Ionic zwitterionic

The initial reaction between a ketene and an enamine is apparently a 1,2 cycloaddition to form an aminocyclobutanone adduct (58) (68-76a). This reaction probably occurs by way of an ionic zwitterion intermediate (75). The thermal stability of this adduct depends upon the nature of substituents Rj, R2, R3, and R. The enolic forms of 58 can exist only if Rj and/or R4 are hydrogens. If the enamine involved in the reaction is an aldehydic enamine with no 3 hydrogens and the ketene involved is di-substituted (i.e., R, R2, R3, and R4 are not hydrogens), then the cyclo-butanone adduct is thermally stable. For example, the reaction of dimethyl-ketene (61) with N,N-dimethylaminoisobutene (10) in isopropyl acetate... 

Future work in this area should focus on further development of novel extraction schemes that exploit one or more of the cited advantages of the nonionic cloud point method. It is worth noting that certain ionic, zwitterionic, microemulsion, and polymeric solutions also have critical consolution points (425,441). There appear to be no examples of the utilization of such media in extractions to date. Consequently, the use of some of these other systems could lead to additional useful concentration methods especially in view of the fact that electrostatic interactions with analyte molecules is possible in such media whereas they are not in the nonionic surfactant systems. The use of the cloud point event should also be useful in that it allows for enhanced thermal lensing methods of detection. 

The second factor is the valency (x) of the head group charge (5 = 0, 1, 2. .. for non-ionic/zwitterionic, monoionic, di-ionic. .. etc.). According to the value of s, the constant A takes the values A=0.5 (5=0), A=0.3 (5=1), A = 1.5-1.8 (5=2). Finally, the valency of the counterions is also important as this influences the value of B. Essentially, the CMC is reduced for multivalent counterions because fewer ions are required close to the micelle surface to (partially) balance the high surface charge density. 

Emulsions are dispersions of one liquid in another liquid, most commonly water-in-oil or oil-in-water. The total interfacial area in an emulsion is very large, and sinee flie interfacial area is associated with a positive free energy (the interfacial tension), the emulsion system is thermodjmam-ically unstable. Nevertheless, it is possible to make emulsions with an excellent long-term stability. This requires the use of emulsifiers that accumulate at the oil/water interface and create an energy barrier towards flocculation and coalescence. The emulsifiers can be ionic, zwitterionic, or nonionic surfactants, proteins, amphiphilic polymers, or combinations of polymers and surfactants. The structure of the adsorbed layer at the water/oil interface may be rather complex, involving several species adsorbed directly to the interface as well as other species adsorbing on top of the first layer. 

Ionic, zwitterionic, and neutral selenium species are present in natural systems. Table 2 shows selenium compounds of biological and clinical interest and species in living organisms are listed in Table 3. Selenoproteins contain selenocysteinyl residues but proteins that contain selenomethionyl residues are not formally classified as selenoproteins. 

The three main types of surfactants are ionic, zwitterionic, and nonionic. Zwitteri-onic surfactants have anionic and cationic constituents. Examples of zwitterionic surfactants include betaines and dimethyl amine oxides. Ionic surfactants are generally classified as cationic if they are positively charged or anionic if they are negatively... 

Figure 10.10 Normal (left) and short contact time (right) N CP/MAS NMR spectra of free base (1), sesquisuccinic acid co-crystal (2), dimalonic acid mixed ionic/zwitterionic complex (3) and dimaleate salt (4) forms of an ErbB2 inhibitor. Only N atoms attached to appear in the short contact time spectra (Adapted from Li et al, ref. 65 Copyright 2006 American Chemical Society).

Lin, J.C.Y., Huang, C.-J., Lee, Y.-T., Lee, K.-M. and Lin, I.J.B., Carboxylic acid functionalized imidazolium salts Sequential formation of ionic, zwitterionic, acid-zwitterionic and lithium salt-zwitterionic liquid crystals, J. Mater. Chem. 21 (22), 8110-8121 (2011). 

Several compounds, among them ionic, zwitterionic and non-ionic surfactants, have been studied on hydrophilic or hydrophobic particles. All investigations reported were performed on surfactants physisorbed to colloidal particles and investigated in concentrated (centrifuged) samples in aqueous environment. In most cases the surfactants were deuterated in or close to the head group, which revealed headgroup dynamics and/or orientation, and in some cases conclusions on the chain dynamics were drawn, which in one study was investigated directly by deuteration in the chain. 

Since protonation plays so significant a role in ionic-zwitterionic interactions, it is to be expected that these interactions will be greatly influenced by the pH of the system. Some data on anionic-zwitterionic interactions are shown in Table 6, together with data on some anionic interactions with non-zwitterionics for comparison. 

