longer than that for O trans to Cl, 2.067(4) ?, which is consistent with the trans influence DMSO-κS > Cl > DMSO-κO of the opposite ligands. Raman and IR absorption spectra were recorded and analyzed and a complete assignment of the vibrational bands was achieved with support by force field calculations. An increase in the Rh-O stretching vibrational frequency corresponded to a decreasing trans-influence from the opposite ligand. The Rh-O force constants obtained were correlated with the Rh-O bond lengths, also including previously obtained values for other M(dmso)(6)(3+) complexes with trivalent metal ions. An almost linear correlation was obtained for the MO stretching force constants vs. the reciprocal square of the MO bond lengths. The results show that the metal ion-oxygen bonding of dimethyl sulfoxide ligands is electrostatically dominated in those complexes and that the stretching force constants provide a useful measure of the relative trans-influence of the opposite ligands in hexa-coordinated Rh(III)-complexes.     

2,2′-Dithiobispyrazine: about the disulfide bond

X-ray diffraction studies reveal that pyrazine-2-thiol undergoes condensation to 2,2′-dithiobispyrazine [systematic name: 2-(pyrazin-2-yldisulfanyl)pyrazine], C₈H₆N₄S₂ ( I ), under aerial conditions. In the molecule of I , the pyrazine rings are arranged in an almost perpendicular manner, with an absolute value of the C—S—S—C torsion angle of −91.45 (6)°. A search in the Cambridge Structural Database confirmed that such a conformation is typical for disulfide compounds. Three different rotamers of disulfide I were studied using quantum theoretical studies. The rotamer of lowest energy was observed in the crystalline state in the structure stabilized by hydrogen-bond, chalcogen-bond and stacking interactions. Further quantum chemical computations confirm that 2,2′-dithiobispyrazine can react according to the S_N2 mechanism.     

Human Lanosterol 14-Alpha Demethylase (CYP51A1) Is a Putative Target for Natural Flavonoid Luteolin 7,3′-Disulfate

Widespread pathologies such as atherosclerosis, metabolic syndrome and cancer are associated with dysregulation of sterol biosynthesis and metabolism. Cholesterol modulates the signaling pathways of neoplastic transformation and tumor progression. Lanosterol 14-alpha demethylase (cytochrome P450(51), CYP51A1) catalyzes one of the key steps in cholesterol biosynthesis. The fairly low somatic mutation frequency of CYP51A1, its druggability, as well as the possibility of interfering with cholesterol metabolism in cancer cells collectively suggest the clinical importance of CYP51A1. Here, we show that the natural flavonoid, luteolin 7,3′-disulfate, inhibits CYP51A1 activity. We also screened baicalein and luteolin, known to have antitumor activities and low toxicity, for their ability to interact with CYP51A1. The K_d values were estimated using both a surface plasmon resonance optical biosensor and spectral titration assays. Unexpectedly, in the enzymatic activity assays, only the water-soluble form of luteolin—luteolin 7,3′-disulfate—showed the ability to potently inhibit CYP51A1. Based on molecular docking, luteolin 7,3′-disulfate binding suggests blocking of the substrate access channel. However, an alternative site on the proximal surface where the redox partner binds cannot be excluded. Overall, flavonoids have the potential to inhibit the activity of human CYP51A1 and should be further explored for their cholesterol-lowering and anti-cancer activity.     

Investigation of the thermal decomposition and flammability of PEEK and its carbon and glass-fibre composites

Conventional thermally durable materials such as metals are being replaced with heat resistant engineering polymers and their composites in applications where burn-through resistance and structural integrity after exposure to fire are required. Poly aryl ether ether ketone (PEEK) is one such engineering polymer. Little work has been published with regards to the flammability of PEEK and its filled composites. The current study aims to assess the flammability and fire behaviour of PEEK and its composites using thermogravimetric analysis, pyrolysis combustion flow calorimetry, limiting oxygen index, a vertical flame resistance test, and fire (cone) calorimetry.     

