Ammonium carboxylate zwitterions

The Akzo Nobel alkyl ammonium carboxylate (zwitterionic)/SDBS (molar ratio 4 1) surfactant mixture is DR in both water and ethylene glycol/water systems. However, better DR results were obtained at lower temperatures by a new alkyl amin-imide zwitterionic in both water and ethylene glycol/water solutions [Zhang et al., 2005b], This was attributed, in part, to the presence of the oleyl chain, which enhances low-temperature solubility. 

The molecule is shown in its zwitterionic ammonium carboxylate form. 

The structure of an amino acid in aqueous solution depends on the pH. Consider, for example, the simplest member of the series, glycine. The major form in neutral solution is the zwitterion. However, in strong acid (pH < 1), glycine exists predominantly as the cationic ammonium carboxylic acid, whereas strongly basic solutions (pH > 13) contain mainly the deprotonated 2-aminocarboxylate ion. These forms interconvert by add-base equihbria (Section 2-2). 

The other three cases are less common. In the case of chemically anchored anionic species, mostly carboxylate [78,79] or sulfonate groups have been investigated. Examples for chemically fixed cation/anion pairs are scarce here, ammonium carboxylate [80,81] or, more related to water-immiscible ionic liquids, guanidinium sulfonimide ion pairs [82] have been reported. The chemical grafting of zwitterionic species is nearly always related to immobilized ammonium or imidazolium sulfonate zwitterions that are easily formed upon reaction of nucleophiles (amines or imidazoles) with sultones [83-85]. 

Glycine (Gly) is the simplest molecule in the 20 amino acids, and the FUV spectrum of Gly is the most appropriate to show the electronic transition of the backbone structure of amino acids. The primary chromophore of Gly is the carboxyl group. Figure 4.13 shows the FUV spectra of 2 M Gly in 1.81 M H2SO4, pure water, and 3.36 M NaOH aqueous solutions. Because the pATai and pK 2 values of Gly are 2.34 and 9.60, the protonation states are ammonium carboxylic acid, zwitterion, and aminocarboxylate anion in acidic solution, pure water, and basic solution, respectively. The intense bands appear at 157.5 nm (molar absorption coefficient e = 5.15x10 M cm ), 168.9 nm (7.76x10 M cm ), and 169.9 nm (7.73 x 10 cm ) for the acid, the zwitterion, and the anion forms,... 

We saw in Sections 20.3 and 24.5 that a carboxyl group is deprotonated and exists as the carboxylate anion at a physiological pH of 7.3, while an amino group is protonated and exists as the ammonium cation. Thus, amino acids exist in aqueous solution primarily in the form of a dipolar ion, or zwitterion (German zwitter, meaning "hybrid"). 

Amino acid zwitterions are internal salts and therefore have many of the physical properties associated with salts. They have large dipole moments, are soluble in water but insoluble in hydrocarbons, and are crystalline substances with relatively high melting points. In addition, amino acids are amphiprotic they can react either as acids or as bases, depending on the circumstances. In aqueous acid solution, an amino acid zwitterion is a base that accepts a proton to yield a cation in aqueous base solution, the zwitterion is an add that loses a proton to form an anion. Note that it is the carboxylate, -C02-, that acts as the basic site and accepts a proton in acid solution, and it is the ammonium cation, -NH3+, that acts as the acidic site and donates a proton in base solution. 

Polytopic macrocyclic receptors 1, 2 (Figure 10.1) are able to complex the zwitterionic form of the amino acids by a double non-covalent charge interaction [28,29]. The unsymmetrical benzocrown sulfonamide derivative, 2 which contains benzo-18-crown-6 and benzo-15-crown-5 moieties was used as a ditopic receptor for multiple molecular recognition of the amino acids, by combining two non-covalent interactions ammonium-crown hydrogen bonding and carboxylate- complexed Na+-benzo-15-crown-5 charge interactions [28,33]. 

Most studies of micellar systems have been carried out on synthetic surfactants where the polar or ionic head group may be cationic, e.g. an ammonium or pyridinium ion, anionic, e.g. a carboxylate, sulfate or sulfonate ion, non-ionic, e.g. hydroxy-compound, or zwitterionic, e.g. an amine oxide or a carboxylate or sulfonate betaine. Surfactants are often given trivial or trade names, and abbreviations based on either trivial or systematic names are freely used (Fendler and Fendler, 1975). Many commercial surfactants are mixtures so that purity can be a major problem. In addition, some surfactants, e.g. monoalkyl sulfates, decompose slowly in aqueous solution. Some examples of surfactants are given in Table 1, together with values of the critical micelle concentration, cmc. This is the surfactant concentration at the onset of micellization (Mukerjee and Mysels, 1970) and can therefore be taken to be the maximum concentration of monomeric surfactant in a solution (Menger and Portnoy, 1967). Its value is related to the change of free energy on micellization (Fendler and Fendler, 1975 Lindman and Wennerstrom, 1980). 

