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3 Acid–Base Behavior of Amino Acids

3 Acid–Base Behavior of Amino Acids

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650



AMINO ACIDS, PROTEINS, AND ENZYMES



H

+



ammonium cation



carboxylate anion



COO−



C



H3N



CH3

alanine

no net charge

A

pH ≈ 6



When strong acid is added to lower the pH to 2 or less, the carboxylate anion gains a proton and

the amino acid has a net positive charge (form B).

The carboxylate anion picks up a proton.

Adding acid:

H

+



H3N



C



H

H+



COO−



+



H3N



CH3



C



pH < 2



COOH



CH3

overall +1 charge

B



A



When strong base is added to A to raise the pH to 10 or higher, the ammonium cation loses a

proton and the amino acid has a net negative charge (form C).

The ammonium cation loses a proton.

Adding base:

H

+



H3N



C



H



−OH



COO−



H2N



C



COO−



+



H2O



pH > 10



CH3



CH3



overall −1 charge

C



A



Thus, alanine exists in one of three different forms depending on the pH of the solution in

which it is dissolved. At the physiological pH of 7.4, neutral amino acids are primarily in their

zwitterionic forms.

• The pH at which the amino acid exists primarily in its neutral form is called its isoelectric

point, abbreviated as p I.



The isoelectric points of neutral amino acids are generally around 6. Acidic amino acids (Table

21.2), which have an additional carboxyl group that can lose a proton, have lower pI values

(around 3). The three basic amino acids, which have an additional basic nitrogen atom that can

accept a proton, have higher pI values (7.6–10.8).



SAMPLE PROBLEM 21.2

ANALYSIS



SOLUTION



Draw the structure of the amino acid glycine at each pH: (a) 6; (b) 2; (c) 11.

A neutral amino acid exists in its zwitterionic form (no net charge) at its isoelectric point, which

is pH ≈ 6. The zwitterionic forms of neutral amino acids appear in Table 21.2. At low pH (≤ 2),

the carboxylate anion is protonated and the amino acid has a net positive (+1) charge. At high pH

(≥ 10), the ammonium cation loses a proton and the amino acid has a net negative (–1) charge.

a.



At pH = 6, the neutral,

zwitterionic form of glycine

predominates.



H3N



smi26573_ch21.indd 650



C



At pH = 2, glycine has a net

+1 charge.



c.



At pH = 11, glycine has a net

−1 charge.



H



H

+



b.



COO−



+



H3N



C



H

COOH



H2N



C



COO−



H



H



H



neutral

pH = 6



+1 charge

pH = 2



−1 charge

pH = 11



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PEPTIDES



651



PROBLEM 21.6



Draw the structure of the amino acid valine at each pH: (a) 6; (b) 2; (c) 11. Which form

predominates at valine’s isoelectric point?



PROBLEM 21.7



Draw the positively charged, neutral, and negatively charged forms for the amino acid

phenylalanine. Which species predominates at pH 11? Which species predominates at pH 1?



21.4 PEPTIDES

When amino acids are joined together by amide bonds, they form larger molecules called peptides and proteins.

• A dipeptide has two amino acids joined together by one amide bond.

• A tripeptide has three amino acids joined together by two amide bonds.

O

+



H3N



CH



C



O

N

H



R1



CH



O

+



O−



C



H3N



R2



CH



C



O

N

H



CH



R1



C



O

N

H



R2



CH



C



O−



R3



tripeptide



dipeptide

[Amide bonds are shown in red.]



Polypeptides and proteins both have many amino acids joined together in long linear chains, but

the term protein is usually reserved for polymers of more than 40 amino acids.

• The amide bonds in peptides and proteins are called peptide bonds.

• The individual amino acids are called amino acid residues.



To form a dipeptide, the –NH3+ group of one amino acid forms an amide bond with the carboxylate (–COO–) of another amino acid, and the elements of H2O are removed. Because

each amino acid has both functional groups, two different dipeptides can be formed. This is

illustrated with alanine (Ala) and serine (Ser) to form dipeptides A and B.

1. The –COO– group of alanine can combine with the –NH3+ group of serine.

new amide bond

O

+



H3N



CH



C



O



H

O−



+



H



CH3



+



N



CH



C



H



CH2OH



O

O−



+



H3N



CH



C



N

H



CH3



CH



C



O−



+



H2O



+



H2O



CH2OH

A



Ser



Ala



O



reacting functional groups



2. The –COO– group of serine can combine with the –NH3+ group of alanine.

new amide bond

O

+



H3N



CH



C



CH2OH



O



H

O−



+



H



+



N



CH



H



CH3



Ser



Ala



C



O

O−



+



H3N



CH



C



O

N

H



CH



C



O−



CH3



CH2OH

B



reacting functional groups



smi26573_ch21.indd 651



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652



AMINO ACIDS, PROTEINS, AND ENZYMES



Dipeptides A and B are constitutional isomers of each other. Both have an ammonium cation

(–NH3+) at one end of their chains and a carboxylate anion (–COO–) at the other.

