Biomimetic Total Synthesis of Hyperjapones A–E and Hyperjaponols A and C

Hyperjapones A–E and hyperjaponols A–C are complex natural products of mixed aromatic polyketide and terpene biosynthetic origin that have recently been isolated from Hypericum japonicum. We have synthesized hyperjapones A–E using a biomimetic, oxidative hetero‐Diels–Alder reaction to couple together dearomatized acylphloroglucinol and cyclic terpene natural products. Hyperjapone A is proposed to be the biosynthetic precursor of hyperjaponol C through a sequence of: 1) epoxidation; 2) acid‐catalyzed epoxide ring‐opening; and 3) a concerted, asynchronous alkene cyclization and 1,2‐alkyl shift of a tertiary carbocation. Chemical mimicry of this proposed biosynthetic sequence allowed a concise total synthesis of hyperjaponol C to be completed in which six carbon–carbon bonds, six stereocenters, and three rings were constructed in just four steps.     

Understanding the Hofmeister effect in interactions between chaotropic anions and lipid bilayers: molecular dynamics simulations

A set of all-atom molecular dynamics simulations have been performed to better understand critical phenomena regarding a Hofmeister series of anions and lipid bilayers. The simulations isolate the effect of anion size and show clear differences in the interactions with the dipolar phoshpatidylcholine headgroup. Cl- anions penetrate into the headgroup region of the bilayer, but the simulations confirm theories which predict that larger anions penetrate more deeply, into a more heterogeneous and hydrophobic molecular region. That anion size leads to such differences in partitioning in the bilayer provides atomic-level support to hypotheses inspired by several experimental studies. The ability of larger anions to bury deep within the bilayer is correlated with a less well-structured hydration shell, shedding of which upon penetration incurs a smaller penalty for the larger anions than for Cl-.     

Asymmetric synthesis of the cis- and trans-stereoisomers of 4-aminopyrrolidine-3-carboxylic acid and 4-aminotetrahydrofuran-3-carboxylic acid

The diastereoselective conjugate addition of lithium (S)-N-benzyl-N-[small alpha]-methylbenzylamide has been successfully applied to the first asymmetric syntheses of cis-(3S,4R)- and trans-(3R,4R)-4-aminotetrahydrofuran-3-carboxylic acids (26% and 25% overall yield respectively, >98% d.e. and >97% e.e. in each case). Furthermore, the most efficient asymmetric synthesis to date of cis-(3R,4R)- and trans-(3R,4S)-4-aminopyrrolidine carboxylic acids is delineated: for cis-(3R,4R), four steps, >98% d.e., 52% overall yield; for trans-(3R,4S), five steps, >98% d.e., 50% overall yield.     

Antagonistic Effects of Endogenous Nitric Oxide in a Glioblastoma Photodynamic Therapy Model

Gliomas are aggressive brain tumors that are resistant to conventional chemotherapy and radiotherapy. Much of this resistance is attributed to endogenous nitric oxide (NO). Recent studies revealed that 5‐aminolevulinic acid (ALA)‐based photodynamic therapy (PDT) has advantages over conventional treatments for glioblastoma. In this study, we used an in vitro model to assess whether NO from glioblastoma cells can interfere with ALA‐PDT. Human U87 and U251 cells expressed significant basal levels of neuronal NO synthase (nNOS) and its inducible counterpart (iNOS). After an ALA/light challenge, iNOS level increased three‐ to fourfold over 24 h, whereas nNOS remained unchanged. Elevated iNOS resulted in a large increase in intracellular NO. Extent of ALA/light‐induced apoptosis increased substantially when an iNOS inhibitor or NO scavenger was present, implying that iNOS/NO was acting cytoprotectively. Moreover, cells surviving a photochallenge exhibited a striking increase in proliferation, migration and invasion rates, iNOS/NO again playing a dominant role. Also observed was a large iNOS/NO‐dependent increase in matrix metalloproteinase‐9 activity, decrease in tissue inhibitor of metalloproteinase‐1 expression and increase in survivin and S100A4 expression, each effect being consistent with accelerated migration/invasion as a prelude to metastasis. Our findings suggest introduction of iNOS inhibitors as pharmacologic adjuvants for glioblastoma PDT.     

