Titration of amino acids

Equivalents of Base Added Figure 23-5 Titration of Amino Acids... 

Chemical modification can be used to verify the assignment of pKa values in titrations of amino acids. For example, by titrating in the presence of formaldehyde, we can show that pKai in Fig. 3-2 belongs to the carboxyl group, and pKa2 to the amino group. 

The corrected pH-ncutralization curves obtained in this manner are shown by the dotted lines in Fig. 108 the inflexions at the equivalence points are now seen to be sufficiently definite for an accurate estimate of the end-point to be possible. This method has been used for the poten-tiometric titration of amino-acids. ... 

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. 

Other methods of titration of amino-acids have been employed that are based on principles already described as with other very weak bases the neutralization by acid may be carried out in glacial acetic acid solution (cf. p. 398), or use may be made of the fact that charged cation acids and anion bases give sharper pH inflexions when neutralized in media of low dielectric constant, e.g., alcohol or acetone (cf. p. 397). ... 

Amino Acid Classes Biologically Active Amino Acids Modified Amino Acids in Proteins Amino Acid Stereoisomers Titration of Amino Acids Amino Acid Reactions PEPTIDES PROTEINS... 

Titration of amino acids and peptides illustrates the effect of pH on their structures. The pH at which a molecule has no net charge is called its isoelectric point. 

Calculating acid-base titration curves Strong acids, strong bases (Table 8.1), p. 266 Spreadsheet calculations, p. 269 Weak acids, weak bases (Table 8.2), p. 272 Spreadsheet calculations, p. 277 Indicators (key equations 8.4, 8.5), p. 270 Titration of Na2C03, p. 280 Titration of polyprotic acids (Table 8.3), p. 281 Titration of amino acids, p. 286... 

Figure 28 (a) Substrate-product pair for four decarboxylases, (b) Fluorescence response upon titrations of amino acids and their... 

In titration of amino acids by this method, it is desirable on theoretical grounds to employ, especially in the proximity of the end point, increments of standard base which are equal and as small as possible although it has been the practice to use increments of 0.25 N base varying from 0.02 to 0.20 ml. 

The titration of amino acids with the dissociable R groups conforms to the same principles but requires three equivalents of bases. The general shape of the titration curve resembles that for the titration of a polyprotic weak acid with strong base (Figure 2.4). The pi of any amino acid can be calculated according to the following formulae ... 

The Sdrenson procedure for the titration of amino acids is perhaps the be.st knoAvn of the analjirical procedures in which formaldehyde is em-plo ed. Reaction with formaldehyde makes it po ible to carry out direct titration of these acids, since the basic properties of the amino radicals are nullified by formaldehyde condensation. The reactions upon which this procedure is based have been already discussed (page 220). 

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