Chemistry
Titration Curve of Amino Acids
The titration curve of amino acids illustrates the pH changes that occur as the amino acid is titrated with a strong base. At the start, the amino acid exists in its acidic form, and as the base is added, the pH rises until the amino acid reaches its isoelectric point, where it becomes electrically neutral. After this point, the pH continues to rise as the base is added.
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9 Key excerpts on "Titration Curve of Amino Acids"
eBook - PDF- Sankhavaram R. Panini(Author)
- 2021(Publication Date)
- Thieme(Publisher)
4.2 Proteins I 41 Fig. 4.3 Titration curves of amino acids. At the indicated p K a points, the concentration of an amino acid prior to and after the loss of a proton at an ionizable group is equal. At the indicated p I points, the sum of the charges of an amino acid is zero. As such, when the pH of the environment is less than the p I, the net charge of the amino acid is positive. Conversely, when the environment ’ s pH is greater than the p I, then the amino acid ’ s net charge is negative. The titration curves presented are for ( a ) glycine, a polar amino acid that has two ionizable groups; ( b ) aspartic acid, an acidic amino acid that has three ionizable groups; and ( c ) arginine, a basic amino acid that has three ionizable groups. Note: Some titration curves have the pH on the y -axis and the NaOH equivalents on the x -axis. Amino Acids and Proteins I 42 4.2.1 Structure The structures of proteins are divided into four levels: primary, secondary, tertiary, and quaternary. Primary Structure The primary structure is the linear arrangement of amino acid residues. The amino acids are linked to each other by peptide bonds formed between the α -carboxyl group of one amino acid and the α -amino group of another. The conventional way of writing a protein ’ s primary sequence is to list the amino acid residues in order from left to right, starting with the N-terminus (which has an amino group, NH 3 + ), and ending with the C-terminus (which has a carboxylate ion, COO -) ( ▶ Fig. 4.4). Foundations Rotation around the polypeptide backbone Although no rotation is possible around the peptide bond (due to its partial double bond character), rotation is allowed along the polypeptide backbone at the bonds attached to the α -carbons of the amino acid residues. This rotation, however, is extremely limited due to steric hindrance between the di ff erent R-groups and the carbonyl oxygens.
- Wallace Brey(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
6-6 AMINO ACIDS AND PROTEINS 2 0 5 given above for the ionization of each of them and to justify the value given for the isoelectric point. Polypeptides and proteins are formed of amino acid units linked together by peptide linkages formed by the elimination of water be-tween the carboxyl group of one acid and the amino group of the next: H O H D , H O I I I / R I I I I I I H R H o H R The side chains denoted by R, R', and R contain various functional groups such as we have seen to be present in the component amino acids, including carboxyl groups, amino groups, phenolic groups, im-idazole rings, and so on. One might expect that the nature of the polar groups in a protein could be determined by dividing the titration curve into sections cor-responding to the p K a values of various sorts of groups. Thus the carboxyl groups should titrate at pH values between 2.5 and 5, the im-idazole and terminal N H 3 + groups between 6 and 8, side-chain amino, phenolic, and sulfhydryl groups between 9 and 10.5, and the guanidyl group at 12 or above. T o a certain extent such divisions can be made, although the sections of the titration curve tend to merge. It should be noted that proteins can buffer over a wide range of pH, although their buffer action is quite limited in the physiological region of pH, be-tween about 7 and 8, where only the imidazole of histidine is effective —hemoglobin, present in red blood cells, is one of the few protein materials containing a large number of histidine units. Detailed interpretation of a protein titration curve is complicated by several factors. One of these is the possible occurrence of a structural change or denaturation process as the pH is changed, and it is neces-sary to be certain that the titration is reversible before interpreting the results in terms of equilibrium constants. A second important point is that the groups in the protein do not ionize completely independently.
