Zwitterionic equilibria

Figure 12.15 Zwitterionic equilibria (a) and that of tautomeric cations (b).

Secondary chemical equilibria, 230,280 see also Secondary equilibria with diprotic acids and zwitterions, equilibrium constants and retention. 241 with micelle ftmtiation, 236... 

QM/EFPl scheme was used for investigating a variety of chemical processes in aqueous environment, including chemical reactions, amino acid neutral/zwitterion equilibrium, solvent effects on properties of a solute such as changes in dipole moment and shifts in vibrational spectrum, and solvatochromic shifts of electronic levels [36, 56, 59-60, 71-79]. Applications of a general QM/EFP scheme were limited so far to studies of electronic excitations and ionization energies in various solvents [56-58]. Extensions of QM/EFP to biological systems have been also developed [80-85]. 

D.A. Hinckley and P.G. Seybold, A spectroscopic/thermodynamic study of the rhodamine B zwitterion equilibrium, Spectrochim. Acta 44A (1988), pp. 1053-1059. 

Theoretical investigation of the neutral/zwitterionic equilibrium of gamma-amino-hutyric acid (GABA) conformers in aqueous solution ... 

So after the addition of aluminum sulfate pH of the solution shifts up, from 2.38 to 2.69. Let us now see how much will the tryptophan acid-base equilibrium change in this new environment. You will use the HH equation to determine the position of the cation-zwitterion equilibrium, as given in Fig. 8.1. 

The effect of the neutral amine substituent on the ionization of substituted carboxylic acids is complicated by the presence of the zwitterion equilibrium. Hence the effect of these groups will be chiefly considered as that of the positive charged nitrogen center. Table 3 indicates that the operative inductive effects of these onium substituents greatly increase the acid strength when compared to XCHgCOOH (X = H, acetic acid pA a = 4 76). It is ako observed + ... 

This is deduced from Hall s assumption of the steric effect on the solvation of tertiary ammonium ions °. The ionization constants of -(substituted-phenyl)gIycines reported recently provide interesting fects on the nature of the substituent, which in conjunction with spectral data, have been used to calculate the zwitterion equilibrium constant and the constants pertaining to the individual ionization process. 

A pKa value of 4.8 is reported for 8 (anion-zwitterion equilibrium) compared to 2.8 for 7b (cation-neutral species equilibrium). ... 

Some 4,5-disubstituted pyridazines exhibit ring-chain isomerism involving heterospiro compounds. For example, 5-(o-aminophenylcarbamoyl)pyridazine-4-carboxylic acid exists in a zwitterionic form in the solid state, but in a solution of DMSO it is almost exclusively 3, 4 -dihydro-3 -oxospiro[pyridazine-5(2//),2 (l //)-quinoxaline]-4-carboxylic acid (134). The equilibrium is strongly influenced by the nature of the solvent, the substituents on the pyridazine ring and the nucleophilicity of the group attached to the phenyl ring (Scheme 48) <80JCS(P2)1339). 

The equilibrium expressed by the preceding equation lies overwhelmingly to the side of the zwitterion. 

The differenee in reaction rates of the amino alcohols to isobutyraldehyde and the secondary amine in strong acidic solutions is determined by the reactivity as well as the concentration of the intermediate zwitterions [Fig. 2, Eq. (10)]. Since several of the equilibrium constants of the foregoing reactions are unknown, an estimate of the relative concentrations of these dipolar species is difficult. As far as the reactivity is concerned, the rate of decomposition is expected to be higher, according as the basicity of the secondary amines is lower, since the necessary driving force to expel the amine will increase with increasing basicity of the secondary amine. The kinetics and mechanism of the hydrolysis of enamines demonstrate that not only resonance in the starting material is an important factor [e.g., if... 

Acridine (9) combines readily with dimethyl acetylenedicarboxylate in methanol yielding the methoxide (11), which is in equilibrium with the corresponding 9-methoxy-9,10-dihydroacridine (12). Presumably the first formed zwitterion (10) abstracts a proton from the solvent,... 

The electronic spectrum of a compound arises from its 7r-electron system which, to a first approximation, is unaffected by substitution of an alkyl group for a hydrogen atom. Thus, comparison of the ultraviolet spectrum of a potentially tautomeric compound with the spectra of both alkylated forms often indicates which tautomer predominates. For example, Fig. 1 shows that 4-mercaptopyridine exists predominantly as pyrid-4-thione. In favorable cases, i.e., when the spectra of the two alkylated forms are very different and/or there are appreciable amounts of both forms present at equilibrium, the tautomeric constant can be evaluated. By using this method, it was shown, for example, that 6-hydroxyquinoline exists essentially as such in ethanol but that it is in equilibrium with about 1% of the zwitterion form in aqueous solution (Fig. 2). 

