Histidine is one of the 20 amino acids found in proteins. Shown here is a fully protonated histidine molecule, where the numbers denote the p K a , values of the acidic groups: (a) Show stepwise ionization of histidine in solution. (Hint: The H - ion will first come off from the strongest acid group followed by the next strongest acid group and so on.) (b) A dipolar ion is one in which the species has an equal number of positive and negative charges. Identify the dipolar ion in part (a), (c) The pH at which the dipolar ion predominates is called the isoelectric point, denoted by p I . The isoelectric point is the average of the p K a values leading to and following the formation of the dipolar ion. Calculate the p I of histidine. (d) The histidine group plays an important role in buffering blood (the pH of blood is about 7.4). Which conjugate acid base pair shown in part (a) is responsible for maintaining the pH of blood?
Histidine is one of the 20 amino acids found in proteins. Shown here is a fully protonated histidine molecule, where the numbers denote the p K a , values of the acidic groups: (a) Show stepwise ionization of histidine in solution. (Hint: The H - ion will first come off from the strongest acid group followed by the next strongest acid group and so on.) (b) A dipolar ion is one in which the species has an equal number of positive and negative charges. Identify the dipolar ion in part (a), (c) The pH at which the dipolar ion predominates is called the isoelectric point, denoted by p I . The isoelectric point is the average of the p K a values leading to and following the formation of the dipolar ion. Calculate the p I of histidine. (d) The histidine group plays an important role in buffering blood (the pH of blood is about 7.4). Which conjugate acid base pair shown in part (a) is responsible for maintaining the pH of blood?
Solution Summary: The author explains the stepwise ionization of histidine in solution, and the conjugate acid–base pair involved in buffering of blood.
Histidine is one of the 20 amino acids found in proteins. Shown here is a fully protonated histidine molecule, where the numbers denote the
p
K
a
, values of the acidic groups:
(a) Show stepwise ionization of histidine in solution. (Hint: The
H
-
ion will first come off from the strongest acid group followed by the next strongest acid group and so on.) (b) A dipolar ion is one in which the species has an equal number of positive and negative charges. Identify the dipolar ion in part (a), (c) The pH at which the dipolar ion predominates is called the isoelectric point, denoted by pI. The isoelectric point is the average of the
p
K
a
values leading to and following the formation of the dipolar ion. Calculate the pI of histidine. (d) The histidine group plays an important role in buffering blood (the pH of blood is about 7.4). Which conjugate acid base pair shown in part (a) is responsible for maintaining the pH of blood?
The simplest amino acid is glycine, H2NCH2CO2H. The common feature of amino acids is that they contain the functional groups: an amine group, –NH2, and a carboxylic acid group, –CO2H. An amino acid can function aseither an acid or a base. For glycine, the acid strength of the carboxyl group is about the same as that of acetic acid, CH3CO2H, and the base strength of the amino group is slightly greater than that of ammonia, NH3.(a) Write the Lewis structures of the ions that form when glycine is dissolved in 1 M HCl and in 1 M KOH.(b) Write the Lewis structure of glycine when this amino acid is dissolved in water.
Discuss the justification for this statement: “Although one does not normally regard NH+4 as an acid, it is actually only slightly weaker as an acid than hydrocyanic acid, HCN, in aqueous solution.”
At 250°C, the equilibrium constant for the dissociation of water is 10–11.16. At this temperature, what is the pH of a neutral solution?
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