eBook - PDF- Allan Blackman, Steven E. Bottle, Siegbert Schmid, Mauro Mocerino, Uta Wille(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
H H H COO – NH 3 + N N Titration of amino acids Values of pK a for the ionisable groups of amino acids are most commonly obtained by acid–base titration (described in the chapter on acids and bases) and by measuring the pH of the solution as a function of added base (or added acid, depending on how the titration is done). To illustrate this experimental procedure, consider a solution containing 1.00 mol of glycine to which has been added enough strong acid so that both the amino and carboxyl groups are fully protonated. Next, the solution is titrated with 1.00 mol L -1 NaOH; the volume of base added and the pH of the resulting solution are recorded and then plotted as shown in figure 24.9. CHAPTER 24 Amino acids, peptides and proteins 1255 FIGURE 24.9 Titration of glycine with sodium hydroxide H 3 NCH 2 COOH H 2 NCH 2 COO – H 3 NCH 2 COO – and H 2 NCH 2 COO – + + H 3 N + CH 2 COO – H 3 NCH 2 COOH and + H 3 NCH 2 COO – + pK a2 = 9.78 pK a1 = 2.35 pI = 6.06 0 1.0 Moles of OH – per mole of amino acid 2.0 pH 14 12 10 8 6 4 2 0 The more acidic group, and the one to react first with added sodium hydroxide, is the carboxyl group. When exactly 0.50 mol of NaOH has been added, the carboxyl group is half neutralised. At this point, the concentration of the zwitterion equals that of the positively charged ion, and the pH of 2.35 equals the pK a value of the carboxyl group (pK a1 ). At pH = pK a1 [H 3 + NCH 2 COOH positive ion ] = [H 3 + NCH 2 COO - zwitterion ] The end point of the first part of the titration is reached when 1.00 mol of NaOH has been added. At this point, the predominant species present is the zwitterion, and the observed pH of the solution is 6.06. The next section of the curve represents titration of the NH 3 + group. When another 0.50 mol of NaOH has been added (bringing the total to 1.50 mol), half of the NH 3 + groups are neutralised and converted to NH 2 .
eBook - PDF- Krish Moorthy(Author)
- 2007(Publication Date)
- CRC Press(Publisher)
Students must try to visualise the pH-titration curve, the progression as NaOH is added, the half- neutralization, the buffering region, the nature of the ionic species, and so forth (see Question 5.16). Finally, in the data book method, tables of values calculated using the Henderson-Hasselbalch equa-tion are provided in data books (e.g., Dawson et al., 1990). These tables provide a list of ratios of acid and base (either as salt or salt to be created) for a range of pH values (of the buffer). It is simply a matter of mixing the two parts together at the appropriate concentration (see Questions 3.21 and 3.22). It is important when following the full-calculation or data book method that the pH of the buffer be checked on a pH meter (or at least with narrow-range pH paper). What if the pH is not exactly the expected value? Minor adjustments can readily be carried out on the pH meter by adding concen-trated acid or base (salt) drop-wise while stirring. Calculating pI of Amino Acids (and Peptides) Organic ionisable groups may become protonated by having protons (positive entities) incorporated into their structures. It is always only one proton that is either added or removed from a group. Hence protonation can render a group + 1 (if it was 0 before) or 0 (if it was −1 before). pI is (the pH) when the molecule as a whole is neutral — that is, when the positive charges and negative charges balance each other. With proteins, this occurrence may involve several groups, but with amino acids it is always one positive against one nega-tive. (See ionic structures of glycine, Figure 5.1.) • • Fundamentals of Biochemical Calculations 64 With simple amino acids (those having only two ionisable groups), we determine the pI by finding the midpoint between the two pK values; we average the two pK values (see Table 5.3).
eBook - PDF- Gary D. Christian, Purnendu K. Dasgupta, Kevin A. Schug(Authors)
- 2020(Publication Date)
- Wiley(Publisher)
Chapter 7 ACID–BASE TITRATIONS KEY THINGS TO LEARN FROM THIS CHAPTER Calculating acid–base titration curves – Strong acids, strong bases (Table 7.1) – Charge balance approach strong acid–strong base, Goal Seek, Solver – Spreadsheet calculations – Weak acids, weak bases (Table 7.2) – Charge balance approach weak acid–strong base, Goal Seek, Solver – Spreadsheet calculations, weak acid–strong base Indicators (key Equations: 7.12, 7.13) Derivative titrations, Goal Seek, Solver, website (7.11) Buffer intensity, buffer capacity Titration of Na 2 CO 3 Titration of polyprotic acids (Table 7.3) – Spreadsheets for deriving titration curves, 7.11 (web- site, Master Polyprotic Acid Titration Using Solver), Problem 66 (website Universal Acid Titrator) Titration of amino acids Kjeldahl analysis of nitrogen-containing compounds, proteins In Chapter 6, we introduced the principles of acid–base equilibria. These are important for the construction and interpretation of titration curves in acid–base titrations. In this chapter, we discuss the various types of acid–base titrations, including the titration of strong acids or bases and of weak acids or bases. The shapes of titration curves obtained are illustrated. Through a description of the theory of indicators, we discuss the selection of a suitable indicator for detecting the completion of a particular titration reaction. The titrations of weak acids or bases with two or more titratable groups and of mixtures of acids or bases are presented. The important Kjeldahl analysis method is described for determining nitrogen in organic and biological samples. What are some uses of acid–base titrations? They are important in a number of industries. They provide precise measurements. While manual titrations are tedious, automated titrators are often used that can perform titrations effortlessly and accu- rately.
