Magnetic circular dichroism spectroscopy of antiferromagnetically coupled hetero-metallic rings [H2NR2][Cr7MF8(O2CCMe3)16

The optical and magnetic properties of the multi-metal rings [NH(2)R(2)][Cr(7)MF(8)(O(2)CCMe(3))(16)], where M = Cd(II), Mn(II) or Ni(II), have been studied using variable-field and variable-temperature magnetic circular dichroism (MCD) in the UV-visible spectra. Spectra of samples were recorded in a frozen organic matrix or cast in a polymethacrylate (PMMA) polymer film between 1.7 and 75 K. The spectra are characteristic of the Cr(III) ion (d(3)) in a rhombic field when M = Cd(II). In the case that M = Ni(II) additional optical transitions arise from the d(8) ion whereas for M = Mn(II) no additional transitions are observed. The influence of magnetic exchange is apparent from a change in the sign of the MCD signal between complexes in which the hetero-atom has a local spin moment greater, or less, than that of Cr(III), S = 3/2, namely, Mn(II), S = 5/2, and Ni(II), S = 1. The exchange coupling generates a manifold of thermally accessible electronic states that give rise to variations in MCD intensity as well as additional spectral features as the temperature is raised. Equations have been derived to relate the splittings observed in the optical spectrum to the single-ion ground state zero-field splittings of chromium(III). There is reasonable agreement between the sign and magnitude of the contribution to the cluster anisotropy from that of the single ion with values estimated from other techniques.     

Ammonium-directed dihydroxylation: metal-free synthesis of the diastereoisomers of 3-aminocyclohexane-1,2-diol

The ammonium-directed, metal-free oxidation of 3-(N,N-dibenzylamino)cyclohex-1-ene with mCPBA in the presence of either trichloroacetic acid or tosic acid has been used as the key step to facilitate the synthesis of all the diastereoisomers of 3-aminocyclohexane-1,2-diol, in >98% de in each case.     

Tandem multi-step synthesis of C-carboxyazlactones promoted by N-heterocyclic carbenes

Cascade reaction sequences incorporating N-heterocyclic carbene-based organocatalysis have been developed that allow the direct preparation of a range of (+/-)-4-phenoxycarbonylazlactones in good isolated yields (66-84%) from the corresponding N-p-anisoyl amino acids.     

Controlling the Assembly of Coiled–Coil Peptide Nanotubes

An ability to control the assembly of peptide nanotubes (PNTs) would provide biomaterials for applications in nanotechnology and synthetic biology. Recently, we presented a modular design for PNTs using α‐helical barrels with tunable internal cavities as building blocks. These first‐generation designs thicken beyond single PNTs. Herein we describe strategies for controlling this lateral association, and also for the longitudinal assembly. We show that PNT thickening is pH sensitive, and can be reversed under acidic conditions. Based on this, repulsive charge interactions are engineered into the building blocks leading to the assembly of single PNTs at neutral pH. The building blocks are modified further to produce covalently linked PNTs via native chemical ligation, rendering ca. 100 nm‐long nanotubes. Finally, we show that small molecules can be sequestered within the interior lumens of single PNTs.     

Nitrosylation of Nitric‐Oxide‐Sensing Regulatory Proteins Containing [4Fe‐4S] Clusters Gives Rise to Multiple Iron–Nitrosyl Complexes

The reaction of protein‐bound iron–sulfur (Fe‐S) clusters with nitric oxide (NO) plays key roles in NO‐mediated toxicity and signaling. Elucidation of the mechanism of the reaction of NO with DNA regulatory proteins that contain Fe‐S clusters has been hampered by a lack of information about the nature of the iron‐nitrosyl products formed. Herein, we report nuclear resonance vibrational spectroscopy (NRVS) and density functional theory (DFT) calculations that identify NO reaction products in WhiD and NsrR, regulatory proteins that use a [4Fe‐4S] cluster to sense NO. This work reveals that nitrosylation yields multiple products structurally related to Roussin's Red Ester (RRE, [Fe₂(NO)₄(Cys)₂]) and Roussin's Black Salt (RBS, [Fe₄(NO)₇S₃]. In the latter case, the absence of ³²S/³⁴S shifts in the Fe?S region of the NRVS spectra suggest that a new species, Roussin's Black Ester (RBE), may be formed, in which one or more of the sulfide ligands is replaced by Cys thiolates.     

