Zhuhua Wu

An enzymatic [4+2] cyclization cascade creates the pentacyclic core of pyrroindomycins

The [4+2] cycloaddition remains one of the most intriguing transformations in synthetic and natural products chemistry. In nature, however, there are remarkably few enzymes known to have this activity. We herein report an unprecedented enzymatic [4+2] cyclization cascade that has a central role in the biosynthesis of pyrroindomycins, which are pentacyclic spirotetramate natural products. Beginning with a linear intermediate that contains two pairs of 1,3-diene and alkene groups, the dedicated cyclases PyrE3 and PyrI4 act in tandem to catalyze the formation of two cyclohexene rings in the dialkyldecalin system and the tetramate spiro-conjugate of the molecules. The two cyclizations are completely enzyme dependent and proceed in a regio- and stereoselective manner to establish the enantiomerically pure pentacyclic core. Analysis of a related spirotetronate pathway confirms that homologs are functionally exchangeable, establishing the generality of these findings and explaining how nature creates diverse active molecules with similar rigid scaffolds.

Spiroketal formation and modification in avermectin biosynthesis involves a dual activity of AveC.

Avermectins (AVEs), which are widely used for the treatment of agricultural parasitic diseases, belong to a family of 6,6-spiroketal moiety-containing, macrolide natural products. AVE biosynthesis is known to employ a type I polyketide synthase (PKS) system to assemble the molecular skeleton for further functionalization. It remains unknown how and when spiroketal formation proceeds, particularly regarding the role of AveC, a unique protein in the pathway that shares no sequence homology to any enzyme of known function. Here, we report the unprecedented, dual function of AveC by correlating its activity with spiroketal formation and modification during the AVE biosynthetic process. The findings in this study were supported by characterizing extremely unstable intermediates, products and their spontaneous derivative products from the simplified chemical profile and by comparative analysis of in vitro biotransformations and in vivo complementations mediated by AveC and MeiC (the counterpart in biosynthesizing the naturally occurring, AVE-like meilingmycins). AveC catalyzes the stereospecific spiroketalization of a dihydroxy-ketone polyketide intermediate and the optional dehydration to determine the regiospecific saturation characteristics of spiroketal diversity. These reactions take place between the closures of the hexene ring and 16-membered macrolide and the formation of the hexahydrobenzofuran unit. MeiC can replace the spirocyclase activity of AveC, but it lacks the independent dehydratase activity. Elucidation of the generality and specificity of AveC-type proteins allows for the rationalization of previously published results that were not completely understood, suggesting that enzyme-mediated spiroketal formation was initially underestimated, but is, in fact, widespread in nature for the control of stereoselectivity.

Insights into Pyrroindomycin Biosynthesis Reveal a Uniform Paradigm for Tetramate/Tetronate Formation

The natural products pyrroindomycins (PYRs), active against various drug-resistant pathogens, possess a characteristic, cyclohexene ring spiro-linked tetramate moiety. In this study, investigation into PYR biosynthesis revealed two new proteins, both of which, phylogenetically distinct from but functionally substitutable to each other in vivo, individually catalyze a Dieckmann cyclization in vitro for converting an N-acetoacetyl-l-alanyl thioester into a tetramate. Their counterparts are commonly present in the biosynthetic pathways of spiro and polyether tetronates, supporting a uniform paradigm for tetronate/tetramate formation, which features an enzymatic way to generate the C-X (X = O or N) bond first and the C-C bond next in building of the 5-membered heterocycle.

1,19-seco-Avermectin analogues from a ΔaveCDE mutant Streptomyces avermectinius strain.

Three new 1,19-seco-avermectin (AVE) analogues were isolated from the ΔaveCDE mutant Streptomyces avermectinius strain. Their structures were elucidated by detailed spectroscopic analysis. This is the first report of 1,19-seco-AVE analogues. In an in vitro assay these compounds displayed cytotoxicity against Saos-2, MG-63, and B16 cell lines.