We can appreciate that ionization of the carboxylie acid is affected by the electron-withdrawing inductive effect of the ammonium residue hence the increased acidity when compared with an alkanoic acid. Similarly, loss of a proton from the ammonium cation of the zwitterion is influenced by the electron-donating inductive effect from the carboxylate anion, which should make the amino group more basic than a typical amine. That this is not the case is thought to be a solvation effect (compare simple amines). 

Many researchers have investigated the use of amines and alcohols as initiators for the ROP of lactones. As a rule, amines and alcohols are not nucleophilic enough to be efficient initiators, and it is then mandatory to use catalysts to perform the polymerization successfully. Nevertheless, highly reactive p-lactones exhibit a particular behavior because their polymerization can be initiated by nucleophilic amines in the absence of any catalyst. As far as tertiary amines are concerned, the initiation step implies the formation of a zwitterion made up of an ammonium cation and a carboxylate anion, as shown in Fig. 20. Authors coined the name zwitterionic polymerization for this process [80]. Nevertheless, this polymerization is not really new because the mechanism is mainly anionic. Interestingly, Rticheldorf and coworkers did not exclude the possibility that, at least at some stage of the polymerization, chain extension takes place by step-growth polycondensation [81]. 

Close proximity of the oppositely charged functional groups can be achieved in ion exchangers with covalently bonded zwitterionic molecules in which two or three methylene chains separate the carboxylate or sulfonate and quaternary ammonium groups [ 13-151. This arrangement results in the establishment of a combination of repulsion and attraction electrostatic forces and such stationary phases have been used for the separation of seven or eight anions and cations [13,141. 

According to this mechanism, initiation includes reaction between the tertiary amine and epoxide, and the primary active centre is represented by a zwitterion with an alkoxide anion and an irreversibly bound amine in the form of an ammonium cation (Eq. (69)). This zwitterion reacts in the next step with the anhydride (Eq. (70)) yielding a carboxylate anion. The growth reactions (Eqs. (71) and (72)) include interactions of the carboxylate anion with epoxide, and of the alkoxide anion with the anhydride. 

Rate and equilibrium constants have been reported for the reactions of butylamine, pyrrolidine, and piperidine with trinitrobenzene, ethyl 2,4,6-trinitrophenyl ether, and phenyl 2,4,6-trinitrophenyl ether in acetonitrile, hi these reactions, leading to cr-adduct formation and/or nucleophilic substitution, proton transfer may be rate limiting. Comparisons with data obtained in DMSO show that, while equilibrium constants for adduct formation are lower in acetonitrile, rate constants for proton transfer are higher. This probably reflects the stronger hydrogen bonding between DMSO and NH+ protons in ammonium ions and in zwitterions.113 Reaction of 1,3,5-trinitrobenzene with indole-3-carboxylate ions in methanol has been shown to yield the re-complex (26), which is the likely precursor of nitrogen- and carbon-bonded cr-adducts expected from the reaction.114 There is evidence for the intermediacy of adducts similar to (27) from the reaction of methyl 3,5-dinitrobenzoate with l,8-diazabicyclo[5.4.0]undec-8-ene (DBU) cyclization eventually yields 2-aminoindole derivatives.115... 

Initiation involves the reaction of the tertiary amine (the most widely used Lewis base) with an epoxy group, giving rise to a zwitterion that contains a quaternary nitrogen atom and an alkoxide anion. The alkoxide reacts at a very fast rate with an anhydride group, leading to a species containing a carboxylate anion as the active center. This ammonium salt can be considered as the initiator of the chainwise copolymerization. 

In 1992, de Mendoza and coworkers reported the synthesis of the bifunction-alized chiral bicyclic guanidinium 34 for the purpose of enantioselectively binding zwitterionic aromatic amino acids (i.e., tryptophan and phenylalanine) [55]. By including binding elements for the carboxylate and ammonium of the amino acids which were noncomplementary, they hoped to avoid the potential... 

The carboxylate-ion-end of the zwitterion is the conjugate base of a weak acid, and the ammonium-ion-end of the zwitterion is the conjugate acid of a weak base. Thus, the zwitterion is a buffer, and solutions of amino acids (and proteins) resist changes in pFl. 

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