Photochemical activity of membrane-localised polyketide derived marine natural products

Marine organisms rely upon complex bio-chemical systems to manage photo-induced oxidative damage as part of a sophisticated chemical defense. Using a combination of synthetic chemistry and biophysical techniques, we provide evidence to support the notion that marine derived polyene γ-pyrone metabolites of sacoglossan molluscs become localised at cell membranes where they influence the total absorption of the harmful radiation by the lipid membrane, thereby functioning as sunscreen agents. We also report two new hyperperoxide analogues of the putative polyene biosynthetic precursors that themselves are likely candidate natural products.     

Novel trifunctional building blocks for fluorescent polymers

[reaction: see text] Herein, we describe the synthesis of fluorescent 2-(arylsulfonyl)methacrylates and its polymers. These novel trifunctional monomers, possessing a fluorescent arylsulfonyl (ArSO(2)) group, an alkyl group (R), and a polymerizable olefin, serve as useful building blocks for functionalized fluorescent polymers.     

Enantiomeric and isomeric separation of herbicides using cyclodextrin-modified capillary zone electrophoresis

Cyclodextrin-modified capillary zone electrophoresis (CD-CZE) was applied successfully to the enantiomeric and isomeric separation of three herbicides (imazaquin, diclofop and imazamethabenz). Commercially available cyclodextrins were evaluated for separation of the enantiomers and isomers of the three herbicides having varied molecular structures. The enantiomers of imazaquin and diclofop, and the isomers of imazamethabenz could be resolved with a resolution of ≥1.5. The resolution was found to depend on pH of the run buffer, cyclodextrin type and cyclodextrin concentration. By employing mixed cyclodextrins in the running buffer, the three herbicides were simultaneously separated in a single run. In addition, rapid (less than 3 min) enantiomeric separation is demonstrated using imazaquin as a model herbicide. The reported capillary electrophoresis (CE) methods are simple, rapid, efficient and reproducible and our results demonstrate that CE provides a powerful analytical tool for enantiomeric and isomeric separation of herbicides.     

The polyhomologation of 1-boraadamantane: mapping the migration pathways of a propagating macrotricyclic trialkylborane

Trialkyl and triaryl organoboranes undergo multiple, repetitive homologations upon reaction with dimethylsulfoxonium methylide (1). This multiple homologation reaction, or polyhomologation, produces polymethylene in a living reaction. Applying the polyhomologation reaction to cyclic and polycyclic organoboranes permits the construction of unique oligomeric and polymeric architectures that are not readily accessible by standard olefin polymerization. The polyhomologation of 1-boraadamantane.THF (2) by ylide 1 generates novel macrotricyclic trialkylboranes (3). The oxidation of these macrocyclic organoboranes generates a three-armed star polymethylene polymer (4) incorporating a cis,cis-1,3,5-trisubstituted cyclohexane core. Interestingly, only one-third of the initiators lead to product formation, resulting in an observed degree of polymerization 3 times higher than expected. Close examination of the initial stages of polymerization show that 1-boraadamantane.THF reacts with 1 equiv of 1 to afford a monohomologated product. Subsequent homologations were found to contain branch points leading to isomeric tricyclic products after the third, fourth, and fifth methylene insertions. At these stages of homologation, all of the propagating species result in tricyclic trialkylborane cages with collapsed, inverted pyramidal boron centers that are substantially less reactive toward ylide. Approximately two-thirds of the species discontinue polymerization at these stages. However, one-third of these species continue to propagate and eventually result in the formation of giant macrotricyclic polymers of narrow polydispersity. Molecular modeling and kinetic simulation have aided in the analysis of the probable pathways through which the reaction proceeds.     

Progress toward the total synthesis of (+)-aldosterone: synthesis of the A-D rings

[reaction: see text] Synthesis of the A-D rings of the cortical hormone (+)-aldosterone is described. The key step incorporates a chiral tether in a type 2 intramolecular Diels-Alder reaction that establishes the absolute configuration of four contiguous asymmetric centers. This approach provides an efficient route for either enantiomer of the steroid skeleton.     

Surface chemistry of quantum-sized metal nanoparticles under light illumination

Size reduction of metal nanoparticles increases the exposure of metal surfaces significantly, favoring heterogeneous chemistry at the surface of the nanoparticles. The optical properties of metal nanoparticles, such as light absorption, also exhibit a strong dependence on their size. It is expected that there will be strong coupling of light absorption and surface chemistry when the metal nanoparticles are small enough. For instance, metal nanoparticles with sizes in the range of 2–10 nm exhibit both surface plasmon resonances, which can efficiently produce high-energy hot electrons near the surface of the nanoparticles under light illumination, and the Coulomb blockade effect, which favors electron transfer from the metal nanoparticles to the surface adsorbates. The synergy of efficient hot electron generation and electron transfer on the surface of small metal nanoparticles leads to double-faced effects: (i) surface (adsorption) chemistry influences optical absorption in the metal nanoparticles, and (ii) optical absorption in the metal nanoparticles promotes (or inhibits) surface adsorption and heterogeneous chemistry. This review article focuses on the discussion of typical quantum phenomena in metal nanoparticles of 2–10 nm in size, which are referred to as “quantum-sized metal nanoparticles”. Both theoretical and experimental examples and results are summarized to highlight the strong correlations between the optical absorption and surface chemistry for quantum-sized metal nanoparticles of various compositions. A comprehensive understanding of these correlations may shed light on achieving high-efficiency photocatalysis and photonics.

