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.