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The Effect of π-Stacking, H-Bonding, and Eectrostatic Interactions on the Ionization Energies of Nucleic Acid Bases Print
Sunday, 14 March 2010 00:00

A combined theoretical and experimental study of the ionized dimers of thymine and adenine, TT, AA, and AT, is presented. Experimentally observed and computed adiabatic and vertical ionization energies (IEs) for monomers and dimers as well as thresholds for the appearance of the protonated species are reported and analyzed. Non-covalent interactions strongly affect the observed IEs. The magnitude and the nature of the effect is different for different isomers of the dimers. The computations reveal that for TT, the largest changes in vertical IEs (0.4 eV) relative to the monomer occur in asymmetric H-bonded and symmetric -stacked isomers, whereas in the lowest-energy symmetric H-bonded dimer the shift in IEs is much smaller (0.2 eV). The origin of the shift and the character of the ionized states is different in asymmetric H-bonded and symmetric stacked isomers. ... Article Link (PDF)

Probing the Hydrogen-Bond Network of Water via Time-Resolved Soft X-Ray Spectroscopy Print
Thursday, 28 May 2009 00:00

We report time-resolved studies of hydrogen bonding in liquid H2O, in response to direct excitation of the O–H stretch mode at 3 m, probed via soft X-ray absorption spectroscopy at the oxygen K-edge. This approach employs a newly developed nanofluidic cell for transient soft X-ray spectroscopy in the liquid phase. Distinct changes in the near-edge spectral region (XANES) are observed, and are indicative of a transient temperature rise of 10 K following transient laser excitation and rapid thermalization of vibrational energy. The rapid heating occurs at constant volume and the associated increase in internal pressure, estimated to be 8 MPa, is manifested by distinct spectral changes that differ from those induced by temperature alone. We conclude that the near-edge spectral shape of the oxygen K-edge is a sensitive probe of internal pressure, opening new possibilities for testing the validity of water models and providing new insight into the nature of hydrogen bonding in water. Article Link (PDF)

Nickel alpha-Keto-beta-Diimine Initiators for Olefin Polymerization Print
Wednesday, 28 January 2009 00:00

In order to design and improve a catalyst, it is important to understand the structure of the present one. Whether they work in the solid state or in solution, if the catalysts can be crystallized, then their structure can be determined by single crystal x-ray diffraction on
station 11.3.1. Some chemical reactions can occur in such a way that two different handedness, i.e., mirror images of the same molecule, are produced. For applications in the pharmaceutical industry, only one of these handedness may have the desired biologically activity, the other
may have negligible or adverse affects. Separation of these mirror image molecules is very difficult and costly, therefore a catalyst which makes only the desired handedness both reduces the energy required for separation and waste at the same time. Article Link (PDF)

Structural Basis of Transcription: Role of the Trigger Loop in Substrate Specificity and Catalysis Print
Friday, 01 December 2006 00:00

Cell CoverNew structures of RNA polymerase II (pol II) transcribing complexes reveal a likely key to transcription. The trigger loop swings beneath a correct nucleoside triphosphate (NTP) in the nucleotide addition site, closing off the active center and forming an extensive network of interactions with the NTP base, sugar, phosphates, and additional pol II residues. A histidine side chain in the trigger loop, precisely positioned by these interactions, may literally "trigger" phosphodiester bond formation. Recognition and catalysis are thus coupled, ensuring the fidelity of transcription. Article Link (PDF)

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