Isomerization

Abstract: 3.2.3. Hydroformylation 2467 3.2.4. Dimerization 2468 3.2.5. Oxidative Cleavage and Ozonolysis 2469 3.2.6. Metathesis 2470 4. Terpenes 2472 4.1. Pinene 2472 4.1.1. Isomerization: R-Pinene 2472 4.1.2. Epoxidation of R-Pinene 2475 4.1.3. Isomerization of R-Pinene Oxide 2477 4.1.4. Hydration of R-Pinene: R-Terpineol 2478 4.1.5. Dehydroisomerization 2479 4.2. Limonene 2480 4.2.1. Isomerization 2480 4.2.2. Epoxidation: Limonene Oxide 2480 4.2.3. Isomerization of Limonene Oxide 2481 4.2.4. Dehydroisomerization of Limonene and Terpenes To Produce Cymene 2481

Abstract: Azobenzene undergoes trans → cisisomerization when irradiated with light tuned to an appropriate wavelength. The reverse cis →transisomerization can be driven by light or occurs thermally in the dark. Azobenzene's photochromatic properties make it an ideal component of numerous molecular devices and functional materials. Despite the abundance of application-driven research, azobenzene photochemistry and the isomerization mechanism remain topics of investigation. Additional substituents on the azobenzene ring system change the spectroscopic properties and isomerization mechanism. This critical review details the studies completed to date on the 3 main classes of azobenzene derivatives. Understanding the differences in photochemistry, which originate from substitution, is imperative in exploiting azobenzene in the desired applications.

Abstract: Peroxynitrite [oxoperoxonitrate(1-), ONOO-] may be formed in vivo from superoxide and nitric oxide. The anion is stable, but the acid (pKa = 6.8) decays to nitrate with a rate of 1.3 s-1 at 25 degrees C. The experimental activation parameters of this process are delta H++ = +18 +/- 1 kcal/mol, delta S++ = +3 +/- 2 cal/(mol.K), and delta G++ = +17 +/- 1 kcal/mol. Peroxynitrite (or its protonated form) oxidizes some compounds such as thiols and thioethers in a biomolecular reaction. The reactions with glutathione and cysteine have activation enthalpies of 10.9 and 9.7 kcal/mol, respectively, which are lower than that of the isomerization reaction. Peroxynitrite reacts with other compounds such as dimethyl sulfoxide and deoxyribose in a unimolecular reaction for which the activation of peroxynitrite is rate-limiting. In theory, activation could involve (1) heterolysis to OH- and NO2+ (delta rxn Gzero' = 13 kcal/mol at pH 7) or (2) homolysis to .OH and .NO2 (delta rxn Gzero = 21 kcal/mol), and these processes also could be involved in the isomerization to nitrate. However, thermodynamic and kinetic considerations indicate that neither process is feasible, although binding to metal ions may reduce the large activation energy associated with heterolysis. An intermediate closely related to the transition state for isomerization of ONOOH to HONO2 may be the strongly oxidizing intermediate responsible for hydroxyl radical-like oxidations mediated by ONOOH. Thus, peroxynitrite reacts with different compounds by at least two distinct mechanisms, and the hydroxyl radical is not involved in either.

Abstract: A reversible intramolecular and co-operative isomerization of double-stranded, oligomeric and poly(dG-dC) · poly(dG-dC) takes place in aqueous solution when the salt concentration at 25 °C and neutral pH is increased to 2.5 m-NaCl, 1.8 m-NaClO4 or 0.7 m-MgCl2. This conformational transition between two different double helical forms is accompanied by changes of optical rotatory dispersion, circular dichroism and ultraviolet absorption. Preliminary X-ray diffraction data are compatible with the existence of two salt-dependent structures. The enthalpy change of the transition is close to zero. The first-order kinetics are in the time range of 102 to 103 seconds and are followed after appropriate shifts in the salt concentration. The activation energy for both over-all rate constants is + 22 ± 2 kcal./mole and is nearly independent of the chain length. In chloride solutions the over-all rate constant for the transition from the high salt L-form to the low salt R-form decreases strongly with increasing salt concentrations. A detailed mechanism is proposed involving for the nucleation of the other helical form the opening or unstacking of a certain number of base pairs at the end of an oligomer. Analytical expressions for the chain length dependence of the degree of transition and the relaxation time are derived, assuming steady-state conditions for intermediates. The experiments show that an isomerization of DNA with a certain base sequence is possible in solution and the proposed model suggests a plausible mechanism. Some biological implications of such conformational transitions in DNA are briefly discussed.

Abstract: The structural parameters of fluid phase bilayers composed of phosphatidylcholines with fully saturated, mixed, and branched fatty acid chains, at several temperatures, have been determined by simultaneously analyzing small-angle neutron and X-ray scattering data. Bilayer parameters, such as area per lipid and overall bilayer thickness have been obtained in conjunction with intrabilayer structural parameters (e.g. hydrocarbon region thickness). The results have allowed us to assess the effect of temperature and hydrocarbon chain composition on bilayer structure. For example, we found that for all lipids there is, not surprisingly, an increase in fatty acid chain trans-gauche isomerization with increasing temperature. Moreover, this increase in trans-gauche isomerization scales with fatty acid chain length in mixed chain lipids. However, in the case of lipids with saturated fatty acid chains, trans-gauche isomerization is increasingly tempered by attractive chain-chain van der Waals interactions with increasing chain length. Finally, our results confirm a strong dependence of lipid chain dynamics as a function of double bond position along fatty acid chains.