Low-lying excited states in crystalline perylene.

Organic materials are promising candidates for advanced optoelectronics and are used in light-emitting diodes and photovoltaics. However, the underlying mechanisms allowing the formation of excited states responsible for device functionality, such as exciton generation and charge separation, are insufficiently understood. This is partly due to the wide range of existing crystalline polymorphs depending on sample preparation conditions. Here, we determine the linear optical response of thin-film single-crystal perylene samples of distinct polymorphs in transmission and reflection geometries. The sample quality allows for unprecedented high-resolution spectroscopy, which offers an ideal opportunity for judicious comparison between theory and experiment. Excellent agreement with first-principles calculations for the absorption based on the GW plus Bethe-Salpeter equation (GW-BSE) approach of many-body perturbation theory (MBPT) is obtained, from which a clear picture of the low-lying excitations in perylene emerges, including evidence of an exciton-polariton stopband, as well as an assessment of the commonly used Tamm-Dancoff approximation to the GW-BSE approach. Our findings on this well-controlled system can guide understanding and development of advanced molecular solids and functionalization for applications.

Characterization of Biomimetic Cofactors According to Stability, Redox Potentials, and Enzymatic Conversion by NADH Oxidase from Lactobacillus pentosus.

Oxidoreductases are attractive biocatalysts that convert achiral substrates into products of higher value, but they are also for the most part dependent on nicotinamide cofactors. Recently, biomimetic nicotinamide derivatives have received attention as less costly alternatives to natural cofactors. However, recycling of biomimetics is still challenging because there are only limited opportunities. Here, we have characterized various biomimetic cofactors with regard to stability and redox potentials to find the best alternative to natural cofactors. Further, the cofactor spectrum of NADH oxidase from Lactobacillus pentosus (LpNox) could be expanded, and the enzymatic activity was also compared to activities with different small-molecule catalysts. As a result, we succeeded in identifying several strategies for regeneration of oxidized biomimetics.

In vitro metabolic engineering for the production of α-ketoglutarate.

α-Ketoglutarate (aKG) represents a central intermediate of cell metabolism. It is used for medical treatments and as a chemical building block. Enzymatic cascade reactions have the potential to sustainably synthesize this natural product. Here we report a systems biocatalysis approach for an in vitro reaction set-up to produce aKG from glucuronate using the oxidative pathway of uronic acids. Because of two dehydrations, a decarboxylation, and reaction conditions favoring oxidation, the pathway is driven thermodynamically towards complete product formation. The five enzymes (including one for cofactor recycling) were first investigated individually to define optimal reaction conditions for the cascade reaction. Then, the kinetic parameters were determined under these conditions and the inhibitory effects of substrate, intermediates, and product were evaluated. As cofactor supply is critical for the cascade reaction, various set-ups were tested: increasing concentrations of the recycling enzyme, different initial NAD+ concentrations, as well as the use of a bubble reactor for faster oxygen diffusion. Finally, we were able to convert 10gL-1 glucuronate with 92% yield of aKG within 5h. The maximum productivity of 2.8gL-1 h-1 is the second highest reported in the biotechnological synthesis of aKG.

Identification and characterization of two new 5-keto-4-deoxy-D-Glucarate Dehydratases/Decarboxylases.

