Poly(guanidyl)silanes as a new class of chelating, N-based ligand

The series of poly(guanidyl)silanes RnSi[hpp](4-n) [hpp = 1,3,4,6,7,8-hexahydro-2Hpyrimido[1,2-a]pyrimidinate; R = Me, n= 1, 2; R = Ph, n= 1] have been synthesised and their coordination behaviour at copper(I) halides investigated.     

Leveraging autocatalytic reactions for chemical domain image classification

Autocatalysis is fundamental to many biological processes, and kinetic models of autocatalytic reactions have mathematical forms similar to activation functions used in artificial neural networks. Inspired by these similarities, we use an autocatalytic reaction, the copper-catalyzed azide–alkyne cycloaddition, to perform digital image recognition tasks. Images are encoded in the concentration of a catalyst across an array of liquid samples, and the classification is performed with a sequence of automated fluid transfers. The outputs of the operations are monitored using UV-vis spectroscopy. The growing interest in molecular information storage suggests that methods for computing in chemistry will become increasingly important for querying and manipulating molecular memory.

Kinetic models of autocatalytic reactions have mathematical forms similar to activation functions used in artificial neural networks. Inspired by these similarities, we use a copper-catalyzed reaction to perform digital image recognition tasks.     

Light-mediated C-C sigma-bond driven crystallization of a phenalenyl radical dimer

Polymorphism-the phenomenon that a given compound forms more than one crystalline arrangement of the molecules in the solid state- plays a crucial role in understanding organic conductors, superconductors, and magnets. We have found that solutions of a new phenalenyl radical can give rise to two (nonpolymorphic) crystalline forms depending on whether the crystallization is allowed to proceed in the presence or absence of light. In both cases the crystals take the form of black shining blades and are indistinguishable by optical microscopy. We have fully characterized these crystalline forms, and we show that they differ by the presence or absence of a C-C sigma-bond between the unpaired electrons of the parent radical. These molecular forms crystallize from the same solvent to give rise to a sigma-dimerized insulator and a monomeric radical semiconductor as dictated by the presence or absence of light.     

Resonance-stabilized 1,2,3-dithiazolo-1,2,3-dithiazolyls as neutral pi-radical conductors

Alkylation of the zwitterionic heterocycle 8-chloro-bis[1,2,3]dithiazolo[4,5-b:5',4'-e]pyridine (ClBP) with alkyl triflates affords 8-chloro-4-alkyl-4H-bis[1,2,3]dithiazolo[4,5-b:5',4'-e]pyridin-2-ium triflates [ClBPR][OTf] (R = Me, Et, Pr). Reduction of these salts with decamethylferrocene affords the corresponding ClBPR radicals as thermally stable crystalline solids. The radicals have been characterized in solution by cyclic voltammetry and EPR spectroscopy. Measured electrochemical cell potentials and computed (B3LYP/6-31G) gas-phase disproportionation enthalpies are consistent with a low on-site Coulombic barrier U to charge transfer in the solid state. The crystal structures of ClBPR (R = Me, Et, Pr) have been determined by X-ray crystallography (at 293 K). All three structures consist of slipped pi-stacks of undimerized radicals, with many close intermolecular S.S contacts. ClBPMe undergoes a phase transition at 93 K to a slightly modified slipped pi-stack arrangement, the structure of which has also been established crystallographically (at 25 K). Variable-temperature magnetic and conductivity measurements have been performed, and the results interpreted in light of extended Hückel band calculations. The room-temperature conductivities of ClBPR systems (sigma(RT) approximately 10(-)(5) to 10(-)(6) S cm(-)(1)), as well as the weak 1D ferromagnetism exhibited by ClBPMe, are interpreted in terms of weak intermolecular overlap along the pi-stacks. The latter is caused by slippage of the molecular plates, a feature necessitated by the steric size of the R and Cl groups on the pyridine ring.     