eBook - PDF- BIOTOL, B C Currell, R C E Dam-Mieras(Authors)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
2.3.3 Ionisation of amino acid sidechains With a sidechain which can ionise, the titration curve is changed. Sidechain pKa values depend entirely on the particular group. These are listed in Table 2.2. So that you do not Amino acids 29 forget that the sidechain ionisation is in addition to -carboxyl and -amino groups, these are listed as well. You need to know which amino acids can ionise, how their charge changes (is it -XH + ^ -X + H + or -YH ^ -Y+ H + ?) and at roughly what pH this occurs. Precise details of structure are much less important. Amino acid Aspartic Glutamic Histidine Cysteine Lysine Tyrosine Arginine acid acid Sidechain -C H 2 -C O O H (carboxyl) -C H 2 -C H 2 -C O O H (carboxyl) H -C = CH + 1 1 HN NH H (imidazole) -C H 2 -S H (sulphydryl) -(CH 2 ) 4 -+ NH 3 (amino) (phenolic) H -( C H 2 ) 3 -N -C -+ NH 2 -OH NH 2 pKa a-COOH 2.0 2.1 1.8 1.9 2.2 2.2 1.8 pKa a-NH 2 9.9 9.5 9.3 10.7 9.1 9.2 9.0 pKa sidechain 3.9 4.1 6.0 8.4 10.5 10.5 12.5 Table 2.2 pKa values of amino acids with ionising sidechains. Figure 2.10 contains the results of titration of aspartic acid and of lysine. Both are found to have an additional region of buffering and require 3 equivalents of NaOH per equivalent of amino acid. This is because there are 3 groups ionising. Aspartic acid has another ionisation at low pH and twice as much NaOH is required to reach pH 6.0 as for alanine. This is because of the sidechain carboxyl group (the ß-carboxyl). After pH 30 Chapter 2 6.0, the titration is like that of alanine. For lysine, the low pH region is like that of alanine (hence a single ionisation in this region), but there is now an additional ionisation at high pH, corresponding to the ionisation of the sidechain amino group (the -amino). 12 10 8 pH 6 4 2 pKa 3 -lys r '-i I pKa 2 -lys / pKa-Hys / / ^ pKai-asp I pKa2-asp I pKa 3 -asp I I 0.5 1.0 1.5 2.0 2.5 3.0 Equivalents of OH-Figure 2.10 Titration curves for L- aspartic acid (continuous line) and L- lysine (dashed line).
eBook - PDF- Douglas Skoog, Donald West, F. Holler, Stanley Crouch, Douglas Skoog(Authors)
- 2021(Publication Date)
- Cengage Learning EMEA(Publisher)
Addition of formaldehyde removes the amine functional group, however, and leaves the carboxylic acid available for titration with a standard base. For example, with glycine, NH 1 3 CH 2 COO 2 1 CH 2 O S CH 2 C w NCH 2 COOH 1 H 2 O The titration curve for the product is that of a typical carboxylic acid. The isoelectric point of a species is the pH at which no net migration occurs in an electric field. The molecular structure of the glycine zwitterion, NH 3 + CH 2 COO 2 . Glycine is one of the so-called nonessential amino acids; it is nonessential in the sense that it is synthesized in the bodies of mammals and so is not generally essen- tial in the diet. Because of its compact structure, glycine acts as a versatile building block in protein synthesis and in the biosynthesis of hemoglobin. A significant fraction of the collagen—or the fibrous protein constituent of bone, cartilage, tendon, and other connective tissue in the human body—is made up of glycine. Glycine is also an inhibitory neurotransmitter and, as a result, has been suggested as a possible therapeutic agent for diseases of the central nervous system such as multiple sclerosis and epilepsy. Glycine is also used in treating schizophrenia, stroke, and benign prostatic hyperplasia. Exercises in Excel The final exercise in Chapter 8 of Applications of Microsoft® Excel® in Analytical Chemistry, 4th ed., considers the titration of an amphiprotic species, phenylalanine. A spreadsheet is developed to plot the titration curve of this amino acid, and the isoelectric pH is calculated. 13H Composition of Polyprotic Acid Solutions as a Function of pH In Section 12E, we showed how alpha values are useful in visualizing the changes in the concentration of various species that occur in a titration of a monoprotic weak acid. Alpha values provide an excellent way of thinking about the properties Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part.
