- Positionally isomeric organic gelators: Structure-gelation study, racemic versus enantiomeric gelators, and solvation effects
-
Low molecular weight gelator molecules consisting of aliphatic acid, amino acid (phenylglycine), and ω-aminoaliphatic acid units have been designed. By varying the number of methylene units in the aliphatic and ωaminoaliphatic acid chains, as defined by descriptors m and n, respectively, a series of positionally isomeric gelators having different positions of the peptidic hydrogen-bonding unit within the gelator molecule has been obtained. The gelation properties of the positional isomers have been determined in relation to a defined set of twenty solvents of different structure and polarity and analyzed in terms of gelator versatility (G ver) and effectiveness (Geff). The results of gelation tests have shown that simple synthetic optimizations of a "lead gelator molecule" by variation of m and n, end-group polarity (carboxylic acid versus sodium carboxylate), and stereochemistry (racemate versus optically pure form) allowed the identification of gelators with tremendously improved versatility (Gver) and effectiveness (Geff). Dramatic differences in Geff values of up to 70 times could be observed between pure racemate/enantiomer pairs of some gelators, which were manifested even in the gelation of very similar solvents such as isomeric xylenes. The combined results of spectroscopic (1H NMR, FTIR), electron microscopy (TEM), and X-ray diffraction studies suggest similar organization of the positionally isomeric gelators at the molecular level, comprising parallel β-sheet hydrogen-bonded primary assemblies that form inversed bilayers at a higher organizational level. Differential scanning calorimetry (DSC) studies of selected enantiomer/racemate gelator pairs and their o- and p-xylene gels revealed the simultaneous presence of different polymorphs in the racemate gels. The increased gelation effectiveness of the racemate compared to that of the single enantiomer is most likely a consequence of its spontaneous resolution into enantiomeric bilayers and their subsequent organization into polymorphic aggregates of different energy. The latter determine the gel fiber thickness and solvent immobilization capacity of the formed gel network.
- Caplar, Vesna,Frkanec, Leo,Vujicic, Natasa Sijakovic,Zinic, Mladen
-
supporting information; experimental part
p. 3066 - 3082
(2010/07/11)
-
- Lewis acid assisted permanganate oxidations
-
Lewis acids combine with permanganate in acetone solutions to form a complex that has enhanced oxidizing capabilities. The use of Lewis acids under these conditions to promote permanganate oxidations is superior to the use of Bronsted acids because the latter promote enolization of the solvent and subsequent unproductive reduction of the oxidant. The products obtained from a variety of alkenes, alkynes, arenes, sulfides, alcohols and ethers have been identified and probable reaction mechanisms proposed.
- Lai, Sheng,Lee, Donald G.
-
p. 9879 - 9887
(2007/10/03)
-
- Hair growth composition containing citric acid esters
-
Triesters of citric acid are used for inducing, maintaining or increasing hair growth. Compositions for topical application to mammalian hair or scalp comprise an effective amount of from 1% to 99% by weight of an ester of citric acid having the structure (1): where, R1, R2 and R3 each independently represent a branched or unbranched alkyl, alkenyl, aryl, alkylaryl or arylalkyl group, each said group having from 1 to 18 carbon atoms, R4 represents -H, or a branched or unbranched saturated or unsaturated acyl, alkyl, aryl, alkylaryl or aylalkyl group having from 1 to 18 carbon atoms, in the presence of a cosmetically acceptable vehicle for the citric acid ester and in the absence of solid absorbent for the ester;, said effective amount of said ester being sufficient to increase hair growth in the rat, when said composition is applied topically thereto over a period of no more than three months, by at least 10% more than that obtainable using a control composition from which the said ester has been omitted, in accordance with the Rat Hair Growth Test.
- -
-
-
- Photosensitive Monolayers. Photochemistry of Long-Chain Diazo and Azide Compounds at the Air-Water Interface
-
The photochemistry of a number of surface-active diazo and azide compounds was investigated in monolayers at the air-water boundary.Irradiation of long-chain α-diazo ketones with ultraviolet light leads to rapid loss of nitrogen.The resulting ketenes react with the subphase to generate carboxylic acids (photochemical Arndt-Eistert reaction) and dimerize to give β-lactones as side-products.A long-chain diester of 2-diazopropanedioic acid loses nitrogen and adds water, yielding the diester of 2-hydroxypropanedioic acid. α-Azido ketones split off nitrogen and the resulting isocyanates react with water (photoche mical Curtius reaction) and undergo further degradation to give complex product mixtures.These reactions lead to pronounced changes in the spreading behavior of the monolayers.Depending on functional group, chain length, substrate pH, and temperature it is possible to achieve changes in compressibility and collapse pressure, disappearance of expanded phases, collapse of monolayers to give oily films, or disappearance of monolayers by dissolution in the subphase.
- Holden, David A.,Ringsdorf, Helmut,Haubs, Michael
-
p. 4531 - 4536
(2007/10/02)
-