BACKGROUND: Hexuronic acids such as D-galacturonic acid and D-glucuronic acid can be utilized via different pathways within the metabolism of microorganisms. One representative, the oxidative pathway, generates α-keto-glutarate as the direct link entering towards the citric acid cycle. The penultimate enzyme, keto-deoxy glucarate dehydratase/decarboxylase, catalyses the dehydration and decarboxylation of keto-deoxy glucarate to α-keto-glutarate semialdehyde. This enzymatic reaction can be tracked continuously by applying a pH-shift assay. RESULTS: Two new keto-deoxy glucarate dehydratases/decarboxylases (EC 4.2.1.41) from Comamonas testosteroni KF-1 and Polaromonas naphthalenivorans CJ2 were identified and expressed in an active form using Escherichia coli ArcticExpress(DE3). Subsequent characterization concerning K m, k cat and thermal stability was conducted in comparison with the known keto-deoxy glucarate dehydratase/decarboxylase from Acinetobacter baylyi ADP1. The kinetic constants determined for A. baylyi were K m 1.0 mM, k cat 4.5 s-1, for C. testosteroni K m 1.1 mM, k cat 3.1 s-1, and for P. naphthalenivorans K m 1.1 mM, k cat 1.7 s-1. The two new enzymes had a slightly lower catalytic activity (increased K m and a decreased k cat) but showed a higher thermal stability than that of A. baylyi. The developed pH-shift assay, using potassium phosphate and bromothymol blue as the pH indicator, enables a direct measurement. The use of crude extracts did not interfere with the assay and was tested for wild-type landscapes for all three enzymes. CONCLUSIONS: By establishing a pH-shift assay, an easy measurement method for keto-deoxy glucarate dehydratase/decarboxylase could be developed. It can be used for measurements of the purified enzymes or using crude extracts. Therefore, it is especially suitable as the method of choice within an engineering approach for further optimization of these enzymes.

Patterned Growth of Organic Semiconductors: Selective Nucleation of Perylene on Self-Assembled Monolayers.

Organic semiconductors (OSC) have received a large amount of attention because they afford the fabrication of flexible electronic devices. However, the limited resistance to radiation and etching of such materials does not permit their patterning by photolithography, which has been a driving force for the development of integrated circuits and therefore requires alternative structuring techniques. One approach is based on precoating the substrate with self-assembled monolayers (SAMs) to control the nucleation of subsequently deposited OSC layers, but the underlying mechanism is barely understood. Here, we used alkanethiols with different chemical terminations to prepare SAMs on gold substrates serving as model systems to identify the mechanism of selective nucleation for the case of the OSC perylene. Using atomic force microscopy and fluorescence microscopy, we demonstrate that the chemical functionalization of the SAMs determines the adhesion forces for the OSC that are smallest for CF3-terminated and largest for OH-terminated SAMs, hence yielding distinctly different sticking probabilities upon perylene deposition at room temperature. Microcontact printing and immersion were employed to prepare SAM patterns that enable the selective growth of polycrystalline perylene films. A quite different situation is found upon printing long-chain thiols with low vapor pressure, which leads to the transfer of multilayers and favors the growth of perylene single crystallites. In a more abstract scenario, patterns of silicone oil droplets were printed on a gold substrate, which was previously covered with a repelling fluorinated SAM. Such droplets provide nucleation centers for liquid-mediated growth, often yielding platelet-shaped perylene single crystallites without unwanted perylene nucleation on the remaining surface.

Novel CAD-like enzymes from Escherichia coli K-12 as additional tools in chemical production.

In analyzing the reductive power of Escherichia coli K-12 for metabolic engineering approaches, we identified YahK and YjgB, two medium-chain dehydrogenases/reductases subgrouped to the cinnamyl alcohol dehydrogenase family, as being important. Identification was achieved using a stepwise purification protocol starting with crude extract. For exact characterization, the genes were cloned into pET28a vector and expressed with N-terminal His tag. Substrate specificity studies revealed that a large variety of aldehydes but no ketones are converted by both enzymes. YahK and and YjgB strongly preferred NADPH as cofactor. The structure of YjgB was modeled using YahK as template for a comparison of the active center giving a first insight to the different substrate preferences. The enzyme activity for YahK, YjgB, and YqhD was determined on the basis of the temperature. YahK showed a constant increase in activity until 60 °C, whereas YjgB was most active between 37 and 50 °C. YqhD achieved the highest activity at 50 °C. Comparing YjgB and Yahk referring to the catalytic efficiency, YjgB achieved for almost all substrates higher rates (butyraldehyde 221 s⁻¹ mM⁻¹, benzaldehyde 1,305 s⁻¹ mM⁻¹). Exceptions are the two substrates glyceraldehydes (no activity for YjgB) and isobutyraldehyde (YjgB 0.26 s⁻¹ mM⁻¹) which are more efficiently converted by YahK (glyceraldehyde 2.8 s⁻¹ mM⁻¹, isobutyraldehyde 14.6 s⁻¹ mM⁻¹). YahK and even more so YjgB are good candidates for the reduction of aldehydes in metabolic engineering approaches and could replace the currently used YqhD.