• The amino acid with the free –NH3+ group on the 𝛂 carbon is called the N-terminal

amino acid.

• The amino acid with the free –COO– group on the 𝛂 carbon is called the C-terminal

amino acid.



By convention, the N-terminal amino acid is always written at the left end of the chain and

the C-terminal amino acid at the right.

A



B



O

+



H3N



CH



C



CH3



O

N



CH



H



CH2OH



N-terminal

amino acid



O

O−



C



C-terminal

amino acid



+



H 3N



CH



O



C



N



CH



CH2OH



H



CH3



N-terminal

amino acid



alanylserine

Ala–Ser



O−



C



Alanine is drawn in blue.

Serine is drawn in red.



C-terminal

amino acid



serylalanine

Ser–Ala



Peptides are named as derivatives of the C-terminal amino acid. To name a peptide:

• Name the C-terminal amino acid using the names in Table 21.2.

• Name all other amino acids from left to right as substituents of the C-terminal amino

acid. Change the -ine or -ic acid ending of the amino acid name to the suffix -yl.



Thus, peptide A, which has serine as its C-terminal amino acid, is named as alanylserine. Peptide

B, which has alanine as its C-terminal amino acid, is named as serylalanine.

The peptide can be abbreviated by writing the one- or three-letter symbols for the amino acids in

the chain from the N-terminal to the C-terminal end. Thus, Ala–Ser has alanine at the N-terminal

end and serine at the C-terminal end, whereas Ser–Ala has serine at the N-terminal end and alanine

at the C-terminal end.



PROBLEM 21.8



Identify the N-terminal and C-terminal amino acid in each peptide.

O



a.



+



H3N



CH



C



CH2



O

N



CH



H



CH3



C



O



O

O−



c.



+



H3N



CH

CH2



C



N



CH



H



CH3



C



O

C



O−



N



CH



H



CH2CH2CONH2



CH(CH3)2



b.



PROBLEM 21.9



smi26573_ch21.indd 652



Arg–His–Asn–Tyr



d.



Val–Thr–Pro–Phe



(a) Identify the N-terminal amino acid in the tetrapeptide alanylglycylleucylmethionine.

(b) What is the C-terminal amino acid? (c) Write the peptide using three-letter symbols for

the amino acids.



12/16/08 2:03:35 PM



PEPTIDES



653



Drawing a dipeptide from component amino acids is illustrated in the stepwise How To procedure. Sample Problem 21.3 illustrates how to identify and name the component amino acids from

a given tripeptide.



HOW TO

EXAMPLE

Step [1]



Draw a Dipeptide from Two Amino Acids

Draw the structure of the dipeptide Val–Gly, and label the N-terminal and C-terminal amino acids.

Draw the structures of the individual amino acids from left to right.

• Use the three-letter symbols to identify the amino acids in the peptide. In this example, Val is valine and Gly is

glycine.

• Draw the structures of the amino acids by placing the –COO– of one amino acid next to the –NH3+ group of the

adjacent amino acid.

• Always draw the –NH3+ group on the left and the –COO– group on the right.

Place these two groups next to each other.

O

+



Draw valine on the left

since its symbol appears

first in the structure.



CH



H3N



O

O−



C



+



+



CH



H3N



CH(CH3)2



O−



H



Val



Step [2]



C



Gly



Join the adjacent –COO– and –NH3+ groups together.

• Form a new amide bond by joining the carbonyl carbon of one amino acid to the N atom of the second amino

acid.



O

+



H3 N



CH



C



CH(CH3)2

Val



N-terminal

amino acid



O

O−



+



+



H3N



CH



C



+



O−



H3N



H



new amide bond

O

CH



O



C



CH(CH3)2



Gly



N



CH



H



H



C



O−

C-terminal

amino acid



Val–Gly



• Valine is the N-terminal amino acid since it contains a free –NH3+ group on the α carbon.

• Glycine is the C-terminal amino acid since it contains a free –COO– group on the α carbon.



PROBLEM 21.10



Draw the structure of each dipeptide: (a) Gly–Phe; (b) Gln–Ile; (c) Leu–Cys.



PROBLEM 21.11



(a) Draw the structures of the two possible dipeptides that can be formed by combining leucine

and asparagine. (b) In each dipeptide label the N- and C-terminal amino acids. (c) Name each

peptide using three-letter abbreviations.