Formation of a heteronuclear hydrolysis complex in the Th(IV)-Fe(III) system

The speciation in the mixed Th(IV)-Fe(III) system has been studied in aqueous solution in the pH range of 2.0-4.8. In the individual systems iron(III) and thorium(IV) hydrolyze easily and hydrolysis products precipitate at approximately pH ≥ 2.0 and 4.0, respectively, at the metal concentrations used in this study, 0.02-0.05 mol dm(-3). In the mixed Th(IV)-Fe(III) system precipitation of ferrihydrite takes place after months of storage at low pH values, 2.0 (six-line ferrihydrite) and 2.3 (two-line ferrihydrite), as identified by X-ray powder diffraction. In the pH range 2.9-4.5 no precipitation was observed after 24 months. Two thorium(IV)-iron(III) solutions with pH = 2.9, C(Th) = 0.02 and 0.05 mol dm(-3) and C(Fe) = 0.02 mol dm(-3), were studied by extended X-ray absorption fine structure, EXAFS, using the Fe K and Th L(3) edges, and a third solution with pH = 2.9 and C(Th) = C(Fe) = 0.40 mol dm(-3) by large angle X-ray scattering, LAXS, to determine the structure of the predominating species. A heteronuclear hydrolysis complex with the composition [Th(2)Fe(2)(μ(2)-OH)(8)(H(2)O)(12)](6+) is proposed to form in solution, with Th···Th, Th···Fe and Fe···Fe distances of 3.94(2) and 3.96(2), 3.41(3) and 3.43(2), 3.04(2) and 3.02(4) ?, as determined by EXAFS and LAXS, respectively.     

Equilibrium behavior and dilational rheology of polyelectrolyte/insoluble surfactant adsorption films: didodecyldimethylammonium bromide and sodium poly(styrenesulfonate)

The surface pressure of monolayers of an insoluble surfactant, didodecyldimethylammonium bromide (DODAB), has been measured onto subphases with different concentrations of poly(styrenesulfonate) (PSS) and at different temperatures. The presence of PSS in the subphase shifts the surface-pressure (Pi) curves to larger areas per DODAB molecule, A, and shifts the surface phase transition to higher Pi's. The presence of PSS chains decreases the surface electric potential; the decrease is higher than expected from the formation of a double layer between the DODAB molecules and the PSS segments. Increasing the temperature shifts the surface-pressure curves to higher areas and also increases the values of Pi of the surface phase transition. The effect of the PSS chains on the Pi versus A curves is contrary to the one induced by the presence of inert electrolytes in the subphase. The behavior is consistent with the existence of a dense layer of PSS segments beneath the DODAB monolayer at low PSS concentrations, c. Two PSS layers exist at higher concentrations, a dense layer immediately below the DODAB and a less-dense layer, below the first one, that protrudes deep into the subphase. The surface-pressure relaxation curves have been found to be bimodal through the whole range of surface pressures and at all the values of polymer concentration studied. These results point out that the adsorption layers behave mainly as elastic bodies, with zero-frequency elasticity, epsilon(omega = 0), which agrees with the equilibrium compressibility modulus. The increase [epsilon(omega = 1) - epsilon(omega = 0)] has been found to be independent of both polymer concentration and molecular weight. The zero-frequency-dilational viscosity, kappa(omega = 0), strongly increases with Pi in the two-dimensional condensed-liquid region. The surface viscosity strongly decreases with increasing frequency; the decreasing rate is higher than the one found for the monolayers of nonionic insoluble polymers. kappa(omega = 0) has also been found to be independent of both polymer concentration and molecular weight. These results seem to indicate that it is the film formed by the DODAB molecules and the first dense polymer layer that determines the surface viscoelastic moduli of this system.     

Processable cyanate ester resin from Cis resveratrol

Trans resveratrol isomerizes to cis resveratrol (cRes) with ultra‐violet light and undergoes cyanation to the new compound cRes tricyanate ( 2 ). Monomer 2 polymerizes to a polycyanurate that is remarkably stable to heat with a glass transition temperature > 350 °C and char yield of 73% in air up to 600 °C. The monomer 2 melts at <80 °C with a wide processing window before polymerization. The new thermosetting resin made from a natural product is a useful matrix material for polymer matrix composites requiring high‐performance characteristics. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 971–980     

Combined Raman and continuous-wave-excited two-photon fluorescence cell imaging

We demonstrate a confocal optical microscope that combines cw two-photon-excited fluorescence microscopy with confocal Raman microscopy. With this microscope fast image acquisition with fluorescence imaging can be used to select areas of interest for subsequent chemical analysis with spontaneous Raman imaging. The distribution of the UV-absorbing fluorophore Hoechst 33342 in the apoptotic HeLa cells is measured in the combined cw two-photon-excited fluorescence and Raman microscopy modes. The 647-nm line of a Kr-ion laser is used to excite both the Raman scattering and the two-photon-excited fluorescence emission. The lateral and axial resolutions in the two imaging modes are compared by use of the Gaussian beam approximation and backprojection of the focal volume through the confocal pinhole.