Bonding in a borylene complex investigated by photoionization and dissociative photoionization

The borylene complex [(OC)(5)Cr=B=N(SiMe(3))(2)] has been investigated by using threshold photoelectron-photoion coincidence spectroscopy with synchrotron radiation. The ionization energy of the parent complex and the 0?K appearance energies of the sequential CO loss channels have been determined. The derived bond-dissociation energies are used to discuss bonding and energetics in this compound.     

ssDNA binding reveals the atomic structure of graphene

We used AFM to investigate the interaction of polyelectrolytes such as ssDNA and dsDNA molecules with graphene as a substrate. Graphene is an appropriate substrate due to its planarity, relatively large surfaces that are detectable via an optical microscope, and straightforward identification of the number of layers. We observe that in the absence of the screening ions deposited ssDNA will bind only to the graphene and not to the SiO(2) substrate, confirming that the binding energy is mainly due to the π-π stacking interaction. Furthermore, deposited ssDNA will map the graphene underlying structure. We also quantify the π-π stacking interaction by correlating the amount of deposited DNA with the graphene layer thickness. Our findings agree with reported electrostatic force microscopy (EFM) measurements. Finally, we inspected the suitability of using a graphene as a substrate for DNA origami-based nanostructures.     

Binding of 8-anilino-1-naphthalenesulfonate to lecithin:cholesterol acyltransferase studied by fluorescence techniques

The solvatochromic fluorescent probe 8-anilino-1-naphthalenesulfonate (ANS) has been used to study the hydrophobicity and conformational dynamics of lecithin:cholesterol acyltransferase (LCAT). The ANS to LCAT binding constant was estimated from titrations with ANS, keeping a constant concentration of LCAT (2 microM). Apparent binding constant was found to be dependent on the excitation. For the direct excitation of ANS at 375 nm the binding constant was 4.7 microM(-1) and for UV excitation at 295 nm was 3.2 microM(-1). In the later case, not only ANS but also tryptophan (Trp) residues of LCAT is being excited. Fluorescence spectra and intensity decays show an efficient energy transfer from tryptophan residues to ANS. The apparent distance from Trp donor to ANS acceptor, estimated from the changes in donor lifetime was about 3 nm and depends on the ANS concentration. Steady-state and time-resolved fluorescence emission and anisotropies have been characterized. The lifetime of ANS bound to LCAT was above 16 ns which is characteristic for it being in a hydrophobic environment. The ANS labeled LCAT fluorescence anisotropy decay revealed the correlation time of 42 ns with a weak residual motion of 2.8 ns. These characteristics of ANS labeled LCAT fluorescence show that ANS is an excellent probe to study conformational changes of LCAT protein and its interactions with other macromolecules.     

Influence of ion pairing on the oxidation of iodide by MLCT excited states

The oxidation of iodide to diiodide, I(2)˙(-), by the metal-to-ligand charge-transfer (MLCT) excited state of [Ru(deeb)(3)](2+), where deeb is 4,4'-(CO(2)CH(2)CH(3))(2)-2,2'-bipyridine, was quantified in acetonitrile and dichloromethane solution at room temperature. The redox and excited state properties of [Ru(deeb)(3)](2+) were similar in the two solvents; however, the mechanisms for excited state quenching by iodide were found to differ significantly. In acetonitrile, reaction of [Ru(deeb)(3)](2+*) and iodide was dynamic (lifetime quenching) with kinetics that followed the Stern-Volmer model (K(D) = 1.0 ± 0.01 × 10(5) M(-1), k(q) = 4.8 × 10(10) M(-1) s(-1)). Excited state reactivity was observed to be the result of reductive quenching that yielded the reduced ruthenium compound, [Ru(deeb(-))(deeb)(2)](+), and the iodine atom, I˙. In dichloromethane, excited state quenching was primarily static (photoluminescence amplitude quenching) and [Ru(deeb(-))(deeb)(2)](+) formed within 10 ns, consistent with the formation of ion pairs in the ground state that react rapidly upon visible light absorption. In both solvents the appearance of I(2)˙(-) could be time resolved. In acetonitrile, the rate constant for I(2)˙(-) growth, 2.2 ± 0.2 × 10(10) M(-1) s(-1), was found to be about a factor of two slower than the formation of [Ru(deeb(-))(deeb)(2)](+), indicating it was a secondary photoproduct. The delayed appearance of I(2)˙(-) was attributed to the reaction of iodine atoms with iodide. In dichloromethane, the growth of I(2)˙(-), 1.3 ± 0.4 × 10(10) M(-1) s(-1), was similar to that in acetonitrile, yet resulted from iodine atoms formed within the laser pulse. These results are discussed within the context of solar energy conversion by dye-sensitized solar cells and storage via chemical bond formation.     

