Optical Control of the GTP Affinity of K-Ras(G12C) by a Photoswitchable Inhibitor.

Photocontrol of protein activity is an emerging field in biomedicine. For optical control of a mutant small GTPase K-Ras(G12C), we developed small-molecule inhibitors with photoswitchable efficacy, where one configuration binds the target protein and exert different pharmacological effects upon light irradiation. The compound design was based on the structure feature of a previously identified allosteric pocket of K-Ras(G12C) and the chemical structure of covalent inhibitors, and resulted in the synthesis and characterization of two representative azobenzene-containing compounds. Nucleotide exchange assays demonstrated the different efficacy to control the GTP affinity by photoswitching of one potent compound PS-C2, which would be a useful tool to probe the conformation of mutational K-Ras. Our study demonstrated the feasibility of designing photoswitchable modulators from allosteric covalent inhibitor of small GTPases.

Effect of humic acids on photodegradation of chloroacetanilide herbicides under UV irradiation.

The effects of two humic acids (HAs) of different origins on the photodegradation of the chloroacetanilide herbicides acetochlor, propisochlor and butachlor were investigated in this study. One of the tested HAs was a standard sample that was purchased from a commercial source, and the other was isolated from the black soil of Northeast China. The photolysis of all three herbicides followed pseudo-first-order kinetics under ultraviolet (UV) irradiation conditions, regardless of whether HAs were present or not. Both HAs improved the photolysis rates of acetochlor in a dose-reversed way, whereas they inhibited butachlor degradation under all experimental concentrations. The two HAs differed in their effects on propisochlor photolysis, changing from enhancement to inhibition, depending on the origin and concentration of HAs. Element and Fourier Transform Infrared spectroscopy analyses showed that the isolated HAs had more polysaccharides and less aliphatic groups than the commercial HAs, and it was indicated that some characteristic radicals (C=O, O-H and phenolic hydroxyls) in HAs were involved in the photolysis of the herbicides. Gas chromatography/mass spectrometry (GC/MS) analyses indicated that the presence of HAs had no effects on the photolysis pathway and photoproduct species of the three herbicides.

Simulation of mefenacet concentrations in paddy fields by an improved PCPF-1 model.

An improved simulation model (PCPF-1) has been evaluated for the prediction of the fate of mefenacet in an experimental paddy field. This model simulates the fate and transport of pesticide in paddy water and the top 1 cm of paddy soil. Observed concentrations of mefenacet in the paddy water and the surface soil exponentially decreased from their maximum concentrations of 0.70 mg litre(-1) and 11.3 mg kg(-1), respectively. Predicted mefenacet concentrations both in the water and surface soil were in excellent agreement with those measured during the first 2 weeks after herbicide application, but concentrations in paddy water were appreciably overestimated thereafter. The model simulated mefenacet losses through runoff, percolation and degradation to be respectively 41.9%, 6.4% and 57.3% of applied, and the mass balance error was about -6%. The model simulation implied that drainage and seepage control, especially shortly after application when herbicide concentrations are high, is essential for preventing pesticide losses from paddy fields. In focusing on pesticide concentrations in this early period the PCPF-1 model can be a beneficial tool for risk assessment of pesticide losses and in the evaluation of agricultural management for reducing pesticide pollution associated with paddy rice production.

Heterogeneous photocatalysed reaction of three selected pesticide derivatives, propham, propachlor and tebuthiuron in aqueous suspensions of titanium dioxide.

Heterogeneous photocatalysed reaction of three selected pesticide derivatives such as propham (1), propachlor (2) and tebuthiuron (3) has been investigated in aqueous suspensions of titanium dioxide by monitoring the change in substrate concentration employing UV Spectroscopic analysis and depletion in Total Organic Carbon (TOC) content as a function of irradiation time. The degradation kinetics was studied under different conditions such as pH, catalyst concentration, substrate concentration, different types of TiO(2) and in the presence of electron acceptors such as hydrogen peroxide (H(2)O(2)), potassium bromate (KBrO(3)) and ammonium persulphate (NH(4))(2)S(2)O(8) besides molecular oxygen. The degradation rates were found to be strongly influenced by all the above parameters. The photocatalyst Degussa P25 was found to be more efficient as compared with other photocatalysts. The pesticide derivative propham (1) was found to degrade faster as compared to propachlor (2) and tebuthiuron (3). An attempt has also been made to identify the products formed during the photooxidation process through GC/MS analysis technique. All the model pollutants showed the formation of several intermediate products, which were identified on the basis of molecular ion and mass spectrometric fragmentation pattern. A probable mechanism for the formation of the products has been proposed.