Plant Antimicrobial Peptides Snakin‐1 and Snakin‐2: Chemical Synthesis and Insights into the Disulfide Connectivity

Antimicrobial peptides and proteins represent an important class of plant defensive compounds against pathogens and provide a rich source of lead compounds in the field of drug discovery. We describe the effective preparation of the cysteine‐rich snakin‐1 and ‐2 antimicrobial peptides by using a combination of solid‐phase synthesis and native chemical ligation. A subsequent cysteine/cystine mediated oxidative folding to form the six internal disulfide bonds concurrently gave the folded proteins in 40–50 % yield. By comparative evaluation of mass spectrometry, HPLC, biological data and trypsin digest mapping of folded synthetic snakin‐2 compared to natural snakin‐2, we demonstrated that synthetic snakin‐2 possesses full antifungal activity and displayed similar chromatographic behaviour to natural snakin‐2. Trypsin digest analysis allowed tentative assignment of three of the purported six disulfide bonds.     

Effects of Substituents on Synthetic Analogs of Chlorophylls. Part 4: How Formyl Group Location Dictates the Spectral Properties of Chlorophylls b,d and f

Photosynthetic organisms are adapted to light characteristics in their habitat in part via the spectral characteristics of the associated chlorophyll pigments, which differ in the position of a formyl group around the chlorin macrocycle (chlorophylls b, d, f) or no formyl group (chlorophyll a). To probe the origin of this spectral tuning, the photophysical and electronic structural properties of a new set of synthetic chlorins are reported. The zinc and free base chlorins have a formyl group at either the 2‐ or 3‐position. The four compounds have fluorescence yields in the range 0.19–0.28 and singlet excited‐state lifetimes of ca 4 ns for zinc chelates and ca 8 ns for the free base forms. The photophysical properties of the 2‐ and 3‐formyl zinc chlorins are similar to those observed previously for 13‐formyl or 3,13‐diformyl chlorins, but differ markedly from those for 7‐formyl analogs. Molecular‐orbital characteristics obtained from density functional theory (DFT) calculations were used as input to spectral simulations employing the four‐orbital model. The analysis has uncovered the key changes in electronic structure engendered by the presence/location of a formyl group at various macrocycle positions, which is relevant to understanding the distinct spectral properties of the natural chlorophylls a, b, d and f.     

Characterization of silica-coated tobacco mosaic virus

The deposition of silica on the surface of tobacco mosaic virus (TMV) is achieved at a higher pH (>7) as a means to enhance its usefulness as a template for the synthesis of nanostructures. Electron energy loss spectroscopy definitively shows the presence of a silica shell on the surface of the TMV while small angle X-ray scattering differentiates successfully between silica-coated TMV and silica particles in the presence of uncoated TMV. Importantly, coating reactions done in a 50% w/v methanol/water solution produce smaller silica nanostructures during the condensation of the hydrolysis intermediates, possibly aiding in obtaining uniform coating. Furthermore, TMV-templated silica coatings are found to enhance the stability of the virus particle in methanol at conditions that would ordinarily disrupt the assembled particle. Combined these findings demonstrate that TMV can function as an efficient template for the controlled deposition of silica at neutral pH.     

What can we learn from N2O isotope data? – Analytics,processes and modelling

The isotopic composition of nitrous oxide (N₂O) provides useful information for evaluating N₂O sources and budgets. Due to the co-occurrence of multiple N₂O transformation pathways, it is, however, challenging to use isotopic information to quantify the contribution of distinct processes across variable spatiotemporal scales. Here, we present an overview of recent progress in N₂O isotopic studies and provide suggestions for future research, mainly focusing on: analytical techniques; production and consumption processes; and interpretation and modelling approaches. Comparing isotope-ratio mass spectrometry (IRMS) with laser absorption spectroscopy (LAS), we conclude that IRMS is a precise technique for laboratory analysis of N₂O isotopes, while LAS is more suitable for in situ/inline studies and offers advantages for site-specific analyses. When reviewing the link between the N₂O isotopic composition and underlying mechanisms/processes, we find that, at the molecular scale, the specific enzymes and mechanisms involved determine isotopic fractionation effects. In contrast, at plot-to-global scales, mixing of N₂O derived from different processes and their isotopic variability must be considered. We also find that dual isotope plots are effective for semi-quantitative attribution of co-occurring N₂O production and reduction processes. More recently, process-based N₂O isotopic models have been developed for natural abundance and ¹⁵N-tracing studies, and have been shown to be effective, particularly for data with adequate temporal resolution. Despite the significant progress made over the last decade, there is still great need and potential for future work, including development of analytical techniques, reference materials and inter-laboratory comparisons, further exploration of N₂O formation and destruction mechanisms, more observations across scales, and design and validation of interpretation and modelling approaches. Synthesizing all these efforts, we are confident that the N₂O isotope community will continue to advance our understanding of N₂O transformation processes in all spheres of the Earth, and in turn to gain improved constraints on regional and global budgets.     

