New fluorescent probes for carbonic anhydrases

We report the synthesis and fluorescence properties of naphthalenesulfonamide derivatives as active site probes for carbonic anhydrases.     

Theoretical investigation of the rates of electron transfer processes Q−I + QII → QI + Q−II and Q−I + Q−II → QI + Q2−II in photosynthesis

A theoretical investigation on the rates of electron-transfer processes Q⁻_I + Q_II → Q⁻_I + Q⁻_II and Q⁻_I + Q⁻_II → Q_I + Q²⁻_II was carried out by using the Marcus theory of long-range electron transfer in solution. The molecular reorganizational parameter λ, the free-energy change ΔG⁰ for the overall reaction, and the electronic matrix element H_DA for these two processes were calculated from the INDO-optimized geometries of molecules Q_I, Q_II, and histidine. Q_I and Q_II are plastoquinones (PQ) which are hydrogen-bonded to a histidine each, and the two histidines may or may not be coordinated to a Fe²⁺ ion. The plastoquinone representing Q_I is additionally flanked by two peptide fragments. Each of the species (Pep)₂Q_I · His and His · Q_II has been considered to be immersed in a dielectric continuum that represents the surrounding molecules and protein folds. INDO calculations confirm the standard reduction potential for the first process (calculated 0.127 V; observed 0.13 V) and predict a midpoint potential of 0.174 V for the second process at 300 K at pH 7 (experimental value remains uncertain but is known to be close to 0.13 V). The plastoquinone fragment carries almost all the net charge (about 95.7%) in [PQ · His]⁻ and the net charge in [PQH · His]⁻. The electron is transferred effectively from the plastoquinone part of [(Pep)₂Q_I · His]⁻ to the plastoquinone moiety of Q_II · His in the first step and to the plastoquinone fragment of HisH⁺ · Q⁻_II in the second step. Therefore, we made use of the formula for the rate of through-space electron transfer from Q_I to Q_II (and to Q⁻_II). The plastoquinones are, of course, electronically coupled to histidines, and the transfer is, in reality, through the molecular bridge consisting of histidines and also Fe²⁺. The through-bridge effect is inherent in our calculation of ΔG⁰, H_DA, and the reorganization parameter λ. We investigated the correlation between half-times for the transfer and (D⁻¹_op− D⁻¹_s), where D_op and D_s are, respectively, optical and static dielectric constants of the condensed phase in the vicinity of the plastoquinones. We found that with reasonable values of D_op (2.6) and D_s (8.5) the experimental rates are adequately explained in terms of transfers from the plastoquinone moiety of Q_I to that of Q_II. The t_1/2 values calculated for the two processes are 247 and 472 μs in the absence of Fe²⁺ and 134 and 181 μs in the presence of Fe²⁺. These are in good agreement with the observed values which are ≈ 100 and ≈ 200 μs when Fe²⁺ is present in the matrix and which are known to be almost twice as large when the Fe²⁺ is evicted from the matrix. The present work also shows that the Marcus-Hush theory of long-range electron transfers can be successfully applied to the investigation of processes occurring in a semirigid condensed phase like the thylakoid membrane region. © 1997 John Wiley & Sons, Inc.     

The Development of Bulky Palladium NHC Complexes for the Most-Challenging Cross-Coupling Reactions

Palladium‐catalyzed cross‐coupling reactions enable organic chemists to form C? C bonds in targeted positions and under mild conditions. Although phosphine ligands have been intensively researched, in the search for even better cross‐coupling catalysts attention has recently turned to the use of N‐heterocyclic carbene (NHC) ligands, which form a strong bond to the palladium center. PEPPSI (pyridine‐enhanced precatalyst preparation, stabilization, and initiation) palladium precatalysts with bulky NHC ligands have established themselves as successful alternatives to palladium phosphine complexes. This Review shows the success of these species in Suzuki–Miyaura, Negishi, and Stille–Migita cross‐couplings as well as in amination and sulfination reactions.     

Synthesis of α,β- and β-Unsaturated Acids and Hydroxy Acids by Tandem Oxidation,Epoxidation, and Hydrolysis/Hydrogenation of Bioethanol Derivatives

We report a reaction platform for the synthesis of three different high-value specialty chemical building blocks starting from bio-ethanol, which might have an important impact in the implementation of biorefineries. First, oxidative dehydrogenation of ethanol to acetaldehyde generates an aldehyde-containing stream active for the production of C₄ aldehydes via base-catalyzed aldol-condensation. Then, the resulting C₄ adduct is selectively converted into crotonic acid via catalytic aerobic oxidation (62 % yield). Using a sequential epoxidation and hydrogenation of crotonic acid leads to 29 % yield of β-hydroxy acid (3-hydroxybutanoic acid). By controlling the pH of the reaction media, it is possible to hydrolyze the oxirane moiety leading to 21 % yield of α,β-dihydroxy acid (2,3-dihydroxybutanoic acid). Crotonic acid, 3-hydroxybutanoic acid, and 2,3-dihydroxybutanoic acid are archetypal specialty chemicals used in the synthesis of polyvinyl-co-unsaturated acids resins, pharmaceutics, and bio-degradable/ -compatible polymers, respectively.     

Pion propagation in real time field theory at finite temperature

We describe how the thermal counterpart of a vacuum two-point function may be obtained in the real time formalism in a simple way by using directly the 2×2 matrices that different elements acquire in this formalism. Using this procedure we calculate the analytic (single component) thermal amplitude for the pion pole term in the ensemble average of two axial-vector currents to two loops in chiral perturbation theory. The general expressions obtained for the effective mass and the decay constants of the pion are evaluated in the chiral and the non-relativistic limits. PACS 11.10.Wx; 12.38.Mh; 12.39.Fe     

External heavy atom effect on the emission of carbazole

External heavy atom induced quenching of luminescence emission of carbazole in n-hexane and methylcyclohexane in the presence of chloro- and bromoacetic acids at 300 K and 77 K is described. A straight line relation between fluorescence quenching rate constant and the acid dissociation constant of chloroacetic acids, which is related to electron affinity, is observed. Different photophysical parameters at 77 K of unperturbed and perturbed fluorescent carbazole in ternary solutions have been determined and suitable reaction mechanism for quenching has been proposed. Inference has been drawn about complex formation in the triplet state of carbazole from the biexponential nature of the phosphorescence decay curve.