Rapid internal conversion by nucleic acid components in solution

The fluorescence excitation spectra of 12 nucleic acid components in ethylene-glycol-H₂O solution at 160 K show quantum yields which are 2 to 10 times higher when excitation is near the 0-0 energy than at the first absorption maximum. Discussion leads to the conclusion that internal conversion which competes with vibrational relaxation is generally responsible.     

The polarization of excimer fluorescence from a dinucleotide

The polarization of the excimer fluorescence from the dinucleotide CpC has been observed to be strikingly different from that of the monomer. The mixed polarization in the absorption reflects significant ground state interaction. The mixed polarization in the fluorescence is most likely due to a component which is polarized between the chromophores, a manifestation of charge resonance character in the excimer state.     

A THEORETICAL STUDY OF THE CYTOSINE EXCIMER STATE: THE ROLE OF GEOMETRY OPTIMIZATION

Abstract A study of the lowest excited dimer (excimer) singlet state of the DNA base cytosine was performed with fully optimized geometry (near the cis-syn form) using a slightly modified version of MOPAC6 (AM1 hamiltonian) and 112 excited configurations involving an active space consisting of the four highest occupied and the two lowest empty MO of the dimer. A binding energy of 3 kcal/mol for the excimer state is demonstrated, and the excimer fluorescence is predicted to be shifted 150 nm to longer wavelengths relative to that of the monomer fluorescence. The excimer geometry has the planes nonparallel (40–45° interplanar angle) and distorted, with the C₅-C₆ bonds showing quite close contact (2.4 Å). Excitation is found to be more localized in the region of the C₅-C₆ bond than expected from the monomer wavefunction. No stable excimer was found if the planes were constrained to be planar, nor if the triplet state was selected. The results suggest that the excimer geometry found in this study may be a precursor to the cis-syn cyclobutane photodimer.     

Reactions of the natural lignan hydroxymatairesinol in basic and acidic nucleophilic media: formation and reactivity of a quinone methide intermediate

The chemical properties and synthetic modifications of the natural lignan hydroxymatairesinol in basic and acidic nucleophilic media were studied. Hydroxymatairesinol presumably reacts via a quinone methide and a carbonium ion mechanism under basic and acidic conditions, respectively. In these conditions the benzylic hydroxyl group was displaced by nucleophiles yielding new 7-substituted butyrolactone lignans. Reactions in alcoholic basic solutions yielded the 7-alkoxy ethers diastereoselectively. Several previously known lignans as well as new lignans and lignan derivatives were synthesised. The transformations were monitored and the products identified by HPLC-MS and NMR.     

Polymorphic modifications of barium metagermanate

Polymorphic transformations of BaGeO₃ were investigated by DTA and X-ray diffraction methods and the data obtained were compared with those previously published. BaGeO₃ undergoes a reversible phase transformation at 1100° from a low-temperature hexagonal polymorph to a medium-temperature polymorph having an unknown crystal structure, which undergoes a further reversible transformation at 1200° to a high-temperature orthorhombic polymorph found to be stable up to the melting temperature of 1287°. On cooling from the melt, the high-temperature form can be retained metastably at room temperature, but transforms to the low-temperature form at 900°.

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Zusammenfassung Polymorphe Umwandlungen von BaGeO₃ wurden unter Anwendung der DTA- und Röntgendiffraktionsmethoden untersucht und die erhaltenen Angaben mit den früher veröffentlichten verglichen. Bei 1100° vollzieht sich eine reversible Phasentransformation der BaGeO₃ von der hexagonalen Struktur bei niedrigen Temperaturen in eine Verbindung unbekannter Kristallstruktur bei mittleren Temperaturen, welche wiederum bei 1200° eine weitere reversible Umwandlung in eine orthorhombische Struktur erfuhr und sich bis zur Schmelztemperatur von 1287° als stabil erwies. Beim Abkühlen der Schmelze kann die Form bei hohen Temperaturen bei Zimmertemperatur in metastabilem Zustand erhalten bleiben, wird jedoch bei 900° in die Form bei niedrigen Temperaturen umgewandelt.

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Résumé On a étudié les transformations polymorphiques de BaGeO₃ par ATD et diffraction des rayons X et comparé les résultats avec ceux obtenus antérieurement. A 1100° C BaGeO₃ subit une transformation de phase réversible de la forme hexagonale stable à basse température à la forme stable à moyenne température, de structure cristalline inconnue, qui se transforme réversiblement à 1200° C en une phase orthorhombique qui reste stable jusqu'à 1287° C. Au refroidissement, la forme stable à haute température peut être maintenue à l'état métastable à la température ambiante, mais elle se transforme à 900° C en la forme stable à basse température.

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BaGeO₃ . BaGeO₃ 1100° - - 1200° , 1287°. - , - 900°.

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This investigation was supported by the Boris Kidri Foundation for Scientific Research     

A single-parameter trial wave function for the mixed valence ground state

The mixed valence trial ground state suggested by Stevens and Brandow is reconsidered in the case of two electrons per atom. The wellknown difficulties due to nonorthogonality are resolved by expanding the trial state in an orthonormal basis. The expansion coefficients are determinants composed of Bloch phase factors, as in the Gutzwiller method. Studying first the limiting case of the Kaplan-Mahanti strongly localized ground state in the Brandow formalism, we derive rules for a simplified handling of the determinants; this opens the way to the more complicated weakly localized ground state. This is handled by expressing theN variational parameters of the Brandow formalism through a single one, the hybridization temperature . The ground state energy is a well-behaved function of the hybridization matrix elementV. The valence and the shift of the Fermi level are calculated to lowest order inV. The band occupation numbers follow a Fermi distribution at temperature V. We argue that the ground state is insulating, with thed-electrons localized into large Wannier-type orbitals centered on the respectivef-holes, as envisaged by Stevens.     

Analysis of ketones by selected ion flow tube mass spectrometry

A selected ion flow tube mass spectrometry (SIFT-MS) study of the reactions of H3O+, NO+ and O2+* ions with the ketones (M) 2-heptanone, 2-octanone, 2-nonanone, 2-undecanone and 2-aminoacetophenone has been conducted in preparation for studies of volatile emissions from bacteria. The H3O+ reactions all proceed rapidly via exothermic proton transfer, producing only MH+ ions that form their monohydrates when water vapour is present in the helium carrier gas. The O2+* reactions proceed rapidly via dissociative charge transfer producing parent cations M+* and some fragment ions. The NO+ reactions form the NO+M adduct ions at rates which are dependent on the pressure of the helium carrier gas. Combining the present NO+ kinetic data with those available from previous SIFT studies, the phenomenon of charge transfer complexing is clearly demonstrated. This results in adduct formation in these NO+/ketone reactions at or near the collisional rate. SIFT-MS spectra are presented to illustrate the simplicity of SIFT-MS analysis of ketones using both H3O+ and NO+ precursor ions.