To beat the heat - engineering of the most thermostable pyruvate decarboxylase to date.

Pyruvate decarboxylase (PDC) is a key enzyme for the production of ethanol at high temperatures and for cell-free butanol synthesis. Thermostable, organic solvent stable PDC was evolved from bacterial PDCs. The new variant shows >1500-fold-improved half-life at 75 °C and >5000-fold-increased half-life in the presence of 9 vol% butanol at 50 °C.

Substrate scope of a dehydrogenase from Sphingomonas species A1 and its potential application in the synthesis of rare sugars and sugar derivatives.

Rare sugars and sugar derivatives that can be obtained from abundant sugars are of great interest to biochemical and pharmaceutical research. Here, we describe the substrate scope of a short-chain dehydrogenase/reductase from Sphingomonas species A1 (SpsADH) in the oxidation of aldonates and polyols. The resulting products are rare uronic acids and rare sugars respectively. We provide insight into the substrate recognition of SpsADH using kinetic analyses, which show that the configuration of the hydroxyl groups adjacent to the oxidized carbon is crucial for substrate recognition. Furthermore, the specificity is demonstrated by the oxidation of d-sorbitol leading to l-gulose as sole product instead of a mixture of d-glucose and l-gulose. Finally, we applied the enzyme to the synthesis of l-gulose from d-sorbitol in an in vitro system using a NADH oxidase for cofactor recycling. This study shows the usefulness of exploring the substrate scope of enzymes to find new enzymatic reaction pathways from renewable resources to value-added compounds.

Thermostabilization of the uronate dehydrogenase from Agrobacterium tumefaciens by semi-rational design.

Aldaric acids represent biobased 'top value-added chemicals' that have the potential to substitute petroleum-derived chemicals. Until today they are mostly produced from corresponding aldoses using strong chemical oxidizing agents. An environmentally friendly and more selective process could be achieved by using natural resources such as seaweed or pectin as raw material. These contain large amounts of uronic acids as major constituents such as glucuronic acid and galacturonic acid which can be converted into the corresponding aldaric acids via an enzyme-based oxidation using uronate dehydrogenase (Udh). The Udh from Agrobacterium tumefaciens (UdhAt) features the highest catalytic efficiency of all characterized Udhs using glucuronic acid as substrate (829 s-1 mM-1). Unfortunately, it suffers from poor thermostability. To overcome this limitation, we created more thermostable variants using semi-rational design. The amino acids for substitution were chosen according to the B factor in combination with four additional knowledge-based criteria. The triple variant A41P/H101Y/H236K showed higher kinetic and thermodynamic stability with a T 5015 value of 62.2 °C (3.2 °C improvement) and a ∆∆GU of 2.3 kJ/mol compared to wild type. Interestingly, it was only obtained when including a neutral mutation in the combination.

Characterization of uronate dehydrogenases catalysing the initial step in an oxidative pathway.

Uronate dehydrogenases catalyse the oxidation of uronic acids to aldaric acids, which represent 'top value-added chemicals' that have the potential to substitute petroleum-derived chemicals. The identification and annotation of three uronate dehydrogenases derived from Fulvimarina pelagi HTCC2506, Streptomyces viridochromogenes DSM 40736 and Oceanicola granulosus DSM 15982 via sequence analysis is described. Characterization and comparison with two known uronate dehydrogenases in regard to substrate spectrum, catalytic activity and pH as well as temperature dependence was performed. The catalytic efficiency was investigated in two different buffer systems; potassium phosphate and Tris-HCl. In addition to the typical and well available substrates glucuronate and galacturonate also mannuronate as part of many structural polysaccharides were tested. The uronate dehydrogenase of Agrobacterium tumefaciens and Pseudomonas syringae showed catalytic dependency on the buffer system resulting in an increased Km especially for glucuronate in potassium phosphate compared with Tris-HCl buffer. Enzyme stability at 37°C of the different Udhs was in the order: P. syringae < S. viridochromogens < A. tumefaciens < F. pelagi < O. granulosus. All enzymes showed activity within a broad pH range from 7.0 to 9.5, only O. granulosus had a very narrow range around 7.0.