Adamantanes as spherical nanosondes in adducts with a chiral dirhodium complex-discriminating enantiomers and probing spatial proximities

Three different kinds of substituted chiral adamantane molecules—adamantanones, dioxolanoadamantanes and dithiolano—adamantanes—were studied in the dirhodium experiment (NMR measurement with 1:1 molar mixtures with Rh^(II)₂[(R)‐(+)‐MTPA]₄ in CDCl₃). Their different behavior in adduct formation is described, and the possibility of determining enantiomeric purities and absolute configurations is explored. Detailed inspection of one‐ and two‐dimensional NMR experiments allowed for an interpretation of steric and electronic intra‐adduct interaction showing that the phenyl groups of Rh* tend to enwrap the bound adamantane ligand so that through‐space effects over a range of 6–7 Å away from the binding rhodium atom can be observed. Even slight differences in the relative orientation of phenyl groups can be monitored when comparing diastereomeric adducts via NMR signal dispersion. Copyright © 2011 John Wiley & Sons, Ltd.     

Electron hopping through the 15 A triple tryptophan molecular wire in DNA photolyase occurs within 30 ps

DNA photolyase is a photoactive flavoprotein that contains three tryptophan residues between the FAD cofactor and the protein surface, the solvent-exposed Trp being located 14.8 A from the flavin. Photoreduction of the neutral radical FADH. form to the catalytically active FADH- form occurs via electron transfer through this chain. The first step in this chain takes 30 ps, the second less than 4 ps. Using a combination of site-directed mutagenesis and femtosecond polarization spectroscopy to discriminate the spectroscopically indistinguishable Trp residues, we show that the third step occurs in less than 30 ps. This implies that the first photoreduction step is rate limiting and that the Trp chain effectively acts as molecular "wire" ensuring rapid and directed long-range charge translocation across the protein. This finding is important for the functioning of the large class of cryptochrome blue-light receptors, where the Trp chain is conserved. In DNA photolyase we make use of the natural photoactivation of the process, but more generally chains of aromatic amino acids may allow very fast long-range electron transfer also in nonphotoactive proteins.     

Preparative free‐flow electrophoretic offline ESI‐Fourier transform ion cyclotron resonance/MS analysis of Suwannee River fulvic acid

Free‐flow electrophoresis (FFE), a preparative free zone electrophoretic method, was used offline in conjunction with ultrahigh‐resolution FT/ion cyclotron resonance ‐MS to resolve the complexity of Suwannee River fulvic acid (SRFA). Before MS, the FFE separation conditions and the compatibility with ESI were optimized. The constituents in SRFA were effectively separated based on their charge states and sizes. The obtained mass spectra were compared by means of van Krevelen diagrams and the calculated aromaticity indices of the individual constituents were used to describe the distribution of aromatic/unsaturated structures across the FFE‐fractionated samples. The consolidated number of ions observed within the individual SRFA fractions were much higher than those of the bulk samples alone, demonstrating extensive ion suppression effects in bulk SRFA likely also operating in the analysis of complex biogeochemical mixtures in flow injection mode. The FFE approach allows for producing sizable amounts of sample from dilute solutions, which can be easily fractionated into dozens of individual samples with the possibility of further in‐depth characterization.     

Determination of vitamin B5 in a range of fortified food products by reversed-phase liquid chromatography-mass spectrometry with electrospray ionisation

Methods for Vitamin B5 determination in food products remain limited by their low sensitivity and poor selectivity. Here, we have developed a liquid chromatography-mass spectrometry (LC-MS) method for Vitamin B5 determination in wide range of fortified food products. Vitamin B5 was extracted from food samples by heat treatment and analysed by LC-MS in the positive mode using electrospray ionisation (ESI). Vitamin B5 was quantified using hopantenic acid (HOPA) as internal standard after their separation on a C18 narrow-bore column with a gradient of mobile phase made of water/acetonitrile and trifluoroacetic acid (TFA) 0.025%. MS with single ion monitoring mode at mass m/z 220 was used for Vitamin B5 quantification. Calibration curve between 0.5 and 10 microg/ml of Vitamin B5 was linear (r2=0.9993) and the detection limit was determined to be 800 pg. The overall quantitative efficiency of the method was evaluated using Nestle reference sample (infant formula). The intra-assay RSD was 4.8% (n=8), the inter-assay RSD 6.4% (n=4) and the recoveries of the spiked samples were above 95%. Application of the LC-MS method to Vitamin B5 determination in wide range of fortified food products including three US National Institute of Standards and Technology (NIST) reference samples (RM 8435, RM 8415 and SRM 1546) shows consistent results with those obtained by microbiology and recoveries of Vitamin B5 between 93 and 104% for the spiked samples.     

Complete Proton Transfer Cycle in GFP and Its T203V and S205V Mutants

Proton transfer is critical in many important biochemical reactions. The unique three‐step excited‐state proton transfer in avGFP allows observations of protein proton transport in real‐time. In this work we exploit femtosecond to microsecond transient IR spectroscopy to record, in D₂O, the complete proton transfer photocycle of avGFP, and two mutants (T203V and S205V) which modify the structure of the proton wire. Striking differences and similarities are observed among the three mutants yielding novel information on proton transfer mechanism, rates, isotope effects, H‐bond strength and proton wire stability. These data provide a detailed picture of the dynamics of long‐range proton transfer in a protein against which calculations may be compared.