Titration curves of alanine

Titration curve of alanine By applying the Henderson-Hasselbalch equation to each dissociable acidic group, it is possi ble to calculate the complete titration curve of a weak acid. Figure 1.11 shows the change in pH that occurs during the addition of base to the fully protonated form of alanine (I) to produce the completely deprotonated form (III). Note the following ... 

Titration curve of alanine. The predominant ionic species at each cardinal point in the titration is indicated. 

What would the titration curve of alanine look like Figure 1.12 shows that the titration curve for alanine looks like that of a diprotic weak acid. From the midpoint of the first titration curve we can calculate the pKai (for the dissociation of carbojqrl group) and from the mid-point of the second titration curve we can calculate the pKa2 (for the dissociation of the amino group). From these two pK<, values we can calculate the pH of the solution of edanine in its zwitterionic form by the equation... 

Figure 26.1 A titration curve for alanine, plotted using the Henderson-Hasselbalch equation. Each of the two legs is plotted separately. At pH < 1, alanine is entirely protonated at pH = 2.34, alanine is a 50 50 mix of protonated and neutral forms at pH 6.01, alanine is entirely neutral at pH = 9.69, alanine is a 50 50 mix of neutral and deprotonated forms at pH > 11.5, alanine is entirely deprotonated.

Dissociation of the carboxyl group The titration curve of an amino acid can be analyzed in the same way as described for acetic acid. Consider alanine, for example, which contains both an a-carboxyl and an a-amino group. At a low (acidic) pH, both of these groups... 

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. 

Titration curve for alanine, showing how its structure varies with pH. Electrostatic potential maps of alanine at low pH (a), at the isoelectric point (b), and at high pH (c) illustrate the difference in charge distribution as a function of pH. 

Figure 17.2 shows a titration curve for alanine, a typical amino acid of this kind. At low pH (acidic solution), the amino acid is in the form of a substituted ammonium ion. At high pH (basic solution), it is present as a substituted carboxylate ion. At some intermediate pH (for alanine, pH 6.02), the amino acid is present as the dipolar ion with an ammonium (—NHj ) and a carboxylate (—CO2 ) unit. A simple rule to remember for any acidic site is that if the pH of the solution is less than the pK, the proton is on if the pH of the solution is greater than the pK, the proton is off. 

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... 

Each leg of the titration curve is calculated separately. The first leg, from pH 1 to 6, corresponds to the dissociation of protonated alanine, H2A+. The second leg, from pH 6 to 11, corresponds to the dissociation of zwitterionic alanine, HA. It s as if we started with H2A+ at low pH and then titrated with NaOH. When 0.5 equivalent of NaOH is added, the deprotonation of H2A+ is 50% done when 1.0 equivalent of NaOH is added, the deprotonation of H2A+ is complete and HA predominates when 1.5 equivalent of NaOH is added, the deprotonation of H A is 50% done and when 2.0 equivalents of NaOH is added, the deprotonation of HA is complete. 

Thymine, electrostatic potential map of, 1104 structure of, 1101 Thyroxine, biosynthesis of, 551 structure of. 1020 TIme-of-flight (TOP) mass spectrometry, 417-418 Titration curve, alanine, 1023 TMS, see Tetramethylsilane see Trimethylsilyl ether Tollens reagent, 701 Tollens test, 992... 

Estimate the titration curve for 0.200 L of 0.120 M diprotic alanine with 3.00 M NaOH. 

B. Although titration curves for proteins are complex because of their multiple acidic and basic groups, their behavior can be illustrated by titration of a simple amino acid such as alanine (Figure 2-1). 

Figure 2-1. Titration of a solution of alanine with a strong base. One equivalent of base is the amount needed to titrate the protons from one group on all the alanine molecules present in the solution. Below the titration curve is a calculation of the pi for alanine derived as the mean of its two pK values.

A simple amino acid with a nonionizable R group gives a complex titration curve with two inflection points. For an example, see the titration of alanine, shown in figure 3.3. At very low pH, alanine carries a single positive charge on the a-amino group. The first inflection point occurs at a pH of 2.3. This is the pK for titration of the carboxyl group,... 

For Questions 4.14-4.17, use the following information. You have isolated an unknown peptide whose titration curve is presented in Figure 4.16. The peptide absorbed ultraviolet light at 280 nm. Treatment with trypsin released free alanine and arginine. Treatment with chymotrypsin resulted in quantitative production of a neutral tripeptide and an alkaline dipeptide. 

Solving the two simultaneous equations gives (HAJ = 0.83 and [A ] =0.17. In other words, at pH = 9.00, 83% of alanine molecules in a 1.00 M solution are neutral (zwitterionic) and 17% are deprotonated. Similar calculations can be done at any other pH and the results plotted to give the titration curve shown in Figure 26.1. 

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