eBook - ePubFood Protein Chemistry
An Introduction for Food Scientists
- Joe Regenstein(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
The degree of reaction is often different in the free amino acids because of the zwitterion nature of the compound. (Zwitterions do not exist in the middle of the protein because the free α -carboxyl and the free α -amino are lost in formation of the protein.) A detailed listing of some individual chemical reactions of amino acids can be found in Appendix 4-2. ACID–BASE PROPERTIES The isoelectric point of an amino acid is the point at which the amino acid has no net electrical charge. It is an important characteristic for any amino acid, because every amino acid has at least two acid–base (titratable) groups. This definition of an isoelectric point is an operational definition, that is, one that specifies an experiment and a particular result which then constitute the defined term. Clearly, the definition depends on the clarity and specificity of the experimental description and results as well as on the accuracy and precision with which the experiment can be done. (Define the difference between accuracy and precision.) Another important property of an amino acid is the association constant of the individual reactive groups. This is usually expressed as a p K, the negative log of K (the dissociation constant). These numbers change as the solution conditions change. It may be helpful to review a few concepts about acids and bases before beginning the more detailed discussion of isoelectric points and p K values. Our first problem is that of nomenclature. Strictly speaking, —CO 2 – and —NH 2 are bases because each of these groups can accept a hydrogen ion. However, the carboxyl group is usually considered acidic and the amino group basic. This is based on the fact that the p K of the carboxyl group is on the acid side of neutral and the p K of the amino group is on the alkaline side. Each acid–base pair must have an acid and a conjugate base (or a base and conjugate acid)
eBook - PDF- Gary D. Christian, Purnendu K. Dasgupta, Kevin A. Schug(Authors)
- 2013(Publication Date)
- Wiley(Publisher)
Chapter Eight ACID – BASE TITRATIONS Chapter 8 URLs Learning Objectives WHAT ARE SOME OF THE KEY THINGS WE WILL LEARN FROM THIS CHAPTER? ● Calculating acid–base titration curves ● Strong acids, strong bases (Table 8.1), p. 282 ● Charge balance approach strong acid–strong base, Goal Seek, Solver, p. 285 ● Spreadsheet calculations, pp. 283, 285 ● Weak acids, weak bases (Table 8.2), p. 290 ● Charge balance approach weak acid–strong base, Goal Seek, Solver, p. 293 ● Spreadsheet calculations, weak acid–strong base, p. 293 ● Indicators (key equations: 8.12, 8.13), pp. 289 ● Derivative titrations, Goal Seek, Solver, p. 304 , website (8.11) ● Buffer intensity, buffer capacity, p. 307 ● Titration of Na 2 CO 3 , p. 296 ● Titration of polyprotic acids (Table 8.3), p. 300 ● Spreadsheets for deriving titration curves, p. 302, 8.11 (website, Master Polyprotic Acid Titration Using Solver), Problem 52 (website Universal Acid Titrator) ● Titration of amino acids, p. 309 ● Kjeldahl analysis of nitrogen-containing compounds, proteins, p. 310 In Chapter 7, we introduced the principles of acid–base equilibria. These are important for the construction and interpretation of titration curves in acid–base titrations. In this chapter, we discuss the various types of acid–base titrations, including the titration of strong acids or bases and of weak acids or bases. The shapes of titration curves obtained are illustrated. Through a description of the theory of indicators, we discuss the selection of a suitable indicator for detecting the completion of a particular titration reaction. The titrations of weak acids or bases with two or more titratable groups and of mixtures of acids or bases are presented. The important Kjeldahl analysis method is described for determining nitrogen in organic and biological samples. What are some uses of acid–base titrations? They are important in a number of (Courtesy of Metrohm AG.) industries.
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