Crystallization of Arthrobacter sp. strain 1C N-(1-D-carboxyethyl)-L-norvaline dehydrogenase and its complex with NAD+

The novel NAD+-linked opine dehydrogenase from a soil isolate Arthrobacter sp. strain 1C belongs to an enzyme superfamily whose members exhibit quite diverse substrate specificites. Crystals of this opine dehydrogenase, obtained in the presence or absence of co-factor and substrates, have been shown to diffract to beyond 1.8 ? resolution. X-ray precession photographs have established that the crystals belong to space group P21212, with cell parameters a = 104.9, b = 80.0, c = 45.5 ? and a single subunit in the asymmetric unit. The elucidation of the three-dimensional structure of this enzyme will provide a structural framework for this novel class of dehydrogenases to enable a comparison to be made with other enzyme families and also as the basis for mutagenesis experiments directed towards the production of natural and synthetic opine-type compounds containing two chiral centres.     

Exciton diffusion and interfacial charge separation in meso-tetraphenylporphyrin/TiO2 bilayers: effect of ethyl substituents

The photoinduced charge separation efficiency in porphyrin/TiO2 bilayers has been determined using the time-resolved microwave conductivity (TRMC) technique. Porphyrins investigated are unsubstituted meso-tetraphenylporphyrin (TPP) and meso-tetra(4-ethylphenyl)porphyrin (TEPP). TEPP/TiO2 bilayers exhibit a charge separation efficiency per incident photon at the Soret band maximum of 6.2%, which is considerably higher than the efficiency of 1.2% found for TPP/TiO2 bilayers. Exciton diffusion lengths of 7 A for TPP and 75 A for TEPP are obtained from fitting a model for the charge separation efficiency to the experimental data. Optical measurements on the porphyrin derivatives on quartz yield a 20 times higher fluorescence quantum yield and a 7 times higher fluorescence rate constant for TEPP layers as compared to TPP layers. The exciton lifetime of 800 ps found for TEPP layers is considerably longer than the lifetime of 260 ps in TPP layers. The exciton diffusion coefficients, determined from the exciton diffusion length and the exciton lifetime, are found to be 2.10(-9) m(2)/s for TPP and 7.10(-8) m(2)/s for TEPP. The difference is discussed in terms of the presence of face-to-face dimers or larger aggregates in TPP layers.     

Observation of bi-polarons in blends of conjugated copolymers and fullerene derivatives

From a fundamental and application point of view it is of importance to understand how charge carrier generation and transport in a conjugated polymer (CP):fullerene blend are affected by the blend morphology. In this work light-induced electron spin resonance (LESR) spectra and transient ESR response signals are recorded on non-annealed and annealed blend layers consisting of alkyl substituted thieno[3,2-b]thiophene copolymers (pATBT) and the soluble fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) at temperatures ranging from 10 to 180 K. Annealing of the blend sample leads to a reduction of the steady state concentration of light-induced PCBM anions within the blend at low temperatures (T = 10 K) and continuous illumination. This is explained on the basis of the reducing interfacial area of the blend composite on annealing, and the high activation energy for electron diffusion in PCBM blends leading to trapped electrons near the interface with the CP. As a consequence, these trapped electrons block consecutive electron transfer from an exciton on a CP to the PCBM domain, resulting in a relatively low concentration charge carriers in the annealed blend. Analysis of the transient ESR data allows us to conclude that in annealed samples diamagnetic bi-polaronic states on the CPs are generated at low temperature. The formation of these states is related to the generation and interaction of multiple positive polarons in the large crystalline polymer domains present in the annealed sample.     

The flow of Newtonian and non-Newtonian liquids through annular converging regions

A coordinate system is introduced in which one of the three sets of coordinate surfaces is constituted by cones. The axes of the cones coincide, but in general the apices of the cones do not. For this so-called special toroidal coordinate system a set of operations is given involving the nabla operator. A few examples of the use of this special toroidal coordinate system will elucidate its advantages for the analysis of the flow of Newtonian and generalized Newtonian liquids in annular convergent regions.Dedicated to the 60th birthday of Prof. Dr. H. Janeschitz-Kriegl     

Tetrahedral n-type materials: efficient quenching of the excitation of p-type polymers in amorphous films

Tetrahedral naphthalenediimide compound 1 has been synthesized as an example of a new class of amorphous n-type materials, in which the nondirectionality obtained by its shape is decoupled from its optoelectronic properties. 1 forms bicontinuous films with p-type polymers and effectively quenches the excited state, yielding long-lived mobile charge carriers on pulsed illumination.     