A water-forming NADH oxidase from Lactobacillus pentosus suitable for the regeneration of synthetic biomimetic cofactors.

The cell-free biocatalytic production of fine chemicals by oxidoreductases has continuously grown over the past years. Since especially dehydrogenases depend on the stoichiometric use of nicotinamide pyridine cofactors, an integrated efficient recycling system is crucial to allow process operation under economic conditions. Lately, the variety of cofactors for biocatalysis was broadened by the utilization of totally synthetic and cheap biomimetics. Though, to date the regeneration has been limited to chemical or electrochemical methods. Here, we report an enzymatic recycling by the flavoprotein NADH-oxidase from Lactobacillus pentosus (LpNox). Since this enzyme has not been described before, we first characterized it in regard to its optimal reaction parameters. We found that the heterologously overexpressed enzyme only contained 13% FAD. In vitro loading of the enzyme with FAD, resulted in a higher specific activity towards its natural cofactor NADH as well as different nicotinamide derived biomimetics. Apart from the enzymatic recycling, which gives water as a by-product by transferring four electrons onto oxygen, unbound FAD can also catalyze the oxidation of biomimetic cofactors. Here a two electron process takes place yielding H2O2 instead. The enzymatic and chemical recycling was compared in regard to reaction kinetics for the natural and biomimetic cofactors. With LpNox and FAD, two recycling strategies for biomimetic cofactors are described with either water or hydrogen peroxide as by-product.

Improving the NADH-cofactor specificity of the highly active AdhZ3 and AdhZ2 from Escherichia coli K-12.

Biocatalysis is a promising tool for the sustainable production of chemicals. When cofactor depending enzymatic reactions are involved the applicability of the right cofactor is a central issue. One important example in this regard is the production of alcohols by nicotinamide cofactor (NAD(P)(+)) depending alcohol dehydrogenases. AdhZ3 from Escherichia coli, which is important for the production of alcohols from biomass, has a preference for NADPH as cofactor. We used a structure guided site-specific random approach, to change the cofactor preference towards NADH and to deduce more general rules for redesigning the cofactor specificity. Transfer of a triplet motif from NADH preferring horse liver ADH to AdhZ3 showed an insufficient switch in the preference towards NADH. A combinatorial site saturation mutagenesis altering three residues at once was applied. Library screening with two different cofactor concentrations (0.1 and 0.3mM) resulted in nine improved variants with AdhZ3-LND having the highest vmax and AdhZ3-CND having the lowest K(m). Asparagine was the most frequent amino acid found in eight of nine triplet motifs. To verify the triplet-motif, two variants of E. coli AdhZ2 DIN and LND were designed and confirmed for improved activity with NADH.

Draft Genome Sequence of Kozakia baliensis SR-745, the First Sequenced Kozakia Strain from the Family Acetobacteraceae.

Kozakia baliensis belongs to the family Acetobacteraceae and was described for the first time in 2002. These acetic acid bacteria are able to produce acetic acid from various carbon sources and 2- and 5-keto-d-gluconate from glucose. The novel K. baliensis strain SR-745 was isolated from a pineapple fruit bought in a German supermarket. The strain produces large amounts of organic acids when grown on glucose-containing medium and accepts also glycerol, fructose, mannitol, and sucrose as a C source. When grown under light and high-oxygen conditions in submerged culture, the production of a pink pigment is observed after 72 h.

Cell-imprinted substrates direct the fate of stem cells.

Smart nanoenvironments were obtained by cell-imprinted substrates based on mature and dedifferentiated chondrocytes as templates. Rabbit adipose derived mesenchymal stem cells (ADSCs) seeded on these cell-imprinted substrates were driven to adopt the specific shape (as determined in terms of cell morphology) and molecular characteristics (as determined in terms of gene expression) of the cell types which had been used as template for the cell-imprinting. This method might pave the way for a reliable, efficient, and cheap way of controlling stem cell differentiation. Data also suggest that besides residual cellular fragments, which are presented on the template surface, the imprinted topography of the templates plays a role in the differentiation of the stem cells.