Tryptophan, reversed micelle

The conformation of bovine myelin basic protein (MBP) in AOT/isooctane/water reversed micellar systems was studied by Waks et al. 67). This MBP is an extrinsic water soluble protein which attains an extended conformation in aqueous solution 68 but is more density packed at the membrane surface. The solubilization of MBP in the AOT reversed micelles depends on the water/AOT-ratio w0 68). The maximum of solubilization was observed at a w0-value as low as 5.56. The same value was obtained for another major protein component of myelin, the Folch-Pi proteolipid 69). According to fluorescence emission spectra of MBP, accessibility of the single tryptophane residue seems to be decreased in AOT reversed micelles. From CD-spectra one can conclude that there is a higher conformational rigidity in reversed micelles and a more ordered aqueous environment. 

The solubilization of amino acids in AOT-reversed micelles has been widely investigated showing the importance of the hydrophobic effect as a driving force in interfacial solubihzation [153-157]. Hydrophilic amino acids are solubilized in the aqueous micellar core through electrostatic interactions. The amino acids with strongly hydrophobic groups are incorporated mainly in the interfacial layer. The partition coefficient for tryptophan and micellar shape are affected by the loading ratio of tryptophan to AOT [158],... 

Some investigations have tested the ability of reversed micelles to act as efficient carriers of molecular species. Solutions of water-containing AOT-reversed micelles have been employed for the selective transport and the efficient separation of the two amino acids tryptophane and j9-iodophenylalanine [160]. 

The spectroscopic probe pyridine-N-oxide was used to characterize polar microdomains in reverse micelles in supercritical ethane from 50 to 300 bar. For both anionic and nonionic surfactants, the polarities of these microdomains were adjusted continuously over a wide range using modest pressure changes. The solubilization of water in the micelles increases significantly with the addition of the cosolvent octane or the co-surfactant octanol. Quantitative solubilities are reported for the first time for hydrophiles in reverse micelles in supercritical fluids. The amino acid tryptophan has been solubilized in ethane at the 0.1 wt.% level with the use of an anionic surfactant, sodium di-2-ethylhexyl sulfosuccinate (AOT). The existence of polar microdomains in aggregates in supercritical fluids at relatively low pressures, along with the adjustability of these domains with pressure, presents new possibilities for separation and reaction processes involving hydrophilic substances. 

The rate-limiting step of the mass transfer for the separated compound(s) in such systems depends on the concentration of the surfactant used [121]. Hebrant and Tondre [120] studied the extraction of tryptophan and p-iodophenylalanine through the reverse-micelle ELM composed of isooctane and the aqueous solution of Aerosol OT, that is, the sodium salt of bis (2-ethylnexyl)sulfosuccinate, as the stripping phase. The extraction efficiencies and transport rates were shown to be amino acid specific. 

Previous work, on the use of a reverse-micelle system for the production of tryptophan reported kinetic data obtained under various conditions (2). Both the water-to-surfactant ratio and the cosurfactant used influenced tryptophan production. The present work reports results from EPR studies of the effect of these parameters on both the water and the enzyme in the reverse micelle. EPR spectra of Mn(H20)g + were recorded to investigate the state of the water in reverse micelles. A nitroxide spin label that reacts with lysine residues was employed to probe the microstructure of tryptophanase in reverse micelles of different w0 values. 

Results and Discussion. Kinetic studies have shown that tryptophan production in reverse micelles is senstitive to w0 and hence to micelle size Q). Figure 4 shows the effect of w0 on tryptophan production. Tryptophanase exhibited its highest activity at a w0 of about 20. 

Hebrant et al, [151] investigated the transport of amino acids (p-iodopheny la la-nine and tryptophan) through AOT/isooctane reverse micelles, and the rates of transport were compared and analyzed in relation to partition coefficients between water and the AOT palisade layer. The rate of uptake of amino acids by the reverse micelles and the influence of the ionization states of the acids on the extent of transport were also presented. Enantioselective transport using chiral AOT was also attempted, but the rigidity of the interfacial film according to them is not sufficient for chiral recognition for enantiomeric enrichment. 

In 1979, Menger and Yamada were the first to report the a-chymotrypsin catalyzed hydrolysis of a synthetic ester, namely Af-acetyl-L-tryptophan methyl ester, in AOT/heptane reverse micelles [55]. Circular dichroism studies of a-chymo-trypsin containing reverse micelles at various water contents showed no major conformational changes of the protein. In this work, for the first time, increased... 

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