Photoelectric features of vacuum-deposited a-Si:H/Alq3 blends

Using three electrode vacuum system for glow discharge of 5% SiH₄ + 95% Ar gas mixture together with thermal evaporation of phosphorus or boric aced, the n- and p-type a-Si:H layers have been deposited. By co-evaporation of phosphorus or boric aced the conductivity of a-Si:H layers was changed in 10^?11–10^?3 Ω^?1 cm^?1 or 10^?11 –10^?8 Ω^?1 cm^?1 range, respectively. Blends of a-Si:H and tris-(8-hydroxyquinoline) aluminum (Alq₃) have been vacuum-deposited by simultaneous glow discharge of 5% SiH₄ + 95 % Ar gas mixture and thermal co-evaporation of Alq₃. Photoluminescence spectrum of a-Si:H/Alq₃ blend coincident with one of Alq₃ was observed at room temperature.     

Enantiocomplementary enzymes: classification, molecular basis for their enantiopreference, and prospects for mirror-image biotransformations

One often-cited weakness of biocatalysis is the lack of mirror-image enzymes for the formation of either enantiomer of a product in asymmetric synthesis. Enantiocomplementary enzymes exist as the solution to this problem in nature. These enzyme pairs, which catalyze the same reaction but favor opposite enantiomers, are not mirror-image molecules; however, they contain active sites that are functionally mirror images of one another. To create mirror-image active sites, nature can change the location of the binding site and/or the location of key catalytic groups. In this Minireview, X-ray crystal structures of enantiocomplementary enzymes are surveyed and classified into four groups according to how the mirror-image active sites are formed.     

Multicoordinational excited state twisting of indan-1,3-dione derivatives

Excited state relaxation of indan-1,3-dione derivatives with different substituents attached to the phenyl ring and with the bridged amino group was investigated by means of the steady-state fluorescence and femtosecond time-resolved absorption pump–probe spectroscopy. Bridging of the amino group increases the fluorescence quantum yield and the excited state lifetime. Analysis of the results indicates that the phenyl ring twisting around a single central bond leads to the nonradiative state formation and to subsequent fast relaxation to the ground state. Double bond twisting takes place in molecules with the bridged amino group and causes a large Stokes shift and slightly slower excited state relaxation.     

Quantitative Screening of Hydrolase Libraries Using pH Indicators: Identifying Active and Enantioselective Hydrolases

Rapid and quantitative screening in 96-well microplates can identify active and enantioselective hydrolases. Hydrolysis of esters releases a proton, which can be detected with pH indicators by colorimetry (figure). Using pure enantiomers, we measured the initial rates of enzyme-catalyzed hydrolysis. The relative initial rate approximates the enantioselectivity. This screening greatly speeds up selection of the best hydrolase for a synthesis.     

Manganese-substituted carbonic anhydrase as a new peroxidase

Carbonic anhydrase is a zinc metalloenzyme that catalyzes the hydration of carbon dioxide to bicarbonate. Replacing the active-site zinc with manganese yielded manganese-substituted carbonic anhydrase (CA[Mn]), which shows peroxidase activity with a bicarbonate-dependent mechanism. In the presence of bicarbonate and hydrogen peroxide, (CA[Mn]) catalyzed the efficient oxidation of o-dianisidine with kcat/KM=1.4 x 10(6) m(-1) s(-1), which is comparable to that for horseradish peroxidase, kcat/KM=57 x 10(6) m(-1) s(-1). CA[Mn] also catalyzed the moderately enantioselective epoxidation of olefins to epoxides (E=5 for p-chlorostyrene) in the presence of an amino-alcohol buffer, such as N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid (BES). This enantioselectivity is similar to that for natural heme-based peroxidases, but has the advantage that CA[Mn] avoids the formation of aldehyde side products. CA[Mn] degrades during the epoxidation limiting the yield of the epoxidations to <12 %. Replacement of active-site residues Asn62, His64, Asn67, Gln92, or Thr200 with alanine by site-directed mutagenesis decreased the enantioselectivity demonstrating that the active site controls the enantioselectivity of the epoxidation.