A linear dichroism study of pyrromethene-, pyrromethenone- and bilatriene-abc-derivatives

The linear dichroism of several pyrromethene, pyrromethenone and bilatriene-abc derivatives embedded in a polyethylene film and oriented by stretching is investigated. On dissolving these pigments in the polymer matrix no changes in their conformations are observed, as compared to solutions in apolar solvents. Polarisation of the absorption bands of the pigments is deduced from the polarisation spectra. To enhance the solubility and orientation in the polymer, a pyrromethenone and a bilatriene-abc bearing a C₁₈-alkyl chain were synthesised for the first time.

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Zum Lineardichroismus von Pyrromethen-, Pyrromethenon- und Bilatrien-abc-Derivaten

Zusammenfassung Der Lineardichroismus einiger Pyrromethen-, Pyrromethenon- und Bilatrienabc-Derivate, die in eine Polyethylenfolie eingebettet vorliegen, wird untersucht. Bei der Lösung dieser Pigmente in der polymeren Matrix wird keine Veränderung der Konformation im Vergleich mit Lösungen in apolaren Lösungsmitteln beobachtet. Aus den Polarisationsspektren wird die Polarisation der Absorptionsbanden dieser Chromophore abgeleitet. Ein Pyrromethenon und ein Bilatrien-abc mit C₁₈-Alkylseitenkette wurden zur Verbesserung der Löslichkeit und Orientierung in der Polymerfolie erstmals dargestellt.

     

Twentyfold increase in alkaline phosphatase activity by sequential reversible activation of the enzyme followed by coupling with a copolyme of ethylene and maleic anhydride

Alkaline phosphatase, APase, (EC 3.1.31) from calf intestine, after shifting the equilibrium by effector molecules towards the dimeric form of the enzyme, was coupled (ratio 1:2, protein: copolymer) to a copolymer of ethylene and maleic anhydride, EMA. The water-soluble APase-EMA was separated from APase and the unbound EMA by DEAE-cellulose ion exchange chromatography. The specific activity of the APase-EMA, compared to APase, increased 26-fold at pH 7.1 and 10-fold at pH 8.6. The pH optimum of APase-EMA was shifted down from pH 9.5 (native APase) to 8.6. This change could be interpreted in terms of polyelectrolyte theory. APase-EMA retained 50–70% of its optimum activity in the pH range 7–8, while APase retained only 5–15% of its optimum activity within the same pH range. Its isoelectric point, pI, was 4.2 (APase 6.0) and it migrated on polyacrylamide gel electrophoresis in a single band, anodic movement twice as fast as APase. Parallel with the kinetic measurements, the reactive-enzyme sedimentation method was used to measure S_20,w values. S_20,w values obtained for APase-EMA, activated APase, and APase dialyzed against wafer were 6.56S, 6.46S, and 5.17S, respectively. Molecular weights, M_r, were determined by equilibrium sedimentation: the values obtained were 180,000, 160,000, and 84,500. M_r values of APase-EMA and APase (native) estimated by Sepharose-4B gel filtrations were essentially the same. The above-mentioned values remained unchanged for APase-EMA after intensive dialysis against water, whereas for the activated APase, separation from the effector molecules caused the equilibrium to shift back to the monomeric, very slightly active enzyme with concomitant changes of S_20,w to 5.15 and M_r to 82,000.     

Novel electronic transitions within excited doublets of dimers

We consider the electronic transition between the split components of the excited electronic doublet of a dimer species when the monomer species have permanent dipole moments. We show that the transition moment is given in terms of the permanent dipole moments of the ground and excited states of the monomers. Extension to ionic dimers and transitions between exciton bands in molecular crystals are suggested.     