Molecular genetic mining of the Aspergillus secondary metabolome: discovery of the emericellamide biosynthetic pathway

The recently sequenced genomes of several Aspergillus species have revealed that these organisms have the potential to produce a surprisingly large range of natural products, many of which are currently unknown. We have found that A. nidulans produces emericellamide A, an antibiotic compound of mixed origins with polyketide and amino acid building blocks. Additionally, we describe the discovery of four previously unidentified, related compounds that we designate emericellamide C-F. Using recently developed gene targeting techniques, we have identified the genes involved in emericellamide biosynthesis. The emericellamide gene cluster contains one polyketide synthase and one nonribosomal peptide synthetase. From the sequences of the genes, we are able to deduce a biosynthetic pathway for the emericellamides. The identification of this biosynthetic pathway opens the door to engineering novel analogs of this structurally complex metabolite.     

Multiconfiguration Pair-Density Functional Theory for Transition Metal Silicide Bond Dissociation Energies,Bond Lengths,and State Orderings

Transition metal silicides are promising materials for improved electronic devices, and this motivates achieving a better understanding of transition metal bonds to silicon. Here we model the ground and excited state bond dissociations of VSi, NbSi, and TaSi using a complete active space (CAS) wave function and a separated-pair (SP) wave function combined with two post-self-consistent field techniques: complete active space with perturbation theory at second order and multiconfiguration pair-density functional theory. The SP approximation is a multiconfiguration self-consistent field method with a selection of configurations based on generalized valence bond theory without the perfect pairing approximation. For both CAS and SP, the active-space composition corresponds to the nominal correlated-participating-orbital scheme. The ground state and low-lying excited states are explored to predict the state ordering for each molecule, and potential energy curves are calculated for the ground state to compare to experiment. The experimental bond dissociation energies of the three diatomic molecules are predicted with eight on-top pair-density functionals with a typical error of 0.2 eV for a CAS wave function and a typical error of 0.3 eV for the SP approximation. We also provide a survey of the accuracy achieved by the SP and extended separated-pair approximations for a broader set of 25 transition metal–ligand bond dissociation energies.     

Charge-transfer transitions in protein circular dichroism calculations

Charge-transfer transitions in proteins play a key role in many biophysical processes, from the behavior of redox proteins to photochemical reactions. We present ab initio calculations on a model dipeptide and more approximate calculations of the electronic excited states of proteins which, taken together, provide the most definitive assignment and characterization of charge-transfer transitions in proteins to date. We have calculated from first principles the electronic circular dichroism (CD) spectra of 31 proteins on the basis of their structures. Compared to previous studies, we achieve more accurate calculated CD spectra between 170 and 190 nm, owing mainly to the importance in alpha-helices of a charge-transfer transition from the lone pair on one peptide group to the pi* orbital on the next peptide group.     

Ionisation in the absence of high voltage using supercritical fluid chromatography: a possible route to increased signal

Supercritical fluid chromatography (SFC) is fast becoming a technique of choice for the analysis of a wide range of compounds and has been found to be complementary to high-performance liquid chromatography (HPLC). The combination of SFC and mass spectrometry (MS) affords a very useful tool in the separation and analysis of compounds. In this study the ionisation of samples in the absence of an applied electrospray voltage has been observed when using SFC/MS, with some compounds showing increased sensitivity when all ionisation source high voltage (HV) is removed. In an attempt to understand the mechanism of ionisation, a series of test compounds were analysed using standard electrospray ionisation (ESI) and atmospheric pressure chemical ionisation (APCI) source configurations and also different API source designs. In both cases, data were acquired with the applied high voltage on (normal conditions) or with the high voltage off, i.e. no voltage spray (novo-spray). The standards were analysed with a range of pressure and modifier percentage conditions. To understand the nature of the ionisation process observed, this was compared with three established liquid-to-gas ionisation mechanisms. These were thermospray (TSP), charge residue model (CRM) of ESI and sonic spray ionisation (SSI). Experiments were undertaken in an attempt to explain this ionisation phenomenon and quantify any observed change in sensitivity. The most important point to note is that enhanced ionisation was observed under novo-spray conditions in a SFC/MS configuration, which in certain cases provides a lowering in the overall limit of detection (LOD).