Glycine, ionic forms

The effect of ionic form on the reaction of the hydrated electron with amino acids has been examined. The cationic form could not be examined since appreciable amounts of H + would have to be present, and with currently available techniques the electron would disappear too rapidly. But by making the solutions alkaline it has been possible to study the anionic form. For glycine (Table I), and several other amino acids and peptides (7), it has been shown that the amino acids are less reactive in the anionic form, agreeing with the conclusion drawn by Garrison. The results for glycine however cannot be interpreted on the basis of the known pK together with assumed rate constants for zwitterion and anion. Other factors are evidently present, and further work is required. 

Ionic Forms of Glycine and Sarcosiine at various pH values ... 

Figure 3.19 The different ionic forms of glycine and the pH at which the form predominates.

Amino acids have two distinct pAa values, the actual ionic form of any amino acid depends on the pH of the solution. The ionic forms of glycine which predominate at various pH values are illustrated in Figure 3.18. Because of the presence of the charged groups, an amino acid in solution will have a positive charge at low pH. As the pH is raised, a point will be reached where the amino acid exists exclusively in the dipolar form, known as the zwitterion. This pH is known as the isoelectric point (pi). 

In the gas phase the neutral form is energetically favored, while in condensed phases, such as solutions in polar solvents and crystals, the ionic form is more stable. Glycine and alanine are zwitterionic in water solutions and their description is a rather challenging task for continuum solvation models. In the following we summarize the results of a study we have published on Theochem on structures and energies of tautomers of glycine and alanine in the framework of the lEF version of the Polarizable Continuum Model. 

It should be carefully noted that the term ligand does not apply to all of the organic material present, but only to that part of it which is in the appropriate ionic form for combining with the metal cation. For ethylenediamine, glycine, and oxalic acid, the ligands are the molecule, the monoanion, and the dianion, respectively. Hence, when stability constants are used to compare the relative... 

At pH = p fj the concentrations of the cation and the amphion are equal, and so each of these two forms amounts to 50% of the total amino acid. At pH = pK2 the concentrations of the amphion and the anion are equal. The dependence of ionic forms of glycine on pH is shown in Figure 2.5. 

While ethylenediamine, and many other chelating agents, form only covalent bonds, there are others which attach by both covalent and ionic bonds. Thus glycine forms with cupric ions (Cu +) the compound copper bisaminoacetate. 

Akers, M. J., Milton, N., Bryn, S. R., Nail, S. L. Glycine crystallization during freezing the effects of salt form, pH, and ionic strength. Pharm Res 12 1457-1461 (1995). 

Fig. 4. Study of the equilibrium between pyruvic acid and glycine. 5.6 x 10 4 M Sodium pyruvate, 0.005% gelatin, glycine buffer pH 9.2. Concentration of the form of glycine with the free amino group is given in the polarogram. Ionic strength kept constant by addition of sodium chloride. Curves starting at —0.8 V, S.C. E., 200 mV/absc.,

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