Solubilization site, probe

Pyrene shown a number of photophysical features that made it an attractive fluorophore to probe the microenvironment in micellar aggregates [19]. For the peaks of pyrene PL, two important peaks at about 373 nm and 390 nm among the five dominant peaks of pyrene fluorescence were numbered as 1 and III, respectively [20]. It has been known that intensity ratio of peak 111 to I (III/I) increased as the polarity at the solubilization site of pyrene decreases. Figure 6 shows fluorescence spectra (A.ex = 310 nm) of pyrene in precursor gel containing TPA and I-IV samples denoted as (a), (b), (c), (d) and (e), respectively. The value of 111/1 of pyrene does not change under silicalite-1 gel due to no formation of micelle. However, in the Fig. 6d (sample II), III/I ratio is rapidly increased, while sample III and IV are decreased slightly again. Previously, Park et al. have reported that 111/1 ratio of pyrene for... 

The reactivity of molecules bound to surfaces, located at various kinds of interfaces, solubilized in microheterogeneous media, or incorporated as "guests" in various "hosts" as inclusion complexes has been the subject of much recent study. Indeed the structure of the medium, the nature of "solubilization sites" and reactivity in these environments have all been the focus of independent or interrelated investigations (1-12). Photochemistry has played a major role in these studies both in terms of studies of the media and also in terms of modified or controlled reactivity (1,5,8,9). In the course of these investigations numerous questions have arisen many of these have developed from differing pictures of solute-environment interactions which are furnished by different studies using different molecules as "probes" (5,10-12). Controversies arising... 

Normal Micelles - Solubilizate Probes. The addition of a probe molecule, usually bearing a C=0 group, to a micelle has been used to asses die solubilization site of the probe (67) and to infer the extent of penetration of water into micelles (68,69). The basis of such studies is the well known decrease in the 0=0 band frequency upon hydrogen bond formation (70 -73). Two important concepts must be addressed, however, when using probes in studies of micelles the solubilization site of the probe (micelle core or palisade layer) and the possibility of probe-induced changes in the micelle. 

Alternatively, the geometrical effects on solubilization sites can be explained by a simple excluded volume effect, forcing the probe closer to the micellar surface in tail end geometry. This explanation is supported by the findings that rather high polarities were observed for a number of vinylic polysoaps of the tail-end type in comparison to their monomers [64, 65, 74, 164, 245]. In contrast, polysoaps of the head type may show similar or even less polar environments than analogous monomers [152, 167, 187]. 

Information about the polarizability of a solvent or of a solubilization site in a microheterogeneous solvent can be obtained fiom absorption or excitation spectra of probe molecules, since the spectral position of an absorption band depends more on the polarizability of the medium, i.e. on the refractive index n, than on the polarity represented by the dielectric constant e. The spectral shift Av that occurs when a molecule is transferred from vacuum to a refracting medium can be expressed as (19S)... 

Working with spectroscopic probes, one needs to know the solubilization site of the probe, which should be determined independently on the spectroscopic effect to be exploited. However, when micelles of a homologous series of surfactants are investigated, information on variation of solubilization sites may be obtained. Roelants et al. (177) concluded from activation energies of quenching processes that the quencher molecule iV-methyl-A7-decylaniline resides a little deeper in TTAC micelles than in CTAC micelles. 

The preferred location of aromatic solutes in micelles cannot be learned unequivocally from the literature. Evidence has been presented for solubilization in the micelle core, at the surface, or at both these sites, depending on the concentration (43,199-204). Therefore the solubilization site of aromatic compounds seems to depend on details of the respective systems. In the case of the probe acridine, site information can be derived from relative quantum yields... 

Various methods for measuring bound water content have been proposed (see Lee and Lee [179], and references therein). A considerable amount of work on the properties of interfacial water of organized assemblies has been done with probes or reactions whose rates are sensitive to the microenvironment [250-252]. These methods often involve hydrophobic solutes whose (average) solubilization site in the micellar pseudophase is stiU a matter of discussion, and which may perturb the aggregate structure [253,254], This is not the case in thermal analysis. 

Recent studies indicate that like the surfactant monomers themselves, the solubilized molecules are not rigidly fixed in the micelle but have a freedom of motion which is dependent to some extent on the solubilization site. In fact, as discussed in Chapter 3, solubilized probes are used as indicators of the fluidity of their micellar micro-environment. In particular, a decrease in the polarization of the fluorescent radiation from fluorescent probes located in the micellar core is indicative of the molecular motion of these probes. Similarly, tumbling of solubilized nitroxide probes proceeds at a more rapid rate than can be accounted for by simple rotation of the micelle itself indicating that the solubilized probe undergoes dynamic motion within the micelle. Such motion produces a characteristic hyperfine pattern on e.s.r. spectra. 

The use of a probe molecule to study the properties of micellar medium requires the knowledge of the micellar binding site of the probe molecule, with absolute certainty under a specific condition, which is almost impossible to achieve by any experimental or theoretical technique. Pyrene, a highly hydrophobic water insoluble hydrocarbon molecule, has been used to determine the polarity of micellar medium, but its precise micellar binding site is not known with absolute certainty. For instance, pulsed Fourier transform NMR and chemical shift analyses used to determine the dynamic solubilization site of the pyrene in cationic micellar solutions show that pyrene is solubilized in the interior of the... 

Ganesh, K.N., Mitra, P., Balasubramanian, D. Solubilization sites of aromatic optical probes in micelles. J. Phys. Chem. 1982, 86(22), 4291-4293. 

The aqueous cores of reverse micelles are of particular interest because of their analogy with the water pockets in bioaggregates and the active sites of enzymes. Moreover, enzymes solubilized in reverse micelles can exhibit an enhanced catalytic efficiency. Figure B4.3.1 shows a reverse micelle of bis(2-ethylhexyl)sulfosuccinate (AOT) in heptane with three naphthalenic fluorescent probes whose excited-state pK values are much lower than the ground-state pK (see Table 4.4) 2-naphthol (NOH), sodium 2-naphthol sulfonate (NSOH), potassium 2-naphthol-6,8-disulfonate (NSOH). The spectra and the rate constants for deprotonation and back-recombination (determined by time-resolved experiments) provide information on the location of the probes and the corresponding ability of their microenvironment to accept a proton , (i) NDSOH is located around the center of the water pool, and at water contents w = [H20]/[A0T] >... 

Fig. 12.12 Top Interaction of 4e with human serum albumin as probed by isothermal titration calorimetry, a single-site model yielded a stoichiometry of 5 1 of 4e HSA with a kn of 2 pM. Middle Inhibition of hemolysis of 4e by HSA. Bottom Identical potency of NO inhibition of 4e solubilized in DMSO or in HSA...

---