Theoretical investigations of the relative stabilities of helical vs extended forms of phenylene ethynylene oligomers established that MMFF molecular mechanics was more useful than AM1 or DFT for calculating helical structures and for estimating relative energies. At the level of MMFF, theory predicts that for o- or m-oligophenylene ethynylenes, helix formation is enthalpically favored for ester and ether-substituted oligomers. In contrast to simple electron-demand predictions, we predict that the position of substituents can make a substantial difference in the tendency to form helices.
[structure: see text] This Letter describes the first published synthesis and characterization of alkoxy-substituted ortho-phenylene ethynylene (o-PE) oligomers. Sonogashira coupling was used to assemble discrete chain lengths, using a key monomer with orthogonal groups. Deprotection or activation allowed stepwise coupling to produce the dimer, trimer, and tetramer, while convergent coupling of appropriately substituted trimers produced the hexamer. The placement of alkoxy side chains renders these oligomers soluble in common organic solvents permitting solution characterization. Absorption and emission spectra of the trimer, tetramer, and hexamer are provided.
In the last decade, our understanding
of the chemistry of olive
oil has dramatically improved. Here, the essential chemistry of olive
oil and its important minor constituents is described and related
to the typical sensory categories used to rate and experience oils:
color, aroma, bitterness, and pungency. We also describe experiments
to explore some of the characteristics of olive oil related to its
status as a new health food. Simple qualitative experiments on olive
oil can be done in conjunction with tastings of the oil. First, we
establish the relationship between the color of an object and the
absorption of light by its molecular constituents using gummy candies
and laser pointers. Then, the color of the various oils can be measured
quantitatively using an iPhone app (Irodori). Illuminating the oil
with a green laser produces a startling red fluorescence in the presence
of the natural chlorophyll in some olive oils. Relatively straightforward
colorimetric assays can reveal the presence of unsaturated fatty acids,
healthy antioxidants such as phenols (unique to olive oils), contaminating
peroxides, and the level of free fatty acid that is a telltale sign
of poorly treated or stale oils. A final comparison of the sensory
observations from tasting with the chemical and spectroscopic analysis
provides an introduction to the science behind food standards and
the sensitivity of our own sensory apparatus.
The confinement of anionic oligoalanine peptides at the surface of cationic membranes can cooperatively reinforce peptide/peptide interactions and induce secondary-structure formation, and, reciprocally, induce chirality expression of the membrane at the mesoscopic level, thus leading to the formation of three-dimensional chiral fibrillar networks. Such a strong binding effect of peptides with cationic membranes and the resulting cooperative assembly behaviors are observed with two different types of cationic surfactant, namely, two-head two-tail gemini and one-head two-tail surfactants. The ensemble of assembly properties, such as critical micellar concentration (cmc), Krafft temperature (T(k) ), molecular area at the air/water interface, molecular organization (as studied by FTIR attenuated total reflectance (ATR) measurements and small-angle X-ray scattering), and morphology of the aggregates (as observed by optical and electron microscopy studies), are reported. The results clearly demonstrate that the molecular organization and mesoscopic supramolecular structures are controlled by a subtle balance between the peptide/peptide interactions, ionic interactions between the membranes and peptides, and the interactions the between surfactant molecules, which are governed by hydrophobicity and steric interactions. Investigation into such cooperative organization can shed light on the mechanism of supramolecular chirality expression in membrane systems and allow understanding of the structure of peptides in interactions with lipid bilayers.
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