Facile synthesis, stabilization, and anti-bacterial performance of discrete Ag nanoparticles using Medicago sativa seed exudates

The biogenic synthesis of metal nanomaterials offers an environmentally benign alternative to the traditional chemical synthesis routes. Colloidal silver (Ag) nanoparticles were synthesized by reacting aqueous AgNO(3) with Medicago sativa seed exudates under non-photomediated conditions. Upon contact, rapid reduction of Ag(+) ions was observed in <1 min with Ag nanoparticle formation reaching 90% completion in <50 min. Effect of Ag concentration, quantity of exudate and pH on the particle size and shape were investigated. At [Ag(+)]=0.01 M and 30°C, largely spherical nanoparticles with diameters in the range of 5-51 nm were generated, while flower-like particle clusters (mean size=104 nm) were observed on treatment at higher Ag concentrations. Pre-dilution of the exudate induced the formation of single-crystalline Ag nanoplates, forming hexagonal particles and nanotriangles with edge lengths of 86-108 nm, while pH adjustment to 11 resulted in monodisperse Ag nanoparticles with an average size of 12 nm. Repeated centrifugation and redispersion enhanced the percentage of nanoplates from 10% to 75% in solution. The kinetics of nanoparticle formation were monitored using ultraviolet-visible spectroscopy and the Ag products were characterized using transmission electron microscopy, selected-area electron diffraction, scanning electron microscopy, X-ray powder diffraction, and atomic force microscopy. X-ray photoelectron spectroscopy was used to investigate the elements and chemical environment in the top layers of the as-synthesized Ag nanoparticles, while the metabolites in the exudate were analyzed using gas chromatography-mass spectroscopy. To our knowledge, this is the first account of M. sativa seed exudate assisted synthesis and stabilization of biogenic Ag nanoparticles; the nanoplates are notably smaller and better faceted compared with those synthesized by vascular plant extracts previously reported. Stabilized films of exudate synthesized Ag nanoparticles were effective anti-bacterial agents.     

Trajectory analysis of single molecules exhibiting non-brownian motion

Four techniques for analyzing single molecule tracking data--confinement level analysis, time series analysis and statistical analysis of lateral diffusion, multistate kinetics, and a newly developed method, radius of gyration evolution analysis--are compared using a set of sample fluorophore trajectories obtained from the lipophilic carbocyanine dye 1,1'-dioctadecyl-3,3,3'3'-tetramethylindocarbocyanine, DiIC(18), partitioned into surface tethered poly(n-isopropylacrylamide). The purpose here is two-fold: first to test that these techniques can be applied to single molecules trajectories, which typically contain a smaller total number of frames than those obtained from other particles, e.g. quantum dots or gold nanoparticles; and second to critically compare the information obtained from each method against the others. A set of five SMT trajectories, ranging in length from 41 to 273 steps with a 30 ms frame transfer exposure, were all successfully analyzed by all four techniques, provided two important criteria were met: enough steps to define the motion were acquired in the trajectory, generally on the order of 50 steps, and the fast and slow diffusion coefficients differ by at least a factor of 5. Beyond that the four trajectory analysis methods studied provide partially confirmatory and partially complementary information. SMT data resulting from more complex physical behavior may well benefit from using these techniques in succession to identify and sort populations.     

Spin crossover in a four-coordinate iron(II) complex

The four-coordinate iron(II) phosphoraniminato complex PhB(MesIm)(3)Fe-N═PPh(3) undergoes an S = 0 to S = 2 spin transition with T(C) = 81 K, as determined by variable-temperature magnetic measurements and Mo?ssbauer spectroscopy. Variable-temperature single-crystal X-ray diffraction revealed that the S = 0 to S = 2 transition is associated with an increase in the Fe-C and Fe-N bond distances and a decrease in the N-P bond distance. These structural changes have been interpreted in terms of electronic structure theory.     

Detection of genetic modification in cured tobacco leaf: proficiency testing using polymerase chain reaction-based methods

The Cooperation Centre for Scientific Research Relative to Tobacco (CORESTA; Paris, France) "Task Force Genetically Modified Tobacco-Detection Methods" investigated the performance of qualitative and quantitative methods based on the polymerase chain reaction (PCR) for the detection and quantitation of genetically modified (GM) tobacco. In the 4 successful rounds of proficiency testing, the cauliflower mosaic virus 35S RNA promoter (CaMV 35S) and the Agrobacterium tumefaciens nopaline synthase terminator (NOS) were selected as target sequences. Blind-coded reference materials containing from 0.1 to 5.0% and from 0.15 to 4% GM tobacco were used in 2 rounds of qualitative and quantitative PCR, respectively. Eighteen laboratories from 10 countries participated in this study. Considering all methods and 2 rounds, the different laboratories were able to detect GM tobacco at the 0.1% level in 46 out of 58 tests in qualitative assays. The results of the proficiency test indicate that both end point screening and real-time quantitative methods are suitable for the detection of genetically modified organisms in tobacco leaf samples having a GM content of 0.1% or higher. The CORESTA proficiency study represents a first step towards the interlaboratory evaluation of accuracy and precision of PCR-based GM tobacco detection, which may lead to the harmonization of analytical procedures and to the enhancement of comparability of testing results produced by different laboratories.     

Structure-reactivity correlations for solid-state enantioselective photochemical reactions established directly from powder X-ray diffraction

A prerequisite for the development of structure-reactivity correlations for photoreactive crystalline materials is to have detailed knowledge of the structural properties of the reactant crystalline phase. In some cases, however, the materials of interest can be prepared only as microcrystalline powders and are not amenable to structural characterization by single-crystal X-ray diffraction. This paper demonstrates the utility of modern powder X-ray diffraction techniques for obtaining structural understanding in such cases, leading to the development of structure-reactivity correlations. In particular, a series of three photoreactive organic salts are considered, which undergo the same photochemical asymmetric reaction but with high enantiomeric excess in two cases and low enantiomeric excess in the other case. The structural properties of the three salts determined from powder X-ray diffraction data are shown to provide a direct rationalization of these observations.