Calculated molecular structures and electronic spectra of the geometric isomers of a model carotenoid molecule, 6,11-dimethylhexadecaheptaene

The molecular structures and the electronic spectra of the geometric isomers of a model carotenoid polyene, 6,11-dimethylhexadecaheptaene, were calculated. It was concluded that solvent effects and conformational isomerization must be taken into account in order to satisfactorily explain the observed spectra. Molecular structures were calculated using molecular mechanics (MM2), and electronic spectra using the VESCF-MO-CI method including all singly-excited configurations. A method based on the calculated and observed spectra of simple linear polyenes was devised to estimate the solvent effects.     

Dietary lipid content influences the activity of lipogenic enzymes in the liver and on whole body delta13C values of Nile tilapia, Oreochromis niloticus (L.)

The use of stable isotope techniques for the reconstruction of diets has increased over the last decade. However, isotopic ratios in an animal are not only affected by the composition of the feed, but also by the amount of feed consumed. An uncertainty of up to 1 per thousand for both delta13C and delta15N values has been observed when the feeding level is unknown. This may have substantial effects on the results of back-calculation. As the feeding level of animals is unknown in nature, an additional indicator for their nutritional status is needed. High feeding levels and a consequent surfeit of dietary energy lead to the synthesis of lipids. In order to test whether the level of lipogenesis could be used as an indicator, Nile tilapia (Oreochromis niloticus) were fed four isonitrogenous and isoenergetic wheat-based semi-synthetic diets with different lipid contents (2.0 %, 4.5 %, 9.5 % and 13.3 %) for eight weeks. Body composition, gross energy content and delta13C values in the lipids and the lipid-free material were determined in diets and fish bodies. The livers of three fish per feeding group were assayed for the activity of two lipogenic enzymes, ATP-citrate lyase and malic enzyme. There was a strong negative correlation between delta13C values in the lipids of the individual fish and the apparent lipid conversion. The activities of lipogenic enzymes decreased with rising lipid content in the diet. The delta13C values in the lipids decreased significantly with increasing specific activity for both enzymes. In this experiment where lipogenesis was influenced by the composition of the diet, it was possible to determine the exact value for the trophic shift in relation to the enzyme activities. Further experiments to investigate the use of enzyme activities in situations where the feeding level of an animal is unknown are recommended.     

A catalytic and iterative route to beta-substituted esters via highly enantioselective conjugate addition of dimethylzinc to unsaturated malonates

Using the chiral phosphoramidite ligand (S,R,R)-L1 in the conjugate addition of dimethylzinc to acyclic unsaturated malonates, enantioselectivities of up to 98% have been obtained for the first time. An iterative and stereodivergent route to 3,5-dimethyl esters that takes advantage of this asymmetric catalysis has been developed.     

Synchrotron and neutron techniques in biological crystallography

Synchrotron radiation (SR) techniques are continuously pushing the frontiers of wavelength range usage, smaller crystal sample size, larger protein molecular weight and complexity, as well as better diffraction resolution. The new research specialism of probing functional states directly in crystals, via time-resolved Laue and freeze trapping structural studies, has been developed, with a range of examples, based on research stretching over some 20 years. Overall, SR X-ray biological crystallography is complemented by neutron protein crystallographic studies aimed at cases where much more complete hydrogen details are needed involving synergistic developments between SR and neutron Laue methods. A big new potential exists in harnessing genome databases for targeting of new proteins for structural study. Structural examples in this tutorial review illustrate new chemistry learnt from biological macromolecules.     

Periodic droplet formation in chemically patterned microchannels

Simulations show that, when a phase-separated binary AB fluid is driven to flow past chemically patterned substrates in a microchannel, the fluid exhibits unique morphological instabilities. For the pattern studied, these instabilities give rise to the simultaneous, periodic formation of monodisperse droplets of A in B and B in A. The system bifurcates between time-independent behavior and different types of regular, nondecaying oscillations in the structural characteristics. The surprisingly complex behavior is observed even in the absence of hydrodynamic interactions and arises from the interplay between the fluid flow and patterned substrate, which introduces nonlinearity into the dynamical system.     

Control of molecular architecture by the degree of deprotonation: self-assembled di- and tetranuclear copper(II) complexes of N,N'-bis(2-pyridylmethyl)pyrazine-2,3-dicarboxamide

In the absence of added base, a deep navy-blue dimeric copper complex [CuII(H2L)(MeCN)]2(BF4)4 (1) of the non-deprotonated bis-terdentate diamide ligand H2L self-assembles whereas in the presence of base a grass-green [2 x 2] grid complex [CuII(HL)]4(BF4)4 (2) of the monodeprotonated ligand HL-, a rare example of a discrete grid of pyrazine-bridged metal ions, is formed.     

Programmable modification of cell adhesion and zeta potential in silica microchips

Spatial patterning of thin polyacrylamide films bonded to self-assembled monolayers on silica microchannels is described as a means for manipulating cell-adhesion and electroosmotic properties in microchips. Streaming potential measurements indicate that the zeta potential is reduced by at least two orders of magnitude at biological pH, and the adhesion of several kinds of cells is reduced by 80-100%. Results are shown for cover slides and in wet-etched silica microchannels. Because the polyacrylamide film is thin and transparent, this film is consistent with optical manipulation of cells and detection of cell contents. The spatial patterning technique is straightforward and has the potential to aid on-chip analysis of single adherent cells.     

What should be impossible: resolution of the mononuclear gallium coordination complex, Tris(benzohydroxamato)gallium(III)

Complexes of Ga3+, a d10 metal ion which lacks ligand-field-stabilization energy, are considered labile. In fact, hexaaquagallium(III) has a ligand exchange rate of 403 s-1, 2.5 times that of the analagous Fe3+ complex (Hugi-Cleary, D.; Helm, L.; Merbach, A. E. J. Am. Chem. Soc. 1987, 109, 4444-4450). Given this lability, resolution of Ga3+ complexes should be impossible. Despite this, we report the resolution of the Lambda and Delta isomers of tris(benzohydroxamate)gallium (III) (1), the first resolution of a mononuclear gallium complex. Not only is resolution possible, but these resolved complexes show remarkable resistance to racemization in aprotic solvents. The unprecedented stability of Lambda- and Delta-1 is a surprise, and as such, alters our understanding of classical coordination chemistry.