The structure of the L3 phase or sponge phase is now well established It is an isotropic phase that consists of a mul-ticonnected bilayer (membrane) that separates the space into two subspaces, each of them self-connected throughout the sample (see Figure 7.1). In phase diagrams the L3 phase often borders on a lamellar phase. It has been reported to occur in many different systems based on ionic,zwitterionic, > and nonionic surfactants. For systems with ionic and zwit-terionic surfactants, the L3 phase is observed in the presence of a cosurfactant, added to tune the curvature of the surfactant bilayer. With nonionic surfactants the cosurfactant is not always needed, as curvature tuning can be achieved by changing the temperature. 

Surfactants are surface-active compounds that consist of a hydrophihc head group attached to a hydrophobic tail (usually a long alkyl chain). They have a high affinity for water or oil depending on the dominant moiety (Walz, 1998 McClements, 2004). When present in sufficiently high concentrations, surfactants form a monolayer at the interface between the oil and water, with the hydrophobic tails of the surfactant orientated towards the oil phase and the hydrophilic head groups towards the aqueous phase. There are four categories of surfactants available in the food industry ionic, non-ionic, zwitterionic and cationic. 

Munoz, M., Rodriguez, A., Graciani, M. del M., Moya, M.L. Micellar medium effects on the hydrolysis of phenyl chloroformate in ionic, zwitterionic, nonionic, and mixed micellar solutions. Int. J. Chem. Kinet. 2002, 34(1), 445-451. 

Schemes for classifying surfactants are based upon physical properties or upon functionality. Charge is tire most prevalent physical property used in classifying surfactants. Surfactants are charged or uncharged, ionic or nonionic. Charged surfactants are furtlier classified as to whetlier tire amphipatliic portion is anionic, cationic or zwitterionic. Anotlier physical classification scheme is based upon overall size and molecular weight. Copolymeric nonionic surfactants may reach sizes corresponding to 10 000-20 000 Daltons. Physical state is anotlier important physical property, as surfactants may be obtained as crystalline solids, amoriDhous pastes or liquids under standard conditions. The number of tailgroups in a surfactant has recently become an important parameter. Many surfactants have eitlier one or two hydrocarbon tailgroups, and recent advances in surfactant science include even more complex assemblies [7, 8 and 9].

Phospholipids. For the removal of ionic contaminants from raw zwitterionic phospholipids, most lipids were purified twice by mixed-bed ionic exchange (Amberlite AB-2) of methanolic solutions. (About Ig of lipid in lOmL of MeOH). With both runs the first ImL of the eluate was discarded. The main fraction of the solution was evaporated at 40°C under dry N2 and recryst three times from n-pentane. The resulting white powder was dried for about 4h at 50° under reduced pressure and stored at 3°. Some samples were purified by mixed-bed ion exchange of aqueous suspensions of the crystal/liquid crystal phase. [Kaatze et al. J Phys Chem 89 2565 7955.]... 

The reactions of electrophilic alkenes (alkenes attached to electron-withdrawing groups) with enamines produce one or more of the following products simple alkylation (2), 1,2 cycloaddition (3), and 1,4 cycloaddition (4). Competition with C alkylation by N alkylation is inconsequential and therefore will be largely ignored (5,7). A stepwise ionic mechanism leading to these products necessarily involves the formation of a zwitterion intermediate (1) as the first step, which is then followed either by one of the... 

FIGURE 4.6 The ionic forms of the amino acids, shown without consideration of any ionizations on the side chain. The cationic form is the low pH form, and the titration of the cationic species with base yields the zwitterion and finally the anionic form. (Irving Geis)... 

More pronounced solvent effects have been observed in special cases where substrates or products possess ionic character. Ito and Matsuda76 found a 35-fold reduction in the rate of addition of the arenethiyl radical 18 to cx-methylstyrene when the solvent was changed from dimelhylsulfoxide to cyclohexane. Rates for addition of other arenethiyl radicals do not show such a marked solvent dependence. The different behavior was attributed to the radical 18 existing partly in a zwitterionic quinonoid form (Scheme 1,7).77... 

The mechanism of decarboxylation of acids containing an amino substituent is further complicated by the possibility of protonation of the substituent and the fact that the species NH2ArCOOH is kinetically equivalent to the zwitterion NHj ArCOO. Both of these species, as well as the anion NH2 ArCOO" and even NH3 ArCOOH must be considered. Willi and Stocker644 investigated by the spectroscopic method the kinetics of the acid-catalysed decarboxylation of 4-aminosalicyclic acid in dilute hydrochloric acid, (ionic strength 0.1, addition of potassium chloride) and also in acetate buffers at 20 °C. The ionisation constants K0 = [HA][H+][H2A+] 1 (for protonation of nitrogen) and Kx = [A"][H+] [HA]-1, were determined at /i = 0.1 and 20 °C. The kinetics followed equation (262)... 

Carbocation-carbanion zwitterionic intermediates were proposed for the thermal cleavage of several cyclic compounds. In most of these reactions the ionically dissociating bond belongs to one of four strained ring systems, i.e. cyclopropane (13), cyclobutane (14), cyclobutene (15) or norbornadiene (16). The mechanism is distinguished from the formation of a diradical intermediate through homolysis in terms of solvent and substituent effects... 

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