SAMPLE PROBLEM 21.3



Identify the individual amino acids used to form the following tripeptide. What is the name of

the tripeptide?

O

+



H3N



CH



C



CH2



O



O

N



CH



H



CH3



C



N

H



CH

CH2



C



O−

OH



CH(CH3)2



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12/16/08 2:03:35 PM



654



AMINO ACIDS, PROTEINS, AND ENZYMES



ANALYSIS



• Locate the amide bonds in the peptide. To draw the structures of the individual amino

acids, break the amide bonds by adding water. Add O– to the carbonyl carbon to form a

carboxylate anion –COO– and 2 H’s to the N atom to form –NH3+.

• Identify the amino acids by comparing with Table 21.2.

• To name the peptide: [1] Name the C-terminal amino acid. [2] Name the other amino acids

as substituents by changing the -ine (or -ic acid) ending to the suffix -yl. Place the names of

the substituent amino acids in order from left to right.



SOLUTION



To determine the amino acids that form the peptide, work backwards. Break the amide bonds in

red that join the amino acids together. This forms leucine, alanine, and tyrosine.

O

+



H3N



CH



O



O



C



CH2



N



CH



H



CH3



C



N

H



CH



O−



C



OH



CH2



CH(CH3)2

O

+



H3N



CH



O

+



O−



C



H3N



CH2



CH



C



O

+



O−



H3N



CH3



C



CH



C-terminal amino acid

O−

OH



CH2



CH(CH3)2

leucine



alanine



tyrosine



The tripeptide is named as a derivative of the C-terminal amino acid, tyrosine, with leucine and

alanine as substituents; thus, the tripeptide is named: leucylalanyltyrosine.



PROBLEM 21.12



Identify the individual amino acids in each dipeptide, and then name the dipeptide using threeletter abbreviations.

O



O



a.



+



H3N



CH

CH3



C



C



O



O

O−



N



CH



H



CH(CH3)CH2CH3



b.



+



H3N



CH

CH2



C



N



CH



C



H



CH(CH3)2



O−



OH



21.5 FOCUS ON THE HUMAN BODY

BIOLOGICALLY ACTIVE PEPTIDES

Many relatively simple peptides have important biological functions.



21.5A NEUROPEPTIDES—ENKEPHALINS AND PAIN RELIEF

Enkephalins, pentapeptides synthesized in the brain, act as pain killers and sedatives by binding

to pain receptors. Two enkephalins that differ in the identity of only one amino acid are known.

Met-enkephalin contains a C-terminal methionine residue, while leu-enkephalin contains a

C-terminal leucine.



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12/16/08 2:03:36 PM



FOCUS ON THE HUMAN BODY: BIOLOGICALLY ACTIVE PEPTIDES



O



O

+



H3N



CH



C



CH2



N



CH



H



H



C



CH



H



H



C



O



O



O

N



655



N



CH



H



CH2



C



C



O−



N



CH



H



CH2CH2SCH3



=



Tyr–Gly–Gly–Phe–Met



OH



met-enkephalin



O



O

+



H3N



CH



C



CH2



OH



N



CH



H



H



C



O

N



CH



H



H



C



O



O

N



CH



H



CH2



C



O−



N



CH



H



CH2CH(CH3)2



C



=



Tyr–Gly–Gly–Phe–Leu

leu-enkephalin



The addictive narcotic analgesics morphine and heroin (Section 18.5) bind to the same receptors

as the enkephalins, and thus produce a similar physiological response. Enkephalins, however,

produce only short-term pain-relieving effects because their peptide bonds are readily hydrolyzed

by enzymes in the brain. Morphine and heroin, on the other hand, are not readily hydrolyzed, so

they are biologically active for a much longer time period.

Enkephalins are related to a group of larger polypeptides called endorphins that contain 16–31

amino acids. Endorphins also block pain and are thought to produce the feeling of well-being

experienced by an athlete after excessive or strenuous exercise.



PROBLEM 21.13



(a) Label the four amide bonds in met-enkephalin. (b) What N-terminal amino acid is present in

both enkephalins? (c) How many chirality centers are present in met-enkephalin?



21.5B PEPTIDE HORMONES—OXYTOCIN AND VASOPRESSIN

Oxytocin and vasopressin are cyclic nonapeptide hormones secreted by the pituitary gland.

Their sequences are identical except for two amino acids, yet this is enough to give them very

different biological activities.

N-terminal amino acid

Cys



Tyr



Ile



S

disulfide

bond



Cys



Gln



S



Asn



S

disulfide

bond



Cys

Pro

oxytocin



smi26573_ch21.indd 655



Tyr



Phe

Gln



S



Asn

Cys



Leu



GlyNH2



Pro



Arg



GlyNH2



vasopressin



12/16/08 2:03:36 PM



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