The ultraviolet spectrum of vitamin Be, or pyridoxine, measured in aqueous ethanol varies with the composition of the solvent indicating that this compound is in equilibrium with the zwitterion form 38. The equilibrium constant in pure water was obtained by extrapolation. Prior to this, equilibria which involved tautomers of type 39 had been suggested for vitamin Be, but see Section VI,A. In the case of pyridoxal, an additional equilibrium, 40 41, occurs (cf. Section VIII) other pyridoxal analogs have also been studied (Table II). 

The equilibrium between neutral a and zwitterionic b forms in the case of nicotinic 6 and isonicotinic 7 acids has been studied by Halle in mixtures of DMSO and water (from 0 to 100%) (Scheme 4). The position of the equilibrium is very sensitive to the composition of the solvent and for more than 80% of DMSO, the a form essentially dominates the equilibrium in solution (96CJC613). An analysis of their data shows a perfect linear relationship (r = 1) between the In Kt of the two acids and moderate linear relationships between In Kt and the percentage of DMSO. Johnston has studied the equilibrium 2-hydroxypyridine/2-pyridone in supercritical fluids (propane at 393 K and 1,1-difluoroethane at 403 K) (89JPC4297). The equilibrium constant Kt (pyridone/hydroxypyridine) increases four-fold for a pressure increase of 40 bar in 1,1-difluoroethane. 

For 5-hydroxythiazoles 159 an equilibrium involving the hydroxy, 159a, 0X0, 159b, and the zwitterionic, 159c, forms was discussed (Scheme 59) [76AHC(S1), p. 367]. 

For 3-hydroxyisoxazol-5-ones 220, a complex equilibrium involving six potential tautomers was considered [75T1861 76AHC(S1), p. 449], In the solid, the tautomeric forms 220b and 220c were observed, while in solution (DMSO, chloroform, ethanol) the zwitterionic tautomer 220f is regarded as the major form (Scheme 73). 

Both the infrared and ultraviolet spectra of pyrrolidine-2,3,5-triones (75) have been interpreted to support their existence as hydroxy-maleimides (76), and the occurrence of a strong OH stretching band in the infrared spectrum of 4-phenylpyrrolidine-2,3,5-trione has been taken as evidence that it too exists in a hydroxy form, probably 76 (R CeHg). However, the trioxo formulation is suggested by t/j the infrared spectra of jV-substituted pyrrolidine-2,3,5-triones, although an equilibrium apparently occurs depending upon the substituents and conditions. The zwitterion formulation 77 has been advanced for 4-aminopyrrolidine-2,3,5-trione. For chemical evidence... 

The pK values for azolediazonium ions (Scheme 12-4) refer to the heterolysis of the NH bond, not to the addition of a hydroxy group. Therefore, these heteroaromatic diazo components may react either as a cation (as shown in Scheme 12-4) or as the zwitterion (after loss of the NH proton). Diener and Zollinger (1986) investigated the relative reactivities of these two equilibrium forms (Scheme 12-5) in the azo coupling reaction of l,3,4-triazole-2-diazonium ion with the tri-basic anion of 2-naphthol-3,6-disulfonic acid. 

Reaching the equilibrium where the amino acid zwitterion predominates is a slow process. After acidifying to pH 6.5, the solution is allowed to stir at 0°C for 25 min during which time the pH of the solution slowly increases. The pH is readjusted to pH 6.5 by slow addition of 2.0N HC1 at 0°C. Repetition of this procedure as many as ten times may be necessary to insure the pH value of the aqueous solution remains at 6.5. 

Equilibrium studies in water were performed for [Mc2Sn(IV)] complexes of zwitterion buffers, such as bicine and tricine (L). The results showed the formation of MLH, ML, ML2, MLH i, and MLH 2 with the hydrolysis products of the... 

The ortho diphenolic structure of apomorphine makes it a strongly reducing substance hence, in acid medium it forms the blue colored or//io-qulnone (6) with iodine or other oxidizing agent which is in equilibrium with its zwitterionic limiting structure (7) (Pellargi s reaction [14]). 

C17-0120. In aqueous solution, amino acids exist as zwitterions (German for double ions ), compounds in which internal proton transfer gives a molecule with two charged functional groups. Use Lewis structures to illustrate the proton transfer equilibrium between the uncharged form of glycine (NH2 CH2 CO2 H) and its zwitterion form. 

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