Acid-base titrations amino acids

Many pharmaceutical compounds are weak acids or bases that can be analyzed by an aqueous or nonaqueous acid-base titration examples include salicylic acid, phenobarbital, caffeine, and sulfanilamide. Amino acids and proteins can be analyzed in glacial acetic acid, using HCIO4 as the titrant. For example, a procedure for determining the amount of nutritionally available protein has been developed that is based on an acid-base titration of lysine residues. ... 

It had been found previously [106] that cell adhesion on the surface of HA copolymers decreased with increasing ambient pH. With polyHEMA, on the other hand, no pH-dependency was observed. We considered the adhesion to be caused primarily by ionic interaction between the lymphocytes and the HA surfaces. The degree of protonation (a) of amino groups in HA copolymer was estimated by acid-base titration of an HCI solution of polyamine macromonomer with NaOH solution. In physiological conditions (pH 7.2-7.4) about 50% of the amino groups of the macromonomer are protonated. In this pH range, the polyamine macromonomer was found to be insoluble. 

As a final example of an acid-base titration, let s consider the gradual addition of NaOH to the protonated form of the amino acid alanine (H2A+), a substance that acts as a diprotic acid. Amino acids (which are discussed in more detail in Chapter 24) are both acidic and basic and can be protonated by strong acids such as HC1, yielding salts such as H2A+C1 . The protonated form of the amino acid has two... 

Comparison of acid-base titrations of native and apohemocyanin has implicated histidine as part of the coordination sphere of the coppers70), but any other coordinated amino acids are not known. 

Since they are amphoteric, these substances can be titrated with either a strong acid or a strong base. Many amino acids are too weak to be titrated in aqueous solutions, but some will give adequate end points, especially if a pH meter is used to construct a titration curve. 

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... 

When the functionalized polymer contains acidic or basic groups, such as those in ion-exchange resins, the acid-base titration methods are the most suitable for functional group determination. Such methods are well-established (Helfferich, 1962), In one report (Schou et aL, 1975), the free amino group in an unblocked resin-peptide was determined by nonaque-ous titration with acid. Similarly, histidine incorporation could be deter-... 

FIGURE 4.6 The ionic forms of the amino acids, shown without consideration of any ionizations on the side chain. The cationic form is the low pH form, and the titration of the cationic species with base yields the zwitterion and finally the anionic form. (Irving Geis)... 

C18-0103. When a solution of leucine (an amino acid) in water is titrated with strong base, the pH before titration is 1.85, the pH at the midpoint of the titration is 2.36, and the pH at the stoichiometric point is 6.00. Determine the value of K. for leucine. 

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... 

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

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. ... 

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. 

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. 

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... 

The molecular weight of 320,000 obtained for the muscle enzyme from sedimentation-diffusion data at 2-6 mg/ml and v = 0.75 (132) is to be compared with 270,000 obtained by Wolfenden et al. from s20,w = 11.1 S and D2 ,w = 3.75 X 10 7 cm2 sec1, and v = 0.731 calculated from the amino acid content (92). The rabbit muscle enzyme has a normal amino acid content, that is, no unusually low or large amount of a particular amino acid was found. Of the 32 cysteine/half-cystine residues per mole based on a molecular weight of 270,000, 6.2 were rapidly titrated with p-mercuribenzoate (92). Typical protein absorption spectra were reported for elasmobranch fish (126), carp (125), rat (127), and rabbit muscle enzyme (68). An E m at 280 nm = 9.13 has been reported for the rabbit muscle enzyme (133). The atypical absorption spectrum with a maximum at 275-276 nm observed by Lee (132) is indicative of contaminating bound nucleotides. 

Consider the titration of 30.0 mL of a 0.0600 M solution of the protonated form of the amino acid methionine (H2A+) with 0.0900 M NaOH. Calculate the pH after addition of 20.0 mL of base. 

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