Improving Surfactin Production in Bacillus subtilis 168 by Metabolic Engineering.

Surfactin is widely used in the petroleum extraction, cosmetics, biopharmaceuticals and agriculture industries. It possesses antibacterial and antiviral activities and can reduce interfacial tension. Bacillus are commonly used as production chassis, but wild-type Bacillus subtilis 168 cannot synthesise surfactin. In this study, the phosphopantetheinyl transferase (PPTase) gene sfp* (with a T base removed) was overexpressed and enzyme activity was restored, enabling B. subtilis 168 to synthesise surfactin with a yield of 747.5 ± 6.5 mg/L. Knocking out ppsD and yvkC did not enhance surfactin synthesis. Overexpression of predicted surfactin transporter gene yfiS increased its titre to 1060.7 ± 89.4 mg/L, while overexpression of yerP, ycxA and ycxA-efp had little or negative effects on surfactin synthesis, suggesting YfiS is involved in surfactin efflux. By replacing the native promoter of the srfA operon encoding surfactin synthase with three promoters, surfactin synthesis was significantly reduced. However, knockout of the global transcriptional regulator gene codY enhanced the surfactin titre to 1601.8 ± 91.9 mg/L. The highest surfactin titre reached 3.89 ± 0.07 g/L, with the yield of 0.63 ± 0.02 g/g DCW, after 36 h of fed-batch fermentation in 5 L fermenter. This study provides a reference for further understanding surfactin synthesis and constructing microbial cell factories.

Recent advances in microbial synthesis of free heme.

Heme is an iron-containing porphyrin compound widely used in the fields of healthcare, food, and medicine. Compared to animal blood extraction, it is more advantageous to develop a microbial cell factory to produce heme. However, heme biosynthesis in microorganisms is tightly regulated, and its accumulation is highly cytotoxic. The current review describes the biosynthetic pathway of free heme, its fermentation production using different engineered bacteria constructed by metabolic engineering, and strategies for further improving heme synthesis. Heme synthetic pathway in Bacillus subtilis was modified utilizing genome-editing technology, resulting in significantly improved heme synthesis and secretion abilities. This technique avoided the use of multiple antibiotics and enhanced the genetic stability of strain. Hence, engineered B. subtilis could be an attractive cell factory for heme production. Further studies should be performed to enhance the expression of heme synthetic module and optimize the expression of heme exporter and fermentation processes, such as iron supply. KEY POINTS: • Strengthening the heme biosynthetic pathway can significantly increase heme production. • Heme exporter overexpression helps to promote heme secretion, thereby further promoting excessive heme synthesis. • Engineered B. subtilis is an attractive alternative for heme production.

Real-Time Exciton Dynamics with Time-Dependent Density-Functional Theory.

Linear-response time-dependent density-functional theory (TDDFT) can describe excitonic features in the optical spectra of insulators and semiconductors, using exchange-correlation (xc) kernels behaving as -1/k^{2} to leading order. We show how excitons can be modeled in real-time TDDFT, using an xc vector potential constructed from approximate, long-range corrected xc kernels. We demonstrate, for various materials, that this real-time approach is consistent with frequency-dependent linear response, gives access to femtosecond exciton dynamics following short-pulse excitations, and can be extended with some caution into the nonlinear regime.

Intrinsic Electric Fields in Two-dimensional Materials Boost the Solar-to-Hydrogen Efficiency for Photocatalytic Water Splitting.

Two-dimensional (2D) materials with the vertical intrinsic electric fields show great promise in inhibiting the recombination of photogenerated carriers and widening light absorption region for the photocatalytic applications. For the first time, we investigated the potential feasibility of the experimentally attainable 2D M2X3 (M = Al, Ga, In; X = S, Se, Te) family featuring out-of-plane ferroelectricity used in photocatalytic water splitting. By using first-principles calculations, all the nine members of 2D M2X3 are verified to be available photocatalysts for overall water splitting. The predicted solar-to-hydrogen efficiency of Al2Te3, Ga2Se3, Ga2Te3, In2S3, In2Se3, and In2Te3 are larger than 10%. Excitingly, In2Te3 is manifested to be an infrared-light driven photocatalyst, and its solar-to-hydrogen efficiency limit using the full solar spectrum even reaches up to 32.1%, which breaks the conventional theoretical efficiency limit.

