Mimicking enzymatic cation-π interactions in hydrazide catalyst design: access to trans-decalin frameworks.

Chiral bicyclic hydrazide organocatalysts have previously been shown to catalyze the cyclization of (Z)-polyene substrates with high enantioselectivity, but with poor selectivity for the corresponding (E)-polyenes. Here we demonstrate that diazapane carboxylates bearing terphenyl groups efficiently catalyze (E)-polyene bicyclization with enantioselectivities up to 94 : 6 er and with high diastereoselectivity for trans-decalin formation. The catalysts function by simultaneously initiating the cyclization via iminium ion formation and stabilizing intermediates/transition states by cation-π interactions.

Identification of triglycerides in liquid and fire debris samples by triple quadrupole liquid chromatography-mass spectrometry.

Analysis of fire debris for triglyceride-based oils may be of interest to fire investigators depending on the circumstances of a particular fire. Such circumstances include accidental or intentionally set cooking oil fires, fires involving triglyceride-based "eco" fire log products, and spontaneous ignition fires that involve drying oils on rags. Many forensic laboratories utilize gas chromatography-mass spectrometry following fatty acid methyl esterification to identify triglyceride residues in fire debris extracts. This study explores an alternate approach, which involves the identification of intact triglycerides by liquid chromatography-mass spectrometry-mass spectrometry (LC-MS/MS). 52 triglyceride-based oils and fats (22 different types) were analyzed by LC-MS/MS using multiple reaction monitoring to investigate variation in triglyceride content between different brands and types of oil and fat. Selected oils were then degraded by exposure to air, typical cooking conditions and/or fire conditions to simulate samples that are typically encountered by Fire Debris Analysts in fire investigation cases. Triglycerides were identified in all pristine and degraded oil samples, and relative peak areas for degraded samples often resembled their pristine oil counterparts. In samples where relative peak area differences were noted, more predominant degradation was observed for triglycerides with a higher proportion of poly-unsaturated fatty acids. Variability in triglyceride content between different brands and types of oil are discussed, as well as factors affecting the identification of triglyceride peaks in commercial oil samples, as compared to the corresponding analytical standard.

Hydrazide-Catalyzed Polyene Cyclization: Asymmetric Organocatalytic Synthesis of cis-Decalins.

Polyene cyclizations offer rapid entry into terpenoid ring systems. Although enantioselective cyclizations of (E)-polyenes to form trans-decalin ring systems are well precedented, highly enantioselective cyclizations of (Z)-polyenes to form the corresponding cis-decalins have not been reported. Here, we describe the first application of iminium catalysis to the initiation of polyene cyclizations. Ethyl 1,2-diazepane-1-carboxylate catalyzes the cyclization of polyenes bearing enal initiators. Moreover, chiral bicyclic hydrazides catalyze the cyclizations of (Z)-polyene substrates to form cis-decalins with enantioselectivities of up to 97:3 er. DFT calculations suggest the catalysts promote the reaction by stabilizing positive charge as it develops during the bicyclization.

Structure of an Intrinsically Disordered Stress Protein Alone and Bound to a Membrane Surface.

Dehydrins are a group of intrinsically disordered proteins that protect plants from damage caused by drought, cold, and high salinity. Like other intrinsically disordered proteins, dehydrins can gain structure when bound to a ligand. Previous studies have shown that dehydrins are able to protect liposomes from cold damage, but the interactions that drive membrane binding and the detailed structure of the bound and unbound forms are not known. We use an ensemble-structure approach to generate models of a dehydrin known as K2 in the presence and absence of sodium dodecyl sulfate micelles, and we docked the bound structure to the micelle. The collection of residual dipolar coupling data, amide protection factors, and paramagnetic relaxation enhancement distances, in combination with chemical shifts and relaxation measurements, allows for determining plausible structures that are not otherwise visible in time-averaged structural data. The results show that in the bound structure, the conserved lysines are important for membrane binding, whereas the flanking hydrophobic residues play a lesser role. The unbound structure shows a high level of disorder and an extended structure. We propose that the structural differences between bound and unbound forms allow dehydrins to act as molecular shields in their unbound state and as membrane protectants in their bound state. Unlike α-synuclein, the significant gain of α-helicity in K2 at low concentrations of sodium dodecyl sulfate is not due to a decrease in the critical micelle concentration. The study provides structural insight into how a disordered protein can interact with a membrane surface.

Structural and Functional Insights into the Cryoprotection of Membranes by the Intrinsically Disordered Dehydrins.

Dehydration can be due to desiccation caused by a lack of environmental water or to freezing caused by a lack of liquid water. Plants have evolved a large family of proteins called LEA (late embryogenesis abundant) proteins, which include the intrinsically disordered dehydrin (dehydration protein) family, to combat these abiotic stresses. Although transcription and translation studies have shown a correlation between dehydration stress and the presence of dehydrins, the biochemical mechanisms have remained somewhat elusive. We examine here the effect and structure of a small model dehydrin (Vitis riparia K2) on the protection of membranes from freeze-thaw stress. This protein is able to bind to liposomes containing phosphatidic acid and protect the liposomes from fusing after freeze-thaw treatment. The presence of K2 did not measurably affect liposome surface accessibility or lipid mobility but did lower its membrane transition temperature by 3 °C. Using sodium dodecyl sulfate as a membrane model, we examined the NMR structure of K2 in the presence and absence of the micelle. Biochemical and NMR experiments show that the conserved, lysine-rich segments are involved in the binding of the dehydrin to a membrane, whereas the poorly conserved φ segments play no role in binding or protection.