In situ liquid secondary ion mass spectrometry (SIMS) enabled by system for analysis at the liquid vacuum interface (SALVI) has proven to be a promising new tool to provide molecular information at solid–liquid and liquid–vacuum interfaces. However, the initial data showed that useful signals in positive ion spectra are too weak to be meaningful in most cases. In addition, it is difficult to obtain strong negative molecular ion signals when m/z>200. These two drawbacks have been the biggest obstacle towards practical use of this new analytical approach. In this study, we report that strong and reliable positive and negative molecular signals are achievable after optimizing the SIMS experimental conditions. Four model systems, including a 1,8-diazabicycloundec-7-ene (DBU)-base switchable ionic liquid, a live Shewanella oneidensis biofilm, a hydrated mammalian epithelia cell, and an electrolyte popularly used in Li ion batteries were studied. A signal enhancement of about two orders of magnitude was obtained in comparison with non-optimized conditions. Therefore, molecular ion signal intensity has become very acceptable for use of in situ liquid SIMS to study solid–liquid and liquid–vacuum interfaces.
CsCTS, a new diterpene synthase from Cephalotaxus sinensis responsible for forming cephalotene, the core skeleton of cephalotane-type diterpenoids with a highly rigid 6/6/5/7 tetracyclic ring system, was functionally characterized. The stepwise cyclization mechanism is proposed mainly based on structural investigation of its derailment products, and further demonstrated through isotopic labeling experiments and density functional theory calculations. Homology modeling and molecular dynamics simulation combined with site-directed mutagenesis revealed the critical amino acid residues for the unique carbocation-driven cascade cyclization mechanism of CsCTS. Altogether, this study reports the discovery of the diterpene synthase that catalyzes the first committed step of cephalotane-type diterpenoid biosynthesis and delineates its cyclization mechanism, laying the foundation to decipher and artificially construct the complete biosynthetic pathway of this type diterpenoids. 相似文献
Summary Analytical methods have been developed for the determination of organic pollutants of intermediate polarity in sewage. Water
samples are first passed through a solid phase adsorption cartridge. The analytes are then extracted from the absorbent with
supercritical CO2 into a small volume of trapping solvent. Finally, the extracts are analyzed directly by capillary gas chromatography and
gas chromatography-mass spectrometer. The various parameters (pressure, temperature, type and concentration of modifiers,
trapping solvent, flow rate and volume of supercritical fluid and equilibrium time) influencing the efficiency of extraction
were studied. Extraction efficiencies for the test compounds are >70%, and relative standard deviations are <4.6% (n=3). The
methods established were applied to the analysis of sewage at the Lanzhou Wastewater Treatment Plant, China. 66 organic pollutants
were detected, of which 15 compounds appeared in the list of priority pollutants suggested by the US EPA. 相似文献
