Significant increase of the photoresponse range and conductivity for a chalcogenide semiconductor by viologen coating through charge transfer.

Widening the photoresponse range while enhancing the electrical properties of semiconductors could reduce the complexity and cost of photodetectors or increase the power conversion efficiency of solar cells. Surface doping through charge transfer with organic species is one of the most effective and widely used approaches to achieve this aim. It usually features easier preparation over other doping methods but is still limited by the low physicochemical stability and high cost of the used organic species or low improvement of electrical properties. This work shows unprecedented surface doping of semiconductors with highly stable, easily obtained, and strong electron-accepting viologen components, realizing the significant improvement of both the photoresponse range and conductivity. Coating the chalcogenide semiconductor KGaS2 with dimethyl viologen dichloride (MV) yields a charge-transfer complex (CTC) on the surface, which broadens the photoresponse range by nearly 300 nm and improves the conductivity by 5 orders of magnitude. The latter value surpasses all records obtained by surface doping through charge transfer with organic species.

Unraveling the Origins of Changing Product Specificity Properties of Arginine Methyltransferase PRMT7 by the E181D and E181D/Q329A Mutations through QM/MM MD and Free-Energy Simulations.

Arginine methylations can regulate important biological processes and affect many cellular activities, and the enzymes that catalyze the methylations are protein arginine methyltransferases (PRMTs). The biological consequences of arginine methylations depend on the methylation states of arginine that are determined by the PRMT's product specificity. Nevertheless, it is still unclear how different PRMTs may generate different methylation states for the target proteins. PRMT7 is the only known member of type III PRMT that produces monomethyl arginine (MMA) product. Interestingly, its E181D and E181D/Q329A mutants can catalyze, respectively, the formation of asymmetrically dimethylated arginine (ADMA) and symmetrically dimethylated arginine (SDMA). The reasons as to why the mutants have the abilities to add the second methyl group and E181D (E181D/Q329A) has the unique product specificity in generating ADMA (SDMA) have not been understood. Here, quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) and potential of mean force (PMF) free-energy simulations are performed for the E181D and E181D/Q329A mutants to understand the origin for their ability to generate, respectively, ADMA and SDMA. The simulations show that the free-energy barrier for adding the second methyl group to MMA in E181D (E181D/Q329A) to produce ADMA (SDMA) is considerably lower than the corresponding barriers in wild type and E181D/Q329A (wild type and E181D), consistent with experimental observations. Some important factors that contribute to the change of the activity and product specificity due to the E181D and E181D/Q329A mutations are identified based on the data from the simulations and analysis. It is shown that the transferable methyl group (from SAM) and Nη2 (the nitrogen atom that is methylated in the substrate MMA) can only form good near-attack conformations in the E181D reaction state for the methyl transfer (not in wild type and E181D/Q329A), while the transferable methyl group and Nη1 (the nitrogen atom that is not methylated in the substrate MMA) can only form good near-attack conformations in E181D/Q329A (not in wild type and E181D). The results suggest that the steric repulsions in the reaction state between the methyl group on MMA and active-site residues (e.g., Q329) and the release of such repulsions (e.g., from the Q329A mutation) may play an important role in generating specific near-attack conformations for the methyl transfer and controlling the product specificity for the mutants. The general principle identified in this work for PRMT7 is expected to be useful for understanding the activity and product specificity of other PRMTs as well.

Photochromic Semiconductive Hydrogen-Bonded Organic Framework (HOF) with Broadband Absorption.

Semiconductors with broadband photoelectric response have important practical needs in many aspects such as solar energy conversion, photocatalysis, and photodetection. We synthesized the first photochromic semiconductive hydrogen-bonded organic framework (HOF), [H2(bpyb)](H2PO4)2·2H2O (1), using the polycyclic viologen cation [H2(bpyb)] (bpyb = 1,4-bis(tetrapyridyl)benzene). After 1 s of xenon lamp irradiation, compound 1 showed a visible color change from the initial yellowish to dark purple after continuous irradiation. The photoinduced radical product has an absorption band covering 200-1700 nm, which is wider than the absorption ranges of silicon and perovskites. It produced photocurrent when irradiated with a xenon lamp or a laser (355, 532, or 808 nm). The on/off ratio of the current (Iirr/Idark) can be as high as 300 times under the irradiation of the 808 nm laser with a power of 1.9 W cm-2. In addition, under the 808 nm light source, the on/off ratio of 1B is 35 times that of 1A.

Catalytic Mechanism and Product Specificity of Protein Arginine Methyltransferase PRMT7: A Study from QM/MM Molecular Dynamics and Free Energy Simulations.

QM/MM molecular dynamics and potential of mean force (PMF) free-energy simulations are performed for wild-type PRMT7 and E172Q, E181Q, and Q329A mutants in this work, and the catalytic mechanism, product specificity, and the role of key residues for the PRMT7 activity are investigated. The main strategies of PRMT7 in reducing the activation barrier for methyl transfer that are found in this study include (1) formation of reactive (near attack) conformations for the substrate Arg, (2) strengthening the active-site interactions at the transition state, and (3) generation of more effective nucleophiles by changing charge distributions on the target Arg through active-site interactions. More importantly, it is shown that it is a combination of these different factors that determines the PRMT7 methylation activity and substrate/product specificity. By taking these factors into consideration, it is possible to provide explanations for the observed effects of some mutations. For E172Q, E181Q, and Q329A, the simulation results suggest that E172Q has the least activity among the three mutants. The free energy barrier increases by 7 and 3 kcal/mol, respectively, as a result of the E181 → Q and Q329 → A mutations. The results showed that PRMT7 has a preference of adding a methyl group to the ω-guanidino nitrogen Nη2 atom of the substrate Arg and that the second methylation reactions cannot occur, which are consistent with previous investigations.