1,4-Aryl migration under copper(I) atom transfer conditions

Reaction of N-alkyl-N-(trichloroacetyl)arylsulfonamides with CuCl/amines leads to N-alkyl-N-(dichloroacetyl)-arylsulfonamides via reduction or N-alkyl-aryldichloroacetamides via 1,4-aryl migration with loss of SO₂. The ratio of reduction to aryl migration is dependent upon the temperature and the ligand utilised. Along with amide bond hydrolysis these reactions may compete when carrying out slow atom transfer radical cyclisation reactions using sulfonamides.     

Diels-Alder reaction of maldoxin with an isopropenylallene

The Diels-Alder reaction of maldoxin with an isopropenylallene at 60-75 °C afforded an adduct closely related to chloropestolide A (24%) and a second adduct (0-11%) that underwent an ene reaction to generate the chloropupukeanolide D (11-22%) skeleton. The Diels-Alder reaction occurred with good selectively (>5:1) from a single face of maldoxin under much milder conditions than previously reported for the analogous dimethoxycyclohexadienone. Furthermore, the ene reaction took place under mild conditions, whereas the analogous Diels-Alder adduct from the dimethoxycyclohexadienone did not undergo an ene reaction.     

New synthetic routes to the kainoids: a synthesis of kainic acid and its analogues

New approaches to the synthesis of kainic acid and its analogues are presented. Two distinctly different approaches are described; the former utilised an intermolecular nitrile oxide addition to a homochiral substrate to furnish epikainate models and the second utilised amino acid chemistry to secure kainic acid.     

Perturbations of aromatic amino acids are associated with iron cluster assembly in ribonucleotide reductase

The β2 subunit of class Ia ribonucleotide reductases (RNR) contains an antiferromagnetically coupled μ-oxo bridged diiron cluster and a tyrosyl radical (Y122?). In this study, an ultraviolet resonance Raman (UVRR) difference technique describes the structural changes induced by the assembly of the iron cluster and by the reduction of the tyrosyl radical. Spectral contributions from aromatic amino acids are observed through UV resonance enhancement at 229 nm. Vibrational bands are assigned by comparison to histidine, phenylalanine, tyrosine, tryptophan, and 3-methylindole model compound data and by isotopic labeling of histidine in the β2 subunit. Reduction of the tyrosyl radical reveals Y122? Raman bands at 1499 and 1556 cm(-1) and Y122 Raman bands at 1170, 1199, and 1608 cm(-1). There is little perturbation of other aromatic amino acids when Y122? is reduced. Assembly of the iron cluster is shown to be accompanied by deprotonation of histidine. A p(2)H titration study supports the assignment of an elevated pK for the histidine. In addition, structural perturbations of tyrosine and tryptophan are detected. For tryptophan, comparison to model compound data suggests an increase in hydrogen bonding and a change in conformation when the iron cluster is removed. pH and (2)H(2)O studies imply that the perturbed tryptophan is in a low dielectric environment that is close to the metal center and protected from solvent exchange. Tyrosine contributions are attributed to a conformational or hydrogen-bonding change. In summary, our work shows that electrostatic and conformational perturbations of aromatic amino acids are associated with metal cluster assembly in RNR. These conformational changes may contribute to the allosteric effects, which regulate metal binding.     

Effect of fluorocarbon molecules confined between sliding self-mated PTFE surfaces

We report on the frictional response and atomic process that occur when molecular fluorocarbon molecules of varying lengths are sheared between two polytetrafluoroethylene (PTFE) surfaces. The thicknesses of the molecular layers are also varied. The approach is classical molecular dynamics simulations using a reactive bond-order potential parametrized for fluorocarbons. The results indicate that the presence of the molecules has a significant impact on the measured friction and wear of the surfaces, and that this impact depends on the nature of the fluorocarbon molecules and the thickness of the molecular film. The molecular mechanisms responsible for these differences are presented.     