Efficient exciton transport in layers of self-assembled porphyrin derivatives

The photosynthetic apparatus of green sulfur bacteria, the chlorosome, is generally considered as a highly efficient natural light-harvesting system. The efficient exciton transport through chlorosomes toward the reaction centers originates from self-assembly of the bacteriochlorophyll molecules. The aim of the present work is to realize a long exciton diffusion length in an artificial light-harvesting system using the concept of self-assembled natural chlorosomal chromophores. The ability to transport excitons is studied for porphyrin derivatives with different tendencies to form molecular stacks by self-assembly. A porphyrin derivative denoted as ZnOP, containing methoxymethyl substituents ({meso-tetrakis[3,5-bis(methoxymethyl)phenyl]porphyrinato}zinc(II)) is found to form self-assembled stacks, in contrast to a derivative with tert-butyl substituents, ZnBuP ({meso-tetrakis[3,5-bis(tert-butyl)phenyl]porphyrinato}zinc(II)). Exciton transport and dissociation in a bilayer of these porphyrin derivatives and TiO2 are studied using the time-resolved microwave conductivity (TRMC) method. For ZnOP layers it is found that excitons undergo diffusive motion between the self-assembled stacks, with the exciton diffusion length being as long as 15 +/- 1 nm, which is comparable to that in natural chlorosomes. For ZnBuP a considerably shorter exciton diffusion length of 3 +/- 1 nm is found. Combining these exciton diffusion lengths with exciton lifetimes of 160 ps for ZnOP and 74 ps for ZnBuP yields exciton diffusion coefficients equal to 1.4 x 10(-6) m2/s and 1 x 10(-7) m2/s, respectively. The larger exciton diffusion coefficient for ZnOP originates from a strong excitonic coupling for interstack energy transfer. The findings show that energy transfer is strongly affected by the molecular organization. The efficient interstack energy transfer shows promising prospects for application of such self-assembled porphyrins in optoelectronics.     

The mechanism of long-range exciton diffusion in a nematically organized porphyrin layer

The exciton diffusion length in a nematically organized meso-tetra(4-n-butylphenyl)porphyrin (TnBuPP) layer was found to exceed 40 nm at a temperature of 90 K and to be equal to 22 +/- 3 nm at 300 K. The exciton diffusion coefficient decreases from > or = 3.1 x 10(-6) m(2)/s at 90 K to (2.5 +/- 0.5) x 10(-7) m(2)/s at 300 K. This thermal deactivation is attributed to exciton motion via a band mechanism. The motion of an exciton is not limited by polaronic effects; that is, the deformation of the atomic lattice around the exciton. The absence of polaronic self-trapping implies that the exciton diffusion coefficient can be enhanced by improvement of structural order and rigidity of the material.     

Photogeneration and decay of charge carriers in hybrid bulk heterojunctions of ZnO nanoparticles and conjugated polymers

The photogeneration and decay of charge carriers in blend films of ZnO nanoparticles (diameter 5 nm) and poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) or poly(3-hexylthiophene) (P3HT) were studied by means of microwave-photoconductance measurements. Excitation of the polymer in the visible spectrum was found to lead to a transient photoconductance due to dissociation of excitons at the interface between ZnO and the conjugated polymer. From the similar decay kinetics of the photoconductance and the effects of UV illumination, it is concluded that the measured photoconductance is due to electrons in ZnO. Increasing the weight fraction of ZnO in the blend films leads to a higher photoconductance. This is attributed to enhanced formation of mobile electrons by interfacial dissociation of excitons at clusters of ZnO nanoparticles rather than at individual nanoparticles. The dependence of the photoconductance on the weight fraction of ZnO is found to be different for ZnO:MDMO-PPV and ZnO:P3HT blends. This is most likely due to the presence of a smaller number of relatively large ZnO clusters in ZnO:P3HT blends and a shorter exciton diffusion length, as compared with ZnO:MDMO-PPV blends. After exposure of the blend films to UV light, a significant increase in the magnitude and the lifetime of the photoconductance is observed. This is explained in terms of the filling of electron traps in ZnO by UV exposure.