Figure The titration curve of 0. The ionic species predominating at key points in the titration are shown above the graph. The pH at the midpoint of this stage is 9. From the titration curve of glycine we can derive several important pieces of information.
First, it gives a quantitative measure of the pKa of each of the two ionizing groups, 2. Note that the carboxyl group of glycine is over times more acidic more easily ionized than the carboxyl group of acetic acid, which has a pK a , of 4. This is due to a stabilizing interaction between opposite charges on the zwitterion and a repulsive interaction between the positive charges of the amino group and the departing proton.
The second piece of information given by the titration curve of glycine Fig. One of these is the relatively flat portion of the curve centered about the first pK a of 2.
Note also that glycine is not a good buffer at the pH of intracellular fluid or blood, about 7. The Henderson-Hasselbalch equation Chapter 4 can be used to calculate the proportions of proton-donor and proton-acceptor species of glycine required to make a buffer at a given pH within the buffering ranges of glycine; it also makes it possible to solve other kinds of buffer problems involving amino acids see Box Another important piece of information derived from the titration curve of an amino acid is the relationship between its net electric charge and the pH of the solution.
At pH 5. For an amino acid such as glycine, which has no ionizable group in the side chain, the isoelectric point is the arithmetic mean of the two pK a values:. As is evident in Figure , glycine has a net negative charge at any pH above its pI and will thus move toward the positive electrode the anode when placed in an electric field. At any pH below its pI, glycine has a net positive charge and will move toward the negative electrode, the cathode. The farther the pH of a glycine solution is from its isoelectric point, the greater the net electric charge of the population of glycine molecules.
At pH 1. At pH 2. The sign and the magnitude of the net charge of any amino acid at any pH can be predicted in the same way. This information has practical importance. For a solution containing a mixture of amino acids, the different amino acids can be separated on the basis of the direction and relative rate of their migration when placed in an electric field at a known pH. This group of amino acids is characterized by having very similar, although not identical, values for pKl the pK of the -COOH group in the range of 1.
Amino acids with an ionizable R group Table have more complex titration curves with three stages corresponding to the three possible ionization steps; thus they have three pK a values. The third stage for the titration of the ionizable R group merges to some extent with the others. The titration curves of two representatives of this group, glutamate and histidine, are shown in Figure The isoelectric points of amino acids in this class reflect the type of ionizing R groups present.
For example, glutamate has a pI of 3. This is a result of the presence of two carboxyl groups which, at the average of their pK a values 3. Similarly, the pI of histidine, with two groups that are positively charged when protonated, is 7.
Most of these amino acids differ only in the nature of the R substituent. The standard amino acids are therefore classified on the basis of these R groups. Amino acids with nonpolar substituents are said to be hydrophobic water-hating. Amino acids with polar R groups that form hydrogen bonds to water are classified as hydrophilic water-loving. The remaining amino acids have substituents that carry either negative or positive charges in aqueous solution at neutral pH and are therefore strongly hydrophilic.
Click here to check your answer to Practice Problem 1. With the exception of glycine, the common amino acids all contain at least one chiral carbon atom. These amino acids therefore exist as pairs of stereoisomers. The structures of the D and L isomers of alanine are shown in the figure below.
Although D amino acids can be found in nature, only the L isomers are used to form proteins. The D isomers are most often found attached to the cell walls of bacteria and in antibiotics that attack bacteria.
The presence of these D isomers protects the bacteria from enzymes the host organism uses to protect itself from bacterial infection by hydrolyzing the proteins in the bacterial cell wall. A few biologically important derivatives of the standard amino acids are shown in the figure below.
Anyone who has used an "anti-histamine" to alleviate the symptoms of exposure to an allergen can appreciate the role that histamine a decarboxylated derivative of histidine plays in mediating the body's response to allergic reactions.
This compound received notoriety a few years ago in the film Awakening , which documented it's use as a treatment for other neurological disorders.
Thyroxine, which is an iodinated ether of tyrosine, is a hormone that acts on the thyroid gland to stimulate the rate of metabolism. Acetic acid and ammonia often play an important role in the discussion of the chemistry of acids and bases. One of these compounds is a weak acid; the other is a weak base.
The zwitterion is the dominant species in aqueous solutions at physiological pH pH 7. The zwitterion can undergo acid-base reactions, howeer, if we add either a strong acid or a strong base to the solution. How is vsepr used to classify molecules? What are the units used for the ideal gas law? How does Charle's law relate to breathing? What is the ideal gas law constant? How do you calculate the ideal gas law constant? How do you find density in the ideal gas law? Does ideal gas law apply to liquids?
Impact of this question views around the world.
0コメント