Amino acid Henderson-Hasselbalch equation

Amino Acids, the Henderson-Hasselbalch Equation, and Isoelectric Points... 

To apply the Henderson-Hasselbalch equation to an amino acid, let s find out what species are present in a 1.00 M solution of alanine at pH = 9.00. According to Table 26.1, protonated alanine [ NCHfCH CC H] has p/Cal =2.34, and neutral zwitteTionlc alanine [+ll3NCH(CH3)C02-] has pK52 = 9.69 ... 

Heat of combustion, 113 Heat of hydrogenation, 186 table of, 187 Heat of reaction, 154 Helicase, DNA replication and, 1106 Hell-Volhard-Zelinskii reaction, 849 amino acid synthesis and. 1025 mechanism of, 849 Heme, biosynthesis of, 966 structure of, 946 Hemiacetal, 717 Hemiketal, 717 Hemithioacetal, 1148 Henderson-Hasselbalch equation,... 

The carboxylic acid groups of amino acids have pAa values in a range from about 1.8 to 2.6 (see Section 13.1). Let us consider a typical carboxylic acid group with pAa 2.0. Using the Henderson-Hasselbalch equation... 

The amino acid histidine contains an imidazole ring. We have just seen that unsubstituted imidazole as a base has p/Ca 7.0. From the Henderson-Hasselbalch equation... 

The imidazole side-chain of histidine has a value of 6.0, making it a weaker base than the unsubstituted imidazole. This reflects the electron-withdrawing inductive effect of the amino group, or, more correctly the ammonium ion, since amino acids at pH values around neutrality exist as doubly charged zwitterionic forms (see Box 4.7). Using the Henderson-Hasselbalch equation, this translates to approximately 9% ionization of the heterocyclic side-chain of histidine at pH 7 (see Box 4.7). In proteins, plCa values for histidine side-chains are estimated to be in range 6-7, so that the level of ionization will, therefore, be somewhere between 9 and 50%, depending upon the protein. 

Ionization State of Amino Acids Each ionizable group of an amino acid can exist in one of two states, charged or neutral. The electric charge on the functional group is determined by the relationship between its pifa and the pH of the solution. This relationship is described by the Henderson-Hasselbalch equation. 

Amino acids in aqueous solution contain weakly acidic a-carboxyl groups and weakly basic a-amino groups. In addition, each of the acidic and basic amino acids contains an ionizable group in its side chain. Thus, both free amino acids and some amino acids combined in pep tide linkages can act as buffers. The quantitative relationship between the concentration of a weak acid (HA) and its conjugate base (A-) is described by the Henderson-Hasselbalch equation. 

Application of the Henderson-Hasselbalch equation The dissociation constant of the carboxyl group of an amino acid is called K1f rather than Ka, because the molecule contains a second titrat-able group. The Henderson-Hasselbalch equation can be used to analyze the dissociation of the carboxyl group of alanine in the same way as described for acetic acid. 

The real importance of the Henderson-Hasselbalch equation, particularly in biochemistry, is that it tells us how the pH affects the percent dissociation of a weak acid. Suppose, for example, that you have a solution containing the amino acid glycine, one of the molecules from which proteins are made, and that the pH of the solution is 2.00 pH units greater than the pKa of glycine ... 

All amino acids contain ionizable groups that act as weak acids or bases, giving off or taking on protons when the pH is altered. As is true of all similar ionizations, these ionizations follow the Henderson-Hasselbalch equation ... 

Cells in the body and bacteria in the gut release the nitrogen of certain amino acids as ammonia or ammonium ion (NILj ) (Fig. 38.5). Because these two forms of nitrogen can be interconverted, the terms are sometimes used interchangeably. Ammonium ion releases a proton to form ammonia by a reaction with a of 9.3 (Fig. 38.6). Therefore, at physiologic pH, the equilibrium favors NH4 by a factor of approximately 100/1 (see Chapter 4, the Henderson-Hasselbalch equation). However, it is important to note that NH3 is also present in the body, because this is the form that can cross cell membranes. For example, NH3 passes into the urine from kidney tubule cells and decreases the acidity of the urine by binding protons, forming NILj. Once the NILj is formed, the compound can no longer freely diffuse across membranes. 

For every amino acid, there is a specific pH value at which it exhibits no net charge. This is called the isoelectric point, pi. At its isoelectric point, an amino acid remains stationary in an applied electric field, i.e. it does not move to the positive or negative pole. The isoelectric point can be estimated via the Henderson-Hasselbalch equation ... 

At neutral pH, amino acids exist as dipolar ions. Using glycine as an example, and given that the p Ta of the carboxyl group is 2.3 and that of the ammonium group is 9.6, predict the predominant form of the molecule at pH 1,7, and 12. Justify your arrswers using the Henderson-Hasselbalch equation (Equation 12.2). 

Noting that the amino acids do have acidic properties, it is of interest to compare these with typical organic acids and bases. Remembering that the pXa of a dissociable function is the pH at which it is half-ionized (see any biochemistry text for the mathematical expression that relates pH and pK, the Henderson-Hasselbalch equation), the pK values of any compound may serve as index of that compound s acidity. 

I Amino Acids and the Henderson-Hasselbalch Equation Isoelectric Points 1051... 

---