Vibrational spectroscopy of triacetone triperoxide (TATP): anharmonic fundamentals, overtones and combination bands

The vibrational spectrum of triacetone triperoxide (TATP) is studied by the correlation-corrected vibrational self-consistent field (CC-VSCF) method which incorporates anharmonic effects. Fundamental, overtone, and combination band frequencies are obtained by using a potential based on the PM3 method and yielding the same harmonic frequencies as DFT/cc-pVDZ calculations. Fundamentals and overtones are also studied with anharmonic single-mode (without coupling) DFT/cc-pVDZ calculations. Average deviations from experiment are similar for all methods: 2.1-2.5%. Groups of degenerate vibrations form regions of numerous combination bands with low intensity: the 5600-5800 cm(-1) region contains ca. 70 overtones and combinations of CH stretches. Anharmonic interactions are analyzed.     

Locating conical intersections relevant to photochemical reactions

A new computerized method for locating conical intersections of interest in photochemistry is presented. The search is based on the Longuet-Higgins phase change theorem (Berry phase) which provides the subspace required for the initial search. The subspace is approximated as a plane containing three stable structures lying on a Longuet-Higgins loop. The search is conducted for a minimum of ΔE, the energy difference between two electronic states. It is started using up to three points within the circle defined by the three structures; symmetry, if relevant, is helpful but not essential. Since a two-dimensional subspace of the large 3N − 6 space is used, the search that uses either Cartesian or internal coordinates is efficient and yields a degeneracy after a few iterations. Given that not all degrees of freedom are included in the search, usually a high lying part of the conical intersection is initially located. The system is subsequently optimized along all coordinates keeping ΔE as close to zero as desired. The method is demonstrated for the symmetric H₃ system and also for the butadiene–cyclobutene–bicyclobutane system in which the three stable structures are not equivalent. The method is general and can be extended to any photochemical system.     

Properties of stable organic bond-stretched non-Lewis molecules

The conditions required for the existence of a stable bond-stretched singlet isomer of hetero derivatives of bicyclo[2.1.0]pentane (which is a cyclopentane-1,3-diyl derivative) are discussed. Such species are non-Lewis systems with a ruptured C-C bond (formally diradicals), in which two electrons occupy the nonbonding orbital. A high-level calculation shows that in contrast with the carbon substituted compounds, in which the open form is a transition state between two classical-bonded closed bicyclic forms, in the heterosubstituted molecules, the open form is calculated to be a stable minimum. The ionization potentials of the open forms are considerably lower than those of their bicyclic isomers and also of regular organic radicals/diradicals. Nitrogen atoms are found to be more effective than oxygen or sulfur in stabilizing the open isomer. In this case, the open isomer is calculated to be a little more stable than the bicyclic compound, and a barrier of approximately 40 kcal/mol is computed for the ring closing reaction. Thus, the open isomer is both thermodynamically and kinetically stable. This result rationalizes some experimental observations that indicated the existence of non-Lewis singlet species.     

Separating the contribution of translational and rotational diffusion to protein association

The association of two proteins is preceded by a mutual diffusional search in solution. The role of translational and rotational diffusion in this process has been studied theoretically for many years. However, systematic experimental verification of theoretical results is still lacking. We report here measurements of association rates of the proteins beta-lactamase (TEM) and beta-lactamase inhibitor protein (BLIP) in solutions of glycerol and poly(ethylene glycol) of increasing viscosity. We also measured translational and rotational diffusion in the same solutions, using fluorescence correlation spectroscopy and fluorescence anisotropy, respectively. It is found that in glycerol both translational and rotational diffusion rates are inversely dependent on viscosity, as predicted by the classical Stokes-Einstein relations, while the association rate depends nonlinearly on viscosity. In contrast, the association rate depends only weakly on the viscosity of the polymer solutions, which results in a similar weak dependence of k(on) on viscosity. The data are modeled using the theory of diffusion-limited association. Deviations from the theory are explained by a short-range solute-induced repulsion between the proteins in glycerol solution and an attractive depletion interaction generated by the polymers. These results open the way to the creation of a unified framework for all nonspecific effects involved in the protein association process, as well as to better theoretical understanding of these effects. Further, they reflect on the complex factors controlling protein association within the crowded environment of cells and suggest that a high concentration of macromolecules does not significantly impede protein association.     

On zero-sum partitions and anti-magic trees

We study zero-sum partitions of subsets in abelian groups, and apply the results to the study of anti-magic trees. Extension to the nonabelian case is also given.