A linear hydroxymethyl tetramate undergoes an acetylation–elimination process for exocyclic methylene formation in the biosynthetic pathway of pyrroindomycins

We herein report the isolation and characterization of a key linear intermediate in the biosynthetic pathway of pyrroindomycins, the potent spirotetramate natural products produced by Streptomyces rugosporus. This polyene intermediate bears a γ-hydroxymethyl group that is exocyclic to the tetramate moiety, indicating that a serine residue serves as the three-carbon unit for tetramate formation and chain-elongation termination. The further conversion involves an acetylation-elimination of the exocyclic γ-hydroxymethyl group to generate a γ-methylene group, which is indispensable for intramolecular [4 + 2] cross-bridging to construct the characteristic pentacyclic core. The findings presented in this study provide new insights into the biosynthesis of pyrroindomycins, and thus suggest a common paradigm for both spirotetramates and spirotetronates in processing the exocyclic γ-hydroxymethyl group of the five-membered heterocycle.

Stress-induced martensitic transformation during tensile test of full-size TF conductor jacket tube at 4.2 K

The toroidal-field (TF) conductor jacket of International Thermonuclear Experimental Reactor (ITER) is made of modified 316LN stainless steel, which is influenced by heat treatment at approximately 650 °C for 200 h to produce Nb3Sn superconducting materials at the final stage. Due to the high electromagnetic forces arising during magnet operation, higher mechanical properties of the jacket materials at cryogenic temperatures are required. In our work, mechanical properties of the full-size TF conductor jacket tube were investigated, which satisfied the ITER requirements. Stress-induced martensitic transformation mechanism during tensile test of the conductor jacket material at 4.2 K was characterized by means of in-situ temperature dependent XRD, vibrating sample magnetometer (VSM) and in conjunction with transmission electron microscopy (TEM). The tensile behavior related to the amount of stress-induced phase transformation at cryogenic temperature was also discussed.The toroidal-field (TF) conductor jacket of International Thermonuclear Experimental Reactor (ITER) is made of modified 316LN stainless steel, which is influenced by heat treatment at approximately 650 °C for 200 h to produce Nb3Sn superconducting materials at the final stage. Due to the high electromagnetic forces arising during magnet operation, higher mechanical properties of the jacket materials at cryogenic temperatures are required. In our work, mechanical properties of the full-size TF conductor jacket tube were investigated, which satisfied the ITER requirements. Stress-induced martensitic transformation mechanism during tensile test of the conductor jacket material at 4.2 K was characterized by means of in-situ temperature dependent XRD, vibrating sample magnetometer (VSM) and in conjunction with transmission electron microscopy (TEM). The tensile behavior related to the amount of stress-induced phase transformation at cryogenic temperature was also discussed.

Rheological behavior and cryogenic properties of cyanate ester/epoxy insulation material for fusion superconducting magnet

In a Tokamak fusion reactor device like ITER, insulation materials for superconducting magnets are usually fabricated by a vacuum pressure impregnation (VPI) process. Thus these insulation materials must exhibit low viscosity, long working life as well as good radiation resistance. Previous studies have indicated that cyanate ester (CE) blended with epoxy has an excellent resistance against neutron irradiation which is expected to be a candidate insulation material for a fusion magnet. In this work, the rheological behavior of a CE/epoxy (CE/EP) blend containing 40% CE was investigated with non-isothermal and isothermal viscosity experiments. Furthermore, the cryogenic mechanical and electrical properties of the composite were evaluated in terms of interlaminar shear strength and electrical breakdown strength. The results showed that CE/epoxy blend had a very low viscosity and an exceptionally long processing life of about 4 days at 60 °C.

Cryogenic thermal expansion and electrical conductivities of Mn3CuN co-doped with Sb and Sn

A series of Sb and Sn co-doped anti-perovskite Mn3CuN materials were fabricated by mechanical ball milling followed by solid-state sintering. Their thermal expansion properties and electrical conductivities were investigated in the temperature range of 77-300 K. The results show that Mn3(CuxSb1-x)N (x=0.4, 0.5, 0.6, 0.7) exhibit negative thermal expansion (NTE) below 150 K, and the NTE temperature range shifts toward the lower temperature with the Sb increased. However, Sn doped in the Mn3(Cu0.5SbxSn1-x)N can lead the NTE behavior shifts to room temperature and the NTE operation-temperature window (ΔT) becomes broader. The electrical conductivities of Mn3(Cu0.5SbxSn1-x)N decrease with the temperature and Sb content increased.