The chalcone synthase superfamily of type III polyketide synthases

This review covers the functionally diverse type III polyketide synthase (PKS) superfamily of plant and bacterial biosynthetic enzymes. from the discovery of chalcone synthase (CHS) in the 1970s through the end of 2001. A broader perspective is achieved by a comparison of these CHS-like enzymes to mechanistically and evolutionarily related families of enzymes, including the type I and type II PKSs, as well as the thiolases and beta-ketoacyl synthases of fatty acid metabolism. As CHS is both the most frequently occurring and best studied type III PKS, this enzyme's structure and mechanism is examined in detail. The in vivo functions and biological activities of several classes of plant natural products derived from chalcones are also discussed. Evolutionary mechanisms of type III PKS divergence are considered, as are the biological functions and activities of each of the known and functionally divergent type III PKS enzymc families (currently twelve in plants and three in bacteria). A major focus of this review is the integration of information from genetic and biochemical studies with the unique insights gained from protein X-ray crystallography and homology modeling. This structural approach has generated a number of new predictions regarding both the importance and mechanistic role of various amino acid substitutions observed among functionally diverse type III PKS enzymes.     

Rotational energy transfer in NO (A 2Sigma+,v'=0) by N2 and O2 at room temperature

State-to-state rotational energy transfer (RET) rate coefficients for NO (A 2Sigma+, v'=0, J=5.5, 11.5, 17.5) were measured for N2 and O2 at room temperature using a pump-probe method. The NO A 2Sigma+ state is prepared by 226 nm light and the RET is monitored by fluorescence from the D 2Sigma+ v'=0 state, following excitation by a time-delayed laser at approximately 1.1 microm. Additionally, total collisional removal and final state distributions were measured exciting in the Q1+P21 band head, to simulate an NO laser-induced fluorescence atmospheric monitoring scheme. Time-resolved modeling is used to understand relaxation mechanisms and predict relaxation times in ambient air. H2O at atmospherically relevant concentrations does not affect the degree of RET in ambient air.     

The influence of second-nearest neighbor interactions on the thermodynamic properties of B2 phases with substitutional defects

In order to explain some discrepancies between the theoretical predictions and the experimental data for the thermodynamic properties of substitutional B 2 phases,Chang's theoretical model is extended by including the influence of second-nearest neighbor interactions. For this purpose a new parameter is introduced which is defined as the ratio of the interchange energies between second-nearest and first-nearest neighbors. Theoretical equations are derived for the compositional dependence of the activity and the partial molar enthalpy. Using literature data, the following phases are re-evaluated in terms of the disorder parameter and the newly introduced parameter : -AuZn, -AuCd, -AgMg, and -NiZn. Very good agreement is found between the theoretical curves and the experimental data for the four systems. The values of obtained range from 0.0 for -NiZn to 0.5 for -AuZn. The inclusion of second-nearest neighbor interactions has little influence on the values of . It is shown that the behavior of the activity curve in -AuZn can be explained in a physically more meaningful way by including interactions between all second-nearest neighbors rather than interactions between gold substitutional defects only, as was done byLibowitz.With 5 Figures     

Mild olefin formation via bio-inspired vitamin B12 photocatalysis

Dehydrohalogenation, or elimination of hydrogen-halide equivalents, remains one of the simplest methods for the installation of the biologically-important olefin functionality. However, this transformation often requires harsh, strongly-basic conditions, rare noble metals, or both, limiting its applicability in the synthesis of complex molecules. Nature has pursued a complementary approach in the novel vitamin B₁₂-dependent photoreceptor CarH, where photolysis of a cobalt–carbon bond leads to selective olefin formation under mild, physiologically-relevant conditions. Herein we report a light-driven B₁₂-based catalytic system that leverages this reactivity to convert alkyl electrophiles to olefins under incredibly mild conditions using only earth abundant elements. Further, this process exhibits a high level of regioselectivity, producing terminal olefins in moderate to excellent yield and exceptional selectivity. Finally, we are able to access a hitherto-unknown transformation, remote elimination, using two cobalt catalysts in tandem to produce subterminal olefins with excellent regioselectivity. Together, we show vitamin B₁₂ to be a powerful platform for developing mild olefin-forming reactions.

Terminal or subterminal olefins can be selectively formed from alkyl electrophiles via bio-inspired vitamin B₁₂ photocatalysis.     

A novel approach to perform metabolite screening during the quantitative LC-MS/MS analyses of in vitro metabolic stability samples using a hybrid triple-quadrupole linear ion trap mass spectrometer

In vitro metabolic stability experiments using microsomes or other liver preparations are important components in the discovery and lead-optimization stages of compound selection in the pharmaceutical industry. Currently, liquid chromatography-tandem mass spectrometric (LC-MS/MS) support of in vitro metabolic stability studies primarily involves the monitoring of disappearance of parent compounds, using selected reaction monitoring (SRM) on triple-quadrupole instruments. If moderate to high turnover is observed, separate metabolite identification experiments are then conducted to characterize the biotransformation products. In this paper, we present a novel method to simultaneously perform metabolite screening in addition to the quantitative stability measurements, both within the same chromatographic run. This is accomplished by combining SRM and SRM-triggered, information-dependent acquisition (IDA) of MS/MS spectra on a hybrid triple-quadrupole linear ion trap (QqQLIT) mass spectrometer. Microsomal stability experiments using model compounds, bufuralol, propranolol, imipramine, midazolam, verapamil and diclofenac, were used to demonstrate the applicability of our approach. This SRM + SRM-IDA approach generated metabolic stability results similar to those obtained by conventional SRM-only approach. In addition, MS/MS spectra from potential metabolites were obtained with the enhanced product ion (EPI) scan function of LIT during the same injection. These spectra were correlated to the spectra of parent compounds to confirm the postulated structures. The time-concentration profiles of identified metabolites were also estimated from the acquired data. This approach has been successfully used to support discovery programs.     

Simple,inexpensive mass spectrometric analyzer for thermogravimetry

A low‐cost mass spectrometer attachment for thermogravimetric analysis has been constructed from readily available commercial instruments and components. The benefits of this set‐up include excellent mass‐flow repeatability, simple design, and significantly lower adoption cost as opposed to ready‐built commercial solutions. The inclusion of an open source software package allows semi‐automated, highly simplified data analysis. The results from the instrument show excellent sensitivity for small volumes of evolved gas, as well as highly reproducible signal strengths. The GUI‐based software package provides data analysis in a way that is very intuitive and that can be easily modified to work with a broad range of TG instruments. Copyright © 2011 John Wiley & Sons, Ltd.     

Design of a microfabricated magnetic cell separator.

A new magnetic separation idea utilizing several ideas from microfabrication and nanomagnetics is presented. The basic idea comes from our earlier work using asymmetry in obstacles and Brownian motion to effect separation of objetcs [10] by moving them in streams whose angle to the hydrodynamic average velocity is a function of the diffusion coefficient of the object. The device we propose here is not technically a Brownian ratchet device but uses the idea of force which acts at angle to the hydrodynamic flow. In our case, the force is generated by a magnetic field gradient which comes from an array of magnetized wires which lie at an angle 0 to a hydrodynamic field flow. The sum of the hydrodynamic force and the magnetic force create a new vector which as in the case of the Brownian ratchet moves the cell out of the main stream direction.