Titration curve, amino acid

See also - Table 5.1, Genetic Code, Glutathione, Metabolism of Serine, Glycine, and Threonine, Essential Amino Acids, Titration Curve of Glycine... 

From these decompositions, amino acid titration curves can be predicted. The evolution of pH is correctly predicted by the model for amino acids such as glycine, aspartic and L-glutamic acids (Figure 3.3). Most predictions for the other amino acids are correct, however, for some, small differences between measurements and predictions have been observed, because they are due to the pKa values that are not generally measured at zero ionic strength and infinite dilution. 

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. 

The appearance of the titration curve of a protein depends primarily upon the relative numbers and dissociation constants of the dissociating groups of its constituent amino acids. The curves observed experimentally differ markedly, however, from a curve derived by merely adding the titra-... 

The potentiometric titration curve shown here was recorded on a 0.4300-g sample of a purified amino acid that was dissolved in 50.00 ml of water and titrated with 0.1036 M NaOH. Identify the amino acid from the possibilities listed in the following table. 

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... 

Typical values for pAlg are in the range of 9.0 to 9.8. At physiological pH, the a-carboxyl group of a simple amino acid (with no ionizable side chains) is completely dissociated, whereas the a-amino group has not really begun its dissociation. The titration curve for such an amino acid is shown in Figure 4.7. 

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,... 

C18-0142. The amine group of an amino acid readily accepts a proton, and the protonated form of an amino acid can be viewed as a diprotic acid. The p Zg values for serine (H2 NCHRCO2 H, i = CH2 OH) are p ra(H3 N"") =9.1 and p (002 H) - 2.2. (a) What is the chemical formula of the species that forms when serine dissolves in pure water (b) If this species is titrated with strong acid, what reaction occurs (c) 10.00 mL of 1.00 M HCl is added to 200. mL of 0.0500 M serine solution. This mixture is then titrated with 0.500 M NaOH. Draw the titration curve, indicating the pH at various stages of this titration. 

The titration curve of penicillamine hydrochloride at 25 °C revealed the presence of three ionizable groups with pKa values of 1.8 (carboxyl group), 7.9 (oc-amino group), and 10.5 (/J-thiol group). Recently, the ionization constants for the acidic functions of (D)-penicillamine were verified by pH titration at 37 °C and 0.15 M ionic strength [2], A 1% solution in water has a pH of 4.5-5.5 [3],... 

With multiple ionizable groups, such as in amino acids and proteins, each group titrates separately according to its pKa. The titration curves shown in Fig. 23-5 are for the amino acids glycine, histidine, and glutamate. 

In the titration curves shown in Fig. 23-5, you start with the fully protonated form of the amino acid. Notice that at pH s that are not near the pKa of any functional group, the pH changes more when base is added. Also notice that there are multiple buffer regions (where the pH doesn t change rapidly when base is added) when there are multiple acid and base groups present. If the pAVs of two groups are close to each... 

Abstract Titration of weak bases in non-aqueous solvents can provide valuable information about these weak bases. Some primary amines 1-aminobutane, 1-aminopropane, 2-aminoheptane, aminocyclohexane, 3-amino-l-phenylbutane were titrated with hydrochloric acid in toluene solvent. All the primary amines gave very well-shaped potentiometric titration curves. The same titrations were done with hydrochloric acid in methanol solvent to show the effect of amphiprotic solvent in the titrations with hydrochloric acid. 

Fig. n.7 Sharp-break PLIMSTEX curves at high protein concentration [23], Line (a) melittin (a 26-amino-acid peptide) titration. Line (b) mastoparan (a 16-amino-acid peptide) titration of 15 pM Ca +-saturated porcine calmodulin (CaM-4Ca) in 50 mM HEPES, 100 mM KCI, 0.49 mM Ca +, 99% D2O, apparent pH 7.4. Data points are based on the average of t /o runs for each titration system, and the breaking point clearly indicates 1 1 protein-ligand binding stochiomet. ... 

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). 

Amino Acids Have Characteristic Titration Curves... 

Titration Curves Predict the Electric Charge of Amino Acids... 

Amino acids vary in their acid-base properties and have characteristic titration curves. Monoamino monocarboxylic amino acids (with nonionizable R groups) are diprotic acids (+H3NCH(R)COOH) at low pH and exist in several different ionic forms as the pH is increased. Amino acids with ionizable R groups have additional ionic species, depending on the pH of the medium and the pIQ of the R group. 

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... 

Ribonuclease is an enzyme with 124 amino acids. Its function is to cleave ribonucleic acid (RNA) into small fragments. A solution containing pure protein, with no other ions present except H+ and OH- derived from the protein and water, is said to be isoionic. From this point near pH 9.6 in the graph, the protein can be titrated with acid or base. Of the 124 amino acids, 16 can be protonated by acid and 20 can lose protons to added base. From the shape of the titration curve, it is possible to deduce the approximate pATa for each titratable group.1-2 This information provides insight into the environment of that amino acid in the protein. In ribonuclease, three tyrosine residues have "normal values of pATa(=10) (Table 10-1) and three others have pA a >12. The interpretation is that three tyrosine groups are accessible to OH, and three are buried inside the protein where they cannot be easily titrated. The solid line in the illustration is calculated from pA"a values for all titratable groups. 

Theoretical titration curves for enzymes can be calculated from known crystal structures and first principles of electrostatics. Key amino acids at the active site have significantly perturbed pK values and unusual regions in which they are partially protonated over a wide pH region.3 In principle, such titration calculations can identify the active site of a protein whose structure is known, but whose function is not. 

In nitrogen heteroaromatics, upfield protonation shifts are found for carbons a to nitrogen, while those in / and y positions are deshielded on protonation [94, 99,100]. This is shown in Fig. 3.5 for quinoline [94]. The protonation shifts for C-/1 and C-y can be rationalized in terms of the cannonical formulae of protonated pyridine [73 d], while the upfield shifts for C-a are probably due to the lower n character of the N — C-a bond. The curves in Fig. 3.5 representing the pH dependence of 13C shifts resemble titration curves. pK values and, in the case of amino acids, the isoelectric points pi can be obtained from the point of inflection of the (5 versus pH plot for each individual carbon [84, 94, 98]. 

NMR studies have been carried out on Schiff bases derived from pyridoxal phosphate and amino acids, since they have been proposed as intermediates in many important biological reactions such as transamination, decarboxylation, etc.90 The pK.d values of a series of Schiff bases derived from pyridoxal phosphate and a-amino adds, most of which are fluorinated (Figure 11), have been derived from H and19F titration curves.91 The imine N atom was found to be more basic and more sensitive to the electron-withdrawing effect of fluorine than the pyridine N atom. Pyridoxal and its phosphate derivative are shown in Figure 12a. The Schiff base formation by condensation of both with octopamine (Figure 12b) in water or methanol solution was studied by 13C NMR. The enolimine form is favoured in methanol, while the ketoamine form predominates in water.92... 

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,... 

---