The effects of unequal diffusion coefficients on periodic travelling waves in oscillatory reaction-diffusion systems

Many oscillatory biological systems show periodic travelling waves. These are often modelled using coupled reaction-diffusion equations. However, the effects of different movement rates (diffusion coefficients) of the interacting components on the predictions of these equations are largely unknown. Here we investigate the ways in which varying the diffusion coefficients in such equations alters the wave speed, time period, wavelength, amplitude and stability of periodic wave solutions. We focus on two sets of kinetics that are commonly used in ecological applications: lambda-omega equations, which are the normal form of an oscillatory coupled reaction-diffusion system close to a supercritical Hopf bifurcation, and a standard predator-prey model. Our results show that changing the ratio of the diffusion coefficients can significantly alter the shape of the one-parameter family of periodic travelling wave solutions. The position of the boundary between stable and unstable waves also depends on the ratio of the diffusion coefficients: in all cases, stability changes through an Eckhaus (‘sideband’) instability. These effects are always symmetrical in the two diffusion coefficients for the lambda-omega equations, but are asymmetric in the predator-prey equations, especially when the limit cycle of the kinetics is of large amplitude. In particular, there are two separate regions of stable waves in the travelling wave family for some parameter values in the predator-prey scenario. Our results also show the existence of a one-parameter family of travelling waves, but not necessarily a Hopf bifurcation, for all values of the diffusion coefficients. Simulations of the full partial differential equations reveals that varying the ratio of the diffusion coefficients can significantly change the properties of periodic travelling waves that arise from particular wave generation mechanisms, and our analysis of the travelling wave families assists in the understanding of these effects.     

Strong Photoluminescence Enhancement from Colloidal Quantum Dot Near Silver Nano-Island Films

We present the fabrication and optical investigation of highly random self-assembled, nano-scale films, probing their influence on the luminescence properties of near surface CdSe/ZnS colloidal quantum dots. When compared to quantum dots distributed on unstructured quartz substrates, the average luminescence intensity is found to be enhanced by a factor of 160×. The silver nanoparticles are prepared using slow thermal evaporation on quartz substrates and post-deposition annealing to produce a randomly-arranged layer of smooth nano-islands. Clear polarization dependent hot spots are observed. Such hot spots deliver a maximal enhancement of the emission intensity of 240× and have a spatial density of (0.050±0.002) μm^− 2. The results show that silver nano-island films strongly enhance the optical efficiency of near quantum dots emitters.     

X-Ray Crystallography and Free Energy Calculations Reveal the Binding Mechanism of A2A Adenosine Receptor Antagonists

We present a robust protocol based on iterations of free energy perturbation (FEP) calculations, chemical synthesis, biophysical mapping and X-ray crystallography to reveal the binding mode of an antagonist series to the A_2A adenosine receptor (AR). Eight A_2AAR binding site mutations from biophysical mapping experiments were initially analyzed with sidechain FEP simulations, performed on alternate binding modes. The results distinctively supported one binding mode, which was subsequently used to design new chromone derivatives. Their affinities for the A_2AAR were experimentally determined and investigated through a cycle of ligand-FEP calculations, validating the binding orientation of the different chemical substituents proposed. Subsequent X-ray crystallography of the A_2AAR with a low and a high affinity chromone derivative confirmed the predicted binding orientation. The new molecules and structures here reported were driven by free energy calculations, and provide new insights on antagonist binding to the A_2AAR, an emerging target in immuno-oncology.     

Quantification of Stereochemical Communication in Metal–Organic Assemblies

The derivation and application of a statistical mechanical model to quantify stereochemical communication in metal–organic assemblies is reported. The factors affecting the stereochemical communication within and between the metal stereocenters of the assemblies were experimentally studied by optical spectroscopy and analyzed in terms of a free energy penalty per “incorrect” amine enantiomer incorporated, and a free energy of coupling between stereocenters. These intra‐ and inter‐vertex coupling constants are used to track the degree of stereochemical communication across a range of metal–organic assemblies (employing different ligands, peripheral amines, and metals); temperature‐dependent equilibria between diastereomeric cages are also quantified. The model thus provides a unified understanding of the factors that shape the chirotopic void spaces enclosed by metal–organic container molecules.     

NMR and X-ray crystallography, complementary tools in structural proteomics of small proteins

NMR spectroscopy and X-ray crystallography, the two primary experimental methods for protein structure determination at high resolution, have different advantages and disadvantages in terms of sample preparation and data collection and analysis. It is therefore of interest to assess their complementarity when applied to small proteins. Structural genomics/proteomics projects provide an ideal opportunity to make such comparisons as they generate data in a systematic manner for large enough numbers of proteins to allow firm conclusions to be drawn. Here we report a comparison for 263 unique proteins screened by both NMR spectroscopy and X-ray crystallography in our structural proteomics pipeline. Only 21 targets (8%) were deemed amenable to both methods based on an initial 2D 15N-HSQC NMR spectrum and optimized crystallization trials. However, the use of both methods in the pipeline increased the total number of targets amenable to structure determination to 107, with 43 amenable to NMR only and 43 amenable to X-ray crystallographic methods only. We did not observe a correlation between 15N-HSQC spectral quality and the success of the same protein in crystallization screens. Similar results were found for an independent set of 159 proteins as reported in the accompanying paper by Snyder et al. Thus, we conclude that both methods are highly complementary, and in order to increase the number of proteins suited for structure determination, we suggest that both methods be used in parallel in screening of all small proteins for structure determination.