Proton coupled electron transfer and redox-active tyrosine Z in the photosynthetic oxygen-evolving complex

Proton coupled electron transfer (PCET) reactions play an essential role in many enzymatic processes. In PCET, redox-active tyrosines may be involved as intermediates when the oxidized phenolic side chain deprotonates. Photosystem II (PSII) is an excellent framework for studying PCET reactions, because it contains two redox-active tyrosines, YD and YZ, with different roles in catalysis. One of the redox-active tyrosines, YZ, is essential for oxygen evolution and is rapidly reduced by the manganese-catalytic site. In this report, we investigate the mechanism of YZ PCET in oxygen-evolving PSII. To isolate YZ(?) reactions, but retain the manganese-calcium cluster, low temperatures were used to block the oxidation of the metal cluster, high microwave powers were used to saturate the YD(?) EPR signal, and YZ(?) decay kinetics were measured with EPR spectroscopy. Analysis of the pH and solvent isotope dependence was performed. The rate of YZ(?) decay exhibited a significant solvent isotope effect, and the rate of recombination and the solvent isotope effect were pH independent from pH 5.0 to 7.5. These results are consistent with a rate-limiting, coupled proton electron transfer (CPET) reaction and are contrasted to results obtained for YD(?) decay kinetics at low pH. This effect may be mediated by an extensive hydrogen-bond network around YZ. These experiments imply that PCET reactions distinguish the two PSII redox-active tyrosines.     

A bright, slow cryogenic molecular beam source for free radicals

We demonstrate and characterize a cryogenic buffer gas-cooled molecular beam source capable of producing bright beams of free radicals and refractory species. Details of the beam properties (brightness, forward velocity distribution, transverse velocity spread, rotational and vibrational temperatures) are measured under varying conditions for the molecular species SrF. Under typical conditions we produce a beam of brightness 1.2 × 10(11) molecules/sr/pulse in the X(2)Σ(+)(v = 0, N(rot) = 0) state, with 140(m/s) forward velocity and a rotational temperature of ≈ 1 K. This source compares favorably to other methods for producing beams of free radicals and refractory species for many types of experiments. We provide details of construction that may be helpful for others attempting to use this method.     

The synthesis and characterization of lipophilic peptide-based carriers for gene delivery

Lipophilic carriers have shown great potential in improving the delivery of gene therapeutics. We have synthesized positively charged peptide-based carriers including lipoamino acids. The carriers were shown to interact with DNA by performing isothermal titration calorimetry and particle size and zeta potential experiments. An exothermic reaction resulted from the titration of carrier into DNA. The particle sizes of the carrier/DNA complexes varied over the different charge ratios from 200-800 nm. The zeta potential was negative at a low charge ratio but positive when the amount of carrier was increased. The utilisation of lipophilic carriers is a promising approach to improve the bioavailability of gene delivery.     

Design of a fraction collector for capillary array electrophoresis

This paper describes a prototype instrument for high-throughput fraction collection with capillary array electrophoresis (CAE). The design of the system was based on a comprehensive collection approach, in which fractions from all capillaries were simultaneously collected in individual collection microwells in predefined time intervals. The location of the fractions in the microwells on the collection plate was determined by monitoring the individual zone velocities close to the end of each capillary. The collection microwell plate was fabricated from buffer-saturated agarose gel, which maintained permanent electrical contact with the separation capillaries during the collection process. Since the collection gel plate consisted of over 90% water, liquid evaporation from the collection wells was minimized. A 12-capillary array instrument was built with two-point detection using a side illumination scheme. The collection performance was demonstrated by reinjection of selected fractions of a double-stranded DNA (dsDNA) separation. The identity of collected DNA fragments was confirmed by PCR and sequencing.     

The thermochemistry of group 15 tetrachloride anions

Bond strengths for a series of Group 15 tetrachloride anions ACl4 (A = P, As, Sb, and Bi) have been determined by measuring thresholds for collision-induced dissociation of the anions in a flowing afterglow-tandem mass spectrometer. The central atoms in these systems have ten electrons, which violates the octet rule: the bond dissociation energies for ACl4- help to clarify the effect of the central atom on hypervalent bond strengths. The 0 K bond energies in kJ mol(-1) are D(Cl3A-CL-) = 90 +/- 7,115 +/- 7,161 +/- 8, and 154 +/- 15, respectively. Computational results using the B3LYP/LANL2DZpd level of theory are higher than the experimental bond energies. Calculations give a geometry for BiCl4 that is essentially tetrahedral rather than the see-saw observed for the other tetrachlorides. NBO calculations predict that the phosphorus and arsenic systems have 3C-4E bonds, while the antimony and bismuth systems are more ionic.