Polarity microenvironmental

Fluorescence spectra of pyrene were employed in order to see the polarity of the environment given by the molecular assemblies mentioned so far. The III/I ratio of vibronic spectra of pyrene, a good measure of microenvironmental polarity(194.2Q). is plotted for the DODACl systems as a function of MEGA-n concentrations in Figure 11. 

Table 1. Binding constants (K) for host-guest complexes of artificial receptors with 11, and microenvironmental polarity parameters ( ) and steady-state fluorescence polarization values (P) for the guest bound to hosts in aqueous solution at 30.0 °C ...

The microenvironmental polarity parameters for ANS and TNS bound to various hosts are listed in Table 3. These values are independent of temperature over a range of 10-40 °C. In the absence of any macrocyclic hosts, ANS is bound to the membrane in its surface domain while TNS to the hydrogen-belt domain [60, 61] interposed between the polar surface region and the hydrophobic domain composed of double-chain segments in the light of the Ej values the microenvironments for the former and the latter are close to that provided by water ( = 1.000) and equivalent to that in ethanol ( = 0.654), respectively. Such a difference in microenvironmental polarity presumably comes from the difference in molecular shape TNS is more slender than ANS. 

The steroid cyclophanes provide less polar microenvironments for ANS and TNS by forming the hybrid assemblies with the peptide lipid. To our surprise, the microenvironment around the ANS molecule incorporated into the hybrid assembly formed with lipid 9 and steroid cyclophane 5 is equivalent to that provided by hexane ( = 0.009). In contrast, the microscopic polarity experienced by TNS in the identical hybrid assembly is as polar as 1-pentanol ( = 0.568). On the other hand, the hybrid assembly formed with lipid 9 and steroid cyclophane 6 binds the TNS molecule in a less polar microenvironment than ANS. Meanwhile, both of the octopus cyclophanes and their hybrid assemblies formed with the peptide lipid incorporate both guest molecules into the comparable binding sites with respect to the microenvironmental polarity. 

Asaoka, S., Wada, T., and Inoue, Y. (2003) Microenvironmental polarity control of electron-transfer photochirogenesis. Enantiodifferentiating polar addition of... 

Figure 8 Microenvironmental polarity control upon enantiodifferentiating polar photi addition of alcohol (ROH) to aromatic olefin (D) sensitized by naphthalenedicarboxylai with saccharide auxiliaries (A ) the local polarity is enhanced around the saccharic moieties, facilitating electron transfer from exited sensitizer (A ) to substrate olefin (I to produce a radical cation (D +). The radical cation produced cannot escape from tl high polarity region around the saccharide to the low-polarity bulk solution and is accor< ingly attacked by ROH in the chiral environment of saccharide to produce the adduct i high ee.

As is apparent from Figure 1, SB shows two absorption bands with maxima at 400 and 318 nm, which are assigned to two tautomeric isomers, C and D in Scheme 2, respectively [7]. Since the relative intensities of these bands sensitively vary depending on the medium polarity, the microenvironmental polarity provided by the octopus cyclophane can be estimated. Figure 2 shows electronic absorption spectra of the SB species formed with PLP and octylamine in the presence and absence of 1, While SB is present exclusively in form C in an aqueous phase, form D is remarkably favored in the presence of 1 the microenvironmental polarity in the cavity of 1 is roughly equivalent to that provided by 2-propanol. Accordingly, it is apparent that PLP bound to the hydrophilic site of the octopus cyclophane moves into a more hydrophobic domain of the host molecule via formation of SB with the hydrophobic alkylammonium substrate as schematically illustrated in Figure 3. 

Studies of oxidations in micellar CTAB and SDS media [31] suggested average microenvironments for ferrocene during electron transfer in micellar solutions that are more polar than in apolar solvents but less polar than in water. Similar microenvironmental polarity is found for hydrophobic solutes in micelles [13]. 

TABLE II. BINDING CONSTANTS (K/mol l dm3) AND MICROENVIRONMENTAL POLARITY PARAMETERS (Ex(30Wkcal mol l) FOR THE INTERACTIONS OF AZACYCLOPHANES WITH VARIOUS GUESTS )... 

The intensity ratio of the third to fin peaks in the vibrational fine structures in pyrene fluorescence spectra (/3//1) is often used as an indicates for the microenvironmental polarity about pyrene. In general, the /3//1 ratio is larger in less polar microenvironments (50). The /3//1 ratio of the pyrene-labeled polymers (Chart 2) with varying /ood increases in the region 10 < fyod < 30 mol %, reaching a... 

Since the shear induces an unusual conformational transition in the PAA to form a self-complexation, it is worthwhile to investigate how the complexation is affected by the interaction with competitive species such as poly(ethylene oxide) [15] and cationic surfactants. With the latter, it is particularly interesting to see how the PAA interchain complexation is interfered with by the presence of a hydrophobic environment due to the bound surfactant molecules, and how their microdomain formation is altered by the presence or absence of shear stress. To monitor the conformational state changes of the PAA, we chose two fluorescence probes, pyrene and AuO the former as a probe for microenvironmental polarity in the absence of shear and the latter as a probe for local rigidity along the PAA chain under shear. 

This effect is attributed to the increased microenvironmental polarity around the sensitizer chro-mophore that stabilizes the exciplex or contact ion pair in nonpolar solvents. As a result of this effect, the stereochemical interaction between the sensitizer and the substrate is more intimate. Because significant enantioselectivities were only observed for dimer 44, an independent cyclodimerization pathway to 44 via an exciplex or contact ion pair of cyclohexadiene and the chiral sensitizer was suggested. Dimer 45 gave much lower ee values even at low temperatures, but the product chirality was inverted within the tested temperature range in favor of enantiomer ent-A5. Similar temperature switching of product chirality has been reported in the enantiodifferentiating photoisomerization of cycloalkenes and in the polar addition of alcohols to 1,1-diphenylalkenes. This effect has been rationalized by a non-zero differential activation entropy of the same sign as the differential activation enthalpy. 

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