Formation of hydrogen-bridged cytosine dimers on Cu(110)

Cytosine was adsorbed onto a Cu(110) surface under UHV conditions. Annealing to 370 K resulted in the formation of a (6 x 6)gg low energy electron diffraction (LEED) pattern, even at submonolayer coverages. Examination of this structure with scanning tunneling microscopy (STM) revealed islands of zigzag chains at low coverages and large ordered domains at monolayer saturation. Further annealing to 480 K initiated a phase transition to a (6 x 2)gg structure observed both by LEED and STM. High resolution electron energy loss spectroscopy spectra for both overlayer structures exhibited mainly in-plane modes suggesting upright/tilted species on the surface. Based on the experimental data and supported by density functional theory calculations, a model is proposed for the (6 x 2)gg structure, which involves the formation of deprotonated hydrogen bridge-bonded cytosine dimers, adsorbed through the oxygen atoms.     

Hyperfine interactions in MgF2:Fe2+ and ZnF2:Fe2+ by Mössbauer spectroscopy

Measurement of the magnetic hyperfine interaction in paramagnetic Fe²⁺ in ZnF₂ and MgF₂ by Mössbauer spectroscopy is reported. The results, ?275 ± 3 kOe and ?270 ± 3 kOe for ZnF₂:Fe²⁺ and MgF₂:Fe²⁺ respectively are compared with a previous analysis of hyperfine interactions in the rutile fluorides.     

Encodamers: unnatural peptide oligomers encoded in RNA

Conventional display libraries are generally limited to the 20 naturally occurring amino acids. Here, we demonstrate that novel unnatural amide-linked oligomers can be constructed and encoded in an attached RNA for the purpose of mRNA display library design. To do this, we translated templates of various lengths in a protein synthesis system modified to promote sense codon suppression. Unnatural residues were escorted to the ribosome as chemically acylated tRNAs added to the translation mixture. Our experiments reveal that unnatural peptide oligomers ("encodamers") consisting of an N-substituted amino acid are readily generated as mRNA-peptide fusions with excellent stepwise efficiency. The N-substituted polyamides have strikingly improved proteolytic stability relative to their naturally encoded counterparts. Overall, our work indicates that the ribosome can be used as a synthesis platform to generate encoded combinatorial chemistry outside the universal genetic code.     

Ab initio interaction energies of hydrogen-bonded amino acid side chain[bond]nucleic acid base interactions

Hydrogen-bonding interactions often make substantial contributions to the specificity of protein-nucleic acid complexes. Using a geometric modeling approach, we previously identified 28 possible doubly hydrogen-bonded interactions to the four unpaired RNA bases. Here we present interaction energies of these models, calculated by ab initio quantum chemical methods, and describe a correlation between the computed energies and observed frequencies of the interactions. In general, interactions with charged side chains show the most favorable energies. An Asp/Glu-G interaction may be especially favorable for recognition of unpaired guanines in RNAs. Asn and Ser/Thr/Tyr side chains are calculated to make iso-energetic interactions to the Hoogsteen face of adenine, but Asn-A interactions are much more common with DNA than RNA, and Ser/Thr/Tyr-A interactions are more common with RNA than DNA. Examination of the known interactions suggests that Ser/Thr/Tyr may be accommodated in a wider variety of protein contexts at RNA-protein interfaces. With these calculated intrinsic affinities, it should be possible to better assess the contributions of bidentate hydrogen-bonding interactions to RNA- and DNA-binding specificity.     

Ueber den Kolloidzustand von Gelatinelösungen und seine Beeinflussung durch Temperaturverschiebungen

Ohne Zusammenfassung     

Tricarbonylchromium-complexed phenylsiloxane polymers as stereoselective hydrogenation catalysts: Preparation and properties

Soluble tricarbonylchromium complexes were made by reacting Cr(CO)₆ with a ladder polyphenylsilsesquioxane and a linear polydiphenylsiloxane. These new polymer Cr(CO)₃ complexes were characterized by elemental Cr, infrared (IR), gel permeation chromatography (GPC), viscosity, and thermal analyses and were evaluated as stereoselective hydrogenation catalysts. Thermogravimetry studies demonstrated that the new complexes were more stable at 180–200°C in N₂ than the corresponding complex from crosslinked polystyrene. These silicone polymer complexes catalyzed stereoselective hydrogenation of methyl sorbate to cis-3-hexenoate in cyclohexane and tetrahydrofuran (THF) and could be recovered from THF for recycling by precipitation-fiItration. Catalytic activity and recyclability, however, were highly influenced by the solvent. Loss of catalytic activity associated with loss of Cr(CO)₃ was observed on recycling. These results support a mechanisim that involves dissociation of Cr(CO)₃, a significant portion of which cannot become reassociated with the polymer phenyl groups.     

Predicted new electromagnetic precursors and altered signal velocity in dispersive media

New optical precursors are predicted in media exhibiting “non-conventional” dispersion characteristics such as spatial dispersion, surface polaritons and surface plasmons. Numerical estimates are given for the exciton and surface polariton precursors in CdS, and surface plasmon in Al. The signal velocity increases in spatially dispersive media and is essentially constant throughout the “stop” gap. Measurement of these effects may provide new spectroscopic information regarding “non-conventional” dispersion.