Amino dissociation behavior

Fig. 3-36. Dissociation behavior of amino acids on the example of glycine.

As an alternative to carbonate/bicarbonate systems, amino adds (a-aminocarb-oxylic acids) may be used as an eluent [47,48]. Their dissociation behavior is depicted in Fig. 3-36. At alkaline pH, amino acids exist in the anionic form due to the dissociation of the carboxyl group and, thus, may act as an eluent ion. The product of the suppressor reaction is the zwitterionic form with a correspondingly low background conductance. This depends on the isoelectric point, pi, of the amino acid. 

The particular structural features of AICAR (Fig. 4.18, 2) that includes an aminoimidazole moiety as well as a ribofuranose residue are reflected in its mass spectrometric dissociation behavior as shown with its product ion mass spectrum (obtained after positive ESI and CID Fig. 4.19b). Numerous product ions originating from consecutive losses of water (-18 Da) and ammonia (-17 Da) were found at m/z 242,241,223,205, and 188, and most abundant fragment ions resulting from the aminoimidazole-carboxamide nucleus were observed at m/z 127 and 110. Assuming an initial protonation of the primary amino function, the neutral loss of the ribose residue (2-hydroxymethyl-2,3-dihydro-furan-3,4-diol, -132 Da) gives rise to the protonated 5-amino-imidazole-4-carboxamide m/z 127) that subsequently releases ammonia (-17 Da) to yield the cation of imidazole-4-carboxamide with m/z 110 (Scheme 4.11b). ... 

The dynamic behavior of the model intermediate rhodium-phosphine 99, for the asymmetric hydrogenation of dimethyl itaconate by cationic rhodium complexes, has been studied by variable temperature NMR LSA [167]. The line shape analysis provides rates of exchange and activation parameters in favor of an intermo-lecular process, in agreement with the mechanism already described for bis(pho-sphinite) chelates by Brown and coworkers [168], These authors describe a dynamic behavior where two diastereoisomeric enamide complexes exchange via olefin dissociation, subsequent rotation about the N-C(olefinic) bond and recoordination. These studies provide insight into the electronic and steric factors that affect the activity and stereoselectivity for the asymmetric hydrogenation of amino acid precursors. 

A way of examining the zwitterionic behavior of amino acids is to study their titration. Suppose, for example, that we begin with a solution of glycine hydrochloride, in which both groups are in their acidic forms. Addition of sodium hydroxide brings about an increase in the pH of the solution, and at the same time, dissociated protons react with the added hydroxyl ions to form water thus allowing further dissociation to take place, as shown in Fig. 3-2. 

The lone pair electrons of water (O atom), ammonia (N atom), and amino groups (N atom) influences the behavior and concentrations of hydrogen ions (H+) in water. Hydrogen ions, produced either by dissociation of water or by dissociation of acids, do not occur as free entities in aqueous solutions. They associate with the lone pair electrons of other water molecules to form hydronium ions, H3O+. This association involves the formation of an electron donor-acceptor bond. 

Sorption behavior of inorganic and carboxylic acids is reported in Table 4.2 (anions), but sorption behavior of amino acids and phenols is reported in Table 4.4 (organic compounds). This classification is arbitrary, e.g. some derivatives of phenol whose sorption is reported in Table 4.4 have higher acidic dissociation... 

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