Zwitterionic Volume

On several occasions the product isolated by the submitters was contaminated with L-cystine dihydrochloride, which was not easily removed by recrystallization. In this event the product was converted to the zwitterionic form and recrystallized in the following manner. The pH of a solution of the product in water was adjusted to 6 with aqueous 2.5N potassium hydroxide. The neutralized solution was evaporated to dryness under reduced pressure at ca. 40°. The residue was dissolved in a minimum amount of hot water, and two volumes of 95% ethanol were added to precipitate S-acetamidomethyl-L-cysteine monohydrate, dec. 187°, [a] 9 — 42.5° (c = 1, water). 

A mixture of the alkylated adducts 39 and 1.5M HC1 (15-20mL-mmol ) was heated to reflux for 2-8h until hydrolysis was complete, as determined by 31P NMR. The mixture was concentrated in vacuo, the residue was dissolved in H20, and the soln was washed with Et20 and reconcentrated in vacuo. The product was finally purified by ion-exchange chromatography or by dissolution in hot EtOH (twice the minimum volume required) followed by cooling to rt and precipitation of the zwitterion by the addition of propylene oxide. The 1-aminoalkylphosphinic acids 33 were isolated as white solids. 

Although a previous chapter in this volume provides a broader perspective on the reactivity of radical cations, in this section we will examine intramolecular electron-transfer reactions coupled with or followed by cleavage of a bond in odd electron species such as radical cations, radical zwitterions and radical anions. In particular, this paragraph will be divided in oxidative and reductive bond-cleavage processes. Because this field is however too large to be covered extensively here, the discussion will be limited to selected examples—for oxidative cleavages, side-chain fragmentation reactions of alkylaromatic radical cations and decarboxylation reactions of radical zwitterions derived from benzoic and arylalkanoic acids, and for reductive... 

From the effect of solvent on the volumes of activation, it was argued that the zwitterion mechanism, (3) and (4), was in better agreement with the data . It is seen from Table 45 that for malonic acid, the volume of activation decreases as the polarity of the solvent decreases. A similar decrease in AF with decreasing solvent polarity was observed in the reaction of n-butyl bromide with pyridine to give N-... 

However, the zwitterion mechanism appears attractive for picolinic and related acids. The volumes of activation for decarboxylation of picolinic are large and positive. This is consistent with release of solvent in going to the transition state as is indicated in reaction (4) of the zwitterion mechanism. The rate data for the decarboxylation of quinaldinic acids fits attractively into the zwitterion mechanism. The ratio of rate coefficients for the decarboxylation of zwitteron ion (I) to that of quinaldinic acid (II) shows a minimum value of 57 in quinoline solvent at 145.8 The anion of quinaldinic acid shows no detectable decomposition in... 

By adding NaOH to the zwitterion, the pA for deprotonating the NH3 group can be determined. Now 20.00 mL of base are required for complete deprotonation. At a volume of 10.00 mL of NaOH added, the pH is equal to the pATg for the amine group, or 9.89. The pK values for other amino acids and more complicated biomolecules such as peptides and proteins can often be obtained in a similar manner. Some amino acids have more than one carboxyl or amine group. Aspartic acid is an example (Fig. 14F-3). 

When a small volume of an aqueous sample containing anions and cations is passed through a column with a zwitterionic stationary phase, neither the cations nor anions can get very close to the opposite charge on the stationary phase. The sample anions and cations are forced into a new state of simultaneous electrostatic attraction and repulsion interaction. Thus the sample ions are somewhat attracted to the zwitterionic stationary phase, but the attraction is weak enough that water alone can serve as the eluent. To preserve electroneutrality, an equal charge of anions and cations must be eluted. The authors have termed this separation method as EKC (electrostatic chromatography). 

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