Amino-acids neutralization curves

If a sample contains groups that can take up or lose a proton, (N//, COO//), then one must expect the pH and the concentration to affect the chemical shift when the experiment is carried out in an acidic or alkaline medium to facilitate dissolution. The pH may affect the chemical shift of more distant, nonpolar groups, as shown by the amino acid alanine (38) in neutral (betaine form 38a) or alkaline solution (anion 38b). The dependence of shift on pH follows the path of titration curves it is possible to read off the pK value of the equilibrium from the point of inflection... 

Look carefully at the titration curve in Figure 26.1. In acid solution, the amino acid is protonated and exists primarily as a cation. In basic solution, the amino acid is deprotonated and exists primarily as an anion. In between the two is an intermediate pH at which the amino acid is exactly balanced between anionic and cationic forms and exists primarily as the neutral,... 

Fig. 8. The effects of basic, neutral and acidic amino acids on the rate of recrystallization of aragonite to calcite the standard rate curve is given for comparison (after Jackson and Bischoff98 )...

You will obtain a titration curve of an amino acid with a neutral side chain such as glycine, alanine, phenylalanine, leucine, or valine. If pH meters are available, you read the pH directly from the instrument after each addition of the base. If a pH meter is not available, you can obtain the pH with the aid of indicator papers. From the titration curve obtained, you can determine the pK values and the isoelectric point. 

The titration curve of ribonuclease (Tanford and Hauenstein, 1956b) is reversible between its acid end point and the onset of alkaline denaturation. All titratable groups, which are expected to be present on the basis of amino acid analysis, are found to be titrated in the expected parts of the titration curve, with the exception of the abnormal phenolic groups mentioned above. The amino and imidazole groups appear to have normal pK s, and the neutral and alkaline regions in which they occur are compatible with the same values of w as are required to fit the titration curves of the three normal phenolic groups. 

The addition of formaldehyde to an aqueous solution of an amino-acid results in no change in the curve showing the variation of pH in the course of the neutralization by acid, but that for the neutralization by alkali is shifted in the direction of increased acid strength, as shown in Fig. 107. It is known that the formaldehyde reacts with the amino-... 

By making an allowance for the extent of hydrolysis, it is possible to obtain hypothetical corrected pH-neutralization curves for glycine and other amino-acids which show appreciable inflexions at the equivalence-points. Two methods of making the necessary corrections have been employed. It will be recalled that hydrolysis represents incomplete neutralization consequently the free strong acid produced as a result of hydrolysis should be subtracted from the total added in the titration in... 

An alternative, simpler, procedure for improving the inflexion in the neutralization of an amino-acid is to add formaldehyde to the solution although this does not affect the acid-titration curve, the one for alkaline titration is changed, as seen in Fig. 107. The effect of the formaldehyde is to increase the strength of the ammonium ion acid which is being titrated, and so the pH inflexion at the equivalence-point becomes much more obvious. This is the basis of the formol titration of amino-acids discovered by Sorensen (1907) approximately 10 per cent of formaldehyde is added to the solution which is then titrated with standard alkali using phenolphthalein as indicator. In the presence of thii concentration of formaldehyde the pH-neutralization curve has a sharp inflexion in the region of pH 9, and so a satisfactory end-point is possible with the aforementioned indicator. 

Fig. 13 The dissolution behavior of triamterene (TRIAM) (A) and hydrochlorothiazide (HCTZ) (B) formulations containing glycine and citric acid (Formula D) on storage at 40 C/85% RH up to 12 weeks. This figure is correlated to Fig. 7 and is from the same reference. The overlapping curves in both cases show that the amino acid/buffer combination is able to neutralize the adverse effect of storage under high humidity and temperature. The figure further depicts that neutralization remains effective even on storing the capsules as long as 12 weeks. (From Ref. p. 498 by courtesy of Marcel Dekker, Inc.)...

LebriUa has developed a chiral mass spectrometry method that has potential to determine enantiomeric excess of mixtures. The method involves an ion—molecule reaction between a neutral amine and a noncovalent charged complex (formed via ESI) consisting of a selector (cyclodextrin) and the desired selectand (chiral substrate, typically an amino acid). The protonated selector—selectand complex undergoes a guest—ligand exchange reaction with the amine. The rate of this reaction is sensitive to the chirality of the bound selectand and thus can be used to the quantify enantiomeric excess of other mixtures using suitable calibration curves. 

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