Asymmetric synthesis of 3,4-anti- and 3,4-syn-substituted aminopyrrolidines via lithium amide conjugate addition

The diastereoselective conjugate addition of homochiral lithium amides to methyl 4-(N-allyl-N-benzylamino)but-2-enoate has been used as the key step in a simple and efficient protocol for the preparation of 3,4-substituted aminopyrrolidines. This protocol provides a complementary and stereoselective route to both anti- and syn-3-amino-4-alkylpyrrolidines as well as anti- and syn-3-hydroxy-4-aminopyrrolidines, in high de and ee viabeta-amino enolate functionalisation. This methodology has been applied to the synthesis of anti-(3S,4S)- and syn-(3R,4S)-3-methoxy-4-(N-methylamino)pyrrolidine.     

Evaluating beta-amino acids as enantioselective organocatalysts of the Hajos-Parrish-Eder-Sauer-Wiechert reaction

A systematic study of the effect of substitution within the beta-amino acid framework indicates that both beta(2)- and beta(3)-amino acids catalyse the Hajos-Parrish-Eder-Sauer-Wiechert reaction with poor to reasonable levels of enantioselectivity. These results led to the evaluation of the conformationally constrained beta-amino acid (1R,2S)-cispentacin, which catalyses the Hajos-Parrish-Eder-Sauer-Wiechert reaction with comparable or higher levels of enantioselectivity to L-proline.     

A segmented radiofrequency-only quadrupole collision cell for measurements of ion collision cross section on a triple quadrupole mass spectrometer

The gas collision cell of a triple quadrupole mass spectrometer has been modified to consist of ten short quadrupole rod segments that allow an axial field to be applied to the cell in order to make measurements of ion mobility. The radiofrequency (rf)-quadrupole field provides effective radial confinement that greatly reduces diffusional losses at low pressure. The mobilities of mass-selected ions from an ionspray source have been measured at a pressure of 8 × 10^?3 torr at electric fields of 0. 1 to 3 V/cm, and used to calculate the collision cross sections of the ions. The measured cross sections compare well with those measured by other techniques.     

Hydrogen-bonding effects,electrostatic potential,and the antitumor activity of flavone acetic acid and related compounds. II. Ab initio studies on the second stable conformations

We previously studied the first stable conformations of flavone acetic acid and related compounds. In this article, a similar investigation was carried out on the second stable conformation of the same compounds. Emphasis is on the conformation dependence of the hydrogen-bonding effects, the molecular electrostatic potential (MEP), and the antitumor activity shown by these compounds. The results show that the second conformation is about 7.0 kcal mol⁻¹ higher in energy and possibly is an inactive conformation as no correlation has been found between the antitumor activity and the MEP features. In addition, a detailed comparison with the first conformation, which is probably the active conformation, has been made of the geometry, the total energy, the Mulliken charges on some important atoms, hydrogen-bonding effects, and the MEP minima and isosurfaces. The role of the hydrogen-bonding effects, which was unclear in our previous work, is clarified in this work. The possible molecular basis of the antitumor activity is suggested. © 1996 John Wiley & Sons, Inc.     

Impact of PV module configuration on energy yield under realistic conditions

Photovoltaic cell and module manufactures optimise their products according to power measurements performed at a set of standard-test conditions. A key parameter for the financing of a solar project is yield under field or realistic conditions. Field testing modules is time consuming and costly. Hence, we develop a methodology for simulating PV module yield based on the optical, thermal and electrical properties of the components, and the module configuration regarding the cell spacing, interconnection and module layers. With our procedure, we model the performance of standard, half cell and encapsulant free modules in different locations. We present results using our cell to module yield framework for 16 different locations in Australia based on one-minute ground measured solar irradiance and ambient temperature values. We find low-light irradiance losses are directly correlated to the number of cloudy days at a given site. The majority of fielded losses are due to temperature effects, which can be predicted by the average temperature at 3 p.m. We note that wind speed is not accounted for and it will be incorporated in future studies.