Size reduction of metal nanoparticles increases the exposure of metal surfaces significantly, favoring heterogeneous photochemistry at the surface of the nanoparticles.     

The influence of different treatments of electrostatic interactions on the thermodynamics of folding of peptides

Replica exchange molecular dynamics simulations were performed to investigate the effects of different electrostatic treatments on the structure and thermodynamics of a small beta-hairpin forming peptide. Three different electrostatic schemes were considered: regular cutoffs, generalized reaction field (GRF), and particle mesh Ewald (PME), with the peptide modeled using OPLS/AA all-atom force field with explicit TIP3P water. Both the GRF and PME methods yielded results consistent with experiment, with free energy surfaces displaying a single minimum corresponding to the native beta-hairpin structure. In contrast, use of straight cutoffs led to the population of an additional local minimum corresponding to nonhairpin conformations that compete with the formation of the native beta-hairpin at low temperatures. This extra minimum would not be apparent in conventional constant-temperature molecular dynamics simulations run for a few nanoseconds. This result points to the critical need of careful sampling of conformational space to assess the quality of different numerical treatments of long-range forces. While differences emerged in the nature of the unfolded states populated using PME and GRF approaches, simulations on the beta-hairpin forming peptide and on two additional control peptides indicate that the GRF treatment of electrostatics offers a satisfactory compromise between accuracy and computational speed for the identification of low-energy conformations. A GRF-based approach emerges as a viable means for treating larger biological systems that would be prohibitively costly to simulate using PME methods.     

Synthesis and chemistry of bridgehead allylsilanes. Stereoselective reactions with aldehydes

A type II intramolecular Diels-Alder reaction provides access to bicyclo[5.3.1] ring systems with an imbedded bridgehead allylsilane. The Lewis acid catalyzed reactions of these compounds with aldehydes proceed efficiently and with control of stereochemistry. [reaction: see text]     

The Type 2 Intramolecular Diels-Alder Reaction: Synthesis and Chemistry of Bridgehead Alkenes

Anti-Bredt alkenes, bicyclic molecules that contain a bridgehead double bond, were for many years regarded as chemical curiosities. The type 2 intramolecular Diels-Alder (IMDA) reaction provides a one-step entry into this fascinating class of molecules. The reaction has made available numerous anti-Bredt alkenes for structural and chemical studies. X-ray crystallography has revealed the magnitude of the deformations associated with the bridgehead double bond, and rate studies of reactions of bridgehead alkenes have allowed quantification of the kinetic consequences of the torsional distortions. More recently, the type 2 intramolecular Diels-Alder reaction and the resulting anti-Bredt alkenes have found application in organic synthesis. The constraints resulting from the connectivity in the Diels-Alder precursor creates a strong regio- and stereochemical bias in the cycloaddition step. The end result of this bias is the stereoselective synthesis of highly substituted six-membered rings. The reaction also achieves a facile synthesis of seven- and eight-membered rings in a single step from acyclic precursors. The utility of this reaction has been verified in recent applications of the type 2 IMDA reaction as a key step in the total synthesis of complex natural products.     

Amyloid beta-protein: monomer structure and early aggregation states of Abeta42 and its Pro19 alloform

The amyloid beta-protein (Abeta) is a seminal neuropathic agent in Alzheimer's disease (AD). Recent evidence points to soluble Abeta oligomers as the probable neurotoxic species. Among the naturally occurring Abeta peptides, the 42-residue form Abeta42 is linked particularly strongly with AD, even though it is produced at approximately 10% of the levels of the more abundant 40-residue form Abeta40. Here, we apply mass spectrometry and ion mobility to the study of Abeta42 and its Pro19 alloform. The Phe19 --> Pro19 substitution blocks fibril formation by [Pro19]Abeta42. Evidence indicates that solution-like structures of Abeta monomers are electrosprayed and characterized. Unfiltered solutions of Abeta42 produce only monomers and large oligomers, whereas [Pro19]Abeta42 solutions produce abundant monomers, dimers, trimers, and tetramers but no large oligomers. When passed through a 10,000 amu filter and immediately sampled, Abeta42 solutions produce monomers, dimers, tetramers, hexamers, and an aggregate of two hexamers that may be the first step in protofibril formation. These results are consistent with recently published photochemical cross-linking data and lend support to recent aggregation mechanisms proposed by Bitan, Teplow, and co-workers [J. Biol. Chem. 2003, 278, 34882-34889].