AMINO ACIDS
Proteins

Amino Acid Molecular Structures
 Elmhurst College
Amino Acid Structures Reactions  Chemistry Department
Amino Acid Review Peptide Bonds  Virtual ChemBook

Review of Characteristics and Properties of Amino Acids
 

Structures of Amino Acids

 Review Polar and Non-polar

 Review of Acid/Base Properties

Amino
Acid
Name
(graphic)

Chime*

 A
b
r
e
v.

 A
b
r
e
v.

 

Structure
of R group (red)

For each answer box, look at structure to determine if:

Polar or Non-Polar

For each answer box, look at structure to determine if:

Acidic, Basic, or Neutral 

Alanine

ala

 A

 Arginine

arg

 R

 

 

 Asparagine

asn

 N

 

 

Aspartic
Acid

asp

 D

 

Cysteine

cys

 C

Slightly

 

Glutamic
Acid

glu

 E

 

 Glutamine

  gln

 Q

 

 Glycine 

gly

 G

 
 Histidine  his  H

 

 
 Isoleucine

 ile

 I

 
 Leucine  leu  L

 
 Lysine  lys  K

 
 Methionine  met  M

 

 Phenyl-
alanine
 phe  F

 
 Proline  pro  P

 
 Serine  ser  S

 
 Threonine  thr  T

 

 Trypto-
phan
 trp  W

 

Slightly

 
 Tyrosine  tyr  Y

 

 
 Valine  Val  V

 

Review of
Characteristics and Properties of Amino Acids

 Principles of Polarity:

The greater the electronegativity difference between atoms in a bond, the more polar the bond. Partial negative charges are found on the most electronegative atoms, the others are partially positive. Review the polarity of functional groups.

Non-Polar Side Chains:

Side chains which have pure hydrocarbon alkyl groups (alkane branches) or aromatic (benzene rings) are non-polar. Examples include valine, alanine, leucine, isoleucine, phenylalanine.

The number of alkyl groups also influences the polarity. The more alkyl groups present, the more non-polar the amino acid will be. This effect makes valine more non-polar than alanine; leucine is more non-polar than valine.

Polar Side Chains:

Side chains which have various functional groups such as acids, amides, alcohols, and amines will impart a more polar character to the amino acid. The ranking of polarity will depend on the relative ranking of polarity for various functional groups as determined in functional groups. In addition, the number of carbon-hydrogens in the alkane or aromatic portion of the side chain should be considered along with the functional group.

Example: Aspartic acid is more polar than serine because an acid functional group is more polar than an alcohol group.

Example: Serine is more polar than threonine since threonine has one more methyl group than serine. The methyl group gives a little more non-polar character to threonine.

Example: Serine is more polar than tyrosine, since tyrosine has the hydrocarbon benzene ring.

 Acid - Base Properties of Amino Acids:

Acidic Side Chains:

If the side chain contains an acid functional group, the whole amino acid produces an acidic solution. Normally, an amino acid produces a nearly