Solubility probes

ESR Measurement. The behavior of a stable free radical added to a medium reflects an average state of molecular rotation of the medium. Oil-soluble free radicals are used commonly to probe the rotation of oil molecules. The mobility of any oil-soluble probe is represented by its rotational correlation time (rc) as calculated from the following equation (28) ... 

The symbols involved in the equation are given in Figure 5. The probes used most widely in the field of food science are of the nitroxide class (29, 30). We selected 2- (10-carboxydecyl) -2-hexyl-4,4-dimethyl-3-oxa-zolodinyloxy methyl ester (Structure I) as an oil-soluble probe. A rc... 

Because monofunctionalization of fullerenes shows that a single hydrophilic addend is insufficient to prevent the strong hydrophobic interactions among the compounds [89, 90, 93, 94], multiple functionalized derivatives were examined as water-soluble probes. In particular, introduction of a second hydrophilic ligand (e.g., pyrrolidinium salts or carboxylates) to the fullerene core enhances the surface coverage of the hydrophobic fullerene surface. In turn, it was expected that fullerene aggregation might be suppressed. It should be stated that these water-soluble derivatives are important alternatives to the y-CD-incorporated and surfactant-embedded fullerenes. 

Figure 5. Self-assembled monolayers as rigid electrochemical spacers. Close packing of the SAM enforces a precise distance between the redox probes and the electrode. (A) In a tethered redox probe system, the probes are covalently attached to the outer surface of the monolayer. The probes may be undiluted (a probe attached to each chain in the monolayer) or mixed (probe-modified chains are diluted with nonelectroactive chains). (B) In a soluble redox probe system, the monolayer typically contains a single component that provides a well-defined separation between the soluble probe and the electrode surface.

From Here to There Conformational Landscapes and Solubility Probes. 174... 

Both of these approaches have their merits, however, use of a labeled polymer offers specific advantages over that of a solublized probe potentially the probe can be sequestered in both the hydrophobic and the more hydrophilic regions of the polyelectrolyte, in addition to the aqueous phase (see e.g., Ref. [20]), while a label allows the tagged site to be monitored directly [12,16,20,22-26]. 

Water-soluble probes have also been reported to undergo marked changes in their fluorescence characteristics when dispersed in polyelectrolyte solutions [18,52,61,72-78]. For example, the cationic dye, auramine O (AuO) is virtually nonfluorescent in aqueous solution but an increase in intensity is observed in the presence of PMAA at low pH as a result of enhanced binding in the hypercoiled state [72,73,75,78]. On neutralization, the fluorescence from AuO decreases as the compact structure breaks down forming the expanded state and the probe is released to the aqueous phase [52]. In a recent extension of this theme, the sensitivity of the emission spectrum of AuO to the environment in which it resides has been further exploited by covalently bonding the dye to PMAA [61]. Figure 2.1 shows the... 

Changes in the fluorescence spectral characteristics of water-soluble probes have also been used to provide information concerning the aqueous solution behavior of PAA [72-75,77], For example, the fluorescence intensity of AuO is low in water but is enhanced in the presence of PMAA, particularly at low degrees of ionization [52,73]. This enhancement was attributed to a higher local viscosity at the molecular level created by the polyelectrolyte chains. The emission from AuO dispersed in PAA, on the other hand, was low across the complete pH range [73] and implies that the probe resides in a water-rich phase, which is consistent with an expanded polymer conformation. 

Alternative approaches to monitoring the solution behavior of PSS through the use of water-soluble probes [147-149] and fluorescently labeled samples [87,144,146] have also appeared in the literature. For example, the fluorescence decay of a VPy-labeled PSS sample [87] was well described by a single-exponential function as in Equation 2.6. This contrasts to the behavior observed for a similarly labeled PMAA [87], which could only be modeled through use of a double-exponential function of the form of Equation 2.7. The authors concluded that a distribution of label environments exists in PMAA, which results in complex decay kinetics. The fact that only one excited state species is present in the PSS sample is indicative that the coil is more homogeneous in nature which is consistent with an extended conformation [136-139] under salt-free conditions. 

Another useful method for investigations of water/C02 emulsions and microemulsions is electron paramagnetic resonance (EPR) spectroscopy, because no transparent samples are necessary [9,13]. Furthermore, data from EPR experiments can provide information about the polarity of the local environment of the EPR-active compound. The diagnostic unpaired electron(s) can be introduced either through stable free radicals or by using transition metal ions such as Mn. The active moieties may be incorporated directly in the surfactant [17,18] or added as a soluble probe molecule such as TEMPO (4-hydroxy-2,2,6,6-tetramethylpiperidin-Toxyl) [9]. 

Chen et al. [74] successftilly used fluorescence polarization of an amphiphilic and an oil-soluble probe to monitor the changes of monolayer curvature within the microemulsion phase in didodecyldimethylammonium bromide (DDAB)-alkane-water. 

Membranes can also be labeled with more water-soluble probes like fluorescein or rhodamine. In such cases the probes can be forced to localize in the membrane by attachment to long acyl chains or to the phospholipids... 

In our initial attempts to study the effect of (DODA) Chloride, (DODA)C, sonicated vesicles on the thiolysis of p-nitrophenylacetate (NPA) by n-heptyl mercaptan (HM) we found that metal from the probe catalyzed the oxidation of the mercaptan [Cuccovia, I.M., unpublished]. In order to avoid mercaptan oxidation we developed an alcohol injection method for the preparation of (DODA)C vesicles, that were characterized by gel filtration and by the incorporation of lipid soluble probes [15]. It is not clear, today, if the aggregates prepared by ethanol injection are highly permeable closed vesicles or other types of bilayer assemblies [16]. 

In contrast to the soluble probes described previously, sensors can be used to measure the concentration of glucose in body tissues or in flowing blood reversibly. Sensors are usually prepared via immobilization of the sensing components in polymer matrices. That is the approach we have taken. Our... 

A lipophilic fluorescent (DPH) was employed as the lipid-soluble probe for the investigation of liposome partitioning in aqueous two-phase systems [10-12]. The DPH-loaded liposomes and the HP-coated liposomes were prepared according to methods described previously [13,14], The final concentration of the liposomal suspension was adjusted to 1.0 X 10 M as the total liposomal lipid concentration. 

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