Penta-Pt2N4: an ideal two-dimensional material for nanoelectronics.

Since the discovery of graphene, two-dimensional (2D) materials have paved new ways to design high-performance nanoelectronic devices. To facilitate applications of such devices, there are three key requirements that a material needs to fulfill: sizeable band gap, high carrier mobility, and robust environmental stability. However, among the most popular 2D materials studied in recent years, graphene is gapless, hexagonal boron nitride has a very large band gap, transition metal dichalcogenides have low carrier mobility, and black phosphorene is ambience-sensitive. Thus far, these three characteristics could seldom be satisfied by only a single material. Therefore, it is a great challenge to find an ideal 2D material that can overcome these limitations. In this study, we theoretically predicted a novel planar 2D material penta-Pt2N4, which was designed using the Cairo pentagonal tiling as well as the rare nitrogen double bonds. Most significantly, 2D penta-Pt2N4 exhibits excellent intrinsic properties, including large direct band gap (up to 1.51 eV), high carrier mobility (up to 105 cm2·V-1·s-1), very high Young's modulus (up to 0.70 TPa), and robust dynamic, thermal, and ambient stabilities. Moreover, penta-Pt2N4 is the global minimum structure among 2D materials with PtN2 stoichiometry. We also propose a CVD/MBE scheme to enable its experimental synthesis. We envision that 2D penta-Pt2N4 may find wide applications in the field of nanoelectronics.

The roles of buckled geometry and water environment in the excitonic properties of graphitic C3N4.

The exciton plays a crucial role in two-dimensional materials involved in photocatalytic water splitting, where its properties are determined not only by the material itself, but also by the surrounding water environment. By the framework of many-body perturbation theory, we investigated the excitonic effects in pure and water-adsorbed g-C3N4. It is shown that the excitonic properties are very sensitive to the geometry of g-C3N4 and the adsorption of water molecules. Firstly, the optical band gap, i.e. the first bright excitonic energy of pure g-C3N4 decreases remarkably from a high symmetry planar structure (3.8 eV) to a P1 buckled configuration (2.7 eV). Secondly, the hydrogen bonds between water and g-C3N4 induce the generation of interface excitons. With a reduced binding energy (at least 0.2 eV), interface excitons can contribute to a more efficient separation of electrons and holes. Our work provides an insight into the excitation mechanism of 2D photocatalysts in a real environment.

Spatial and thickness dependence of coupling interaction of surface states and influence on transport and optical properties of few-layer Bi2Se3.

Coupling interaction between the bottom and top surface electronic states and the influence on transport and optical properties of Bi2Se3 thin films with 1-8 quintuple layers (QLs) have been investigated by first principles calculations. Obvious spatial and thickness dependences of coupling interaction are found by analyzing hybridization of two surface states. In the thin film with a certain thickness, from the outer to inner atomic layers, the coupling interaction exhibits an increasing trend. On the other hand, as thickness increases, the coupling interaction shows a disproportionate decrease trend. Moreover, the system with 3 QLs exhibits stronger interaction than that with 2 QLs. The presence of coupling interaction would suppress destructive interference of surface states and enhance resistance in various degrees. In view of the inversely proportional relation to transport channel width, the resistance of thin films should show disproportionate thickness dependence. This prediction is qualitatively consistent with the transport measurements at low temperature. Furthermore, the optical properties also exhibit obvious thickness dependence. Especially as the thickness increases, the coupling interaction results in red and blue shifts of the multiple-peak structures in low and high energy regions of imaginary dielectric function, respectively. The red shift trend is in agreement with the recent experimental observation and the blue shift is firstly predicted by the present calculation. The present results give a concrete understanding of transport and optical properties in devices based on Bi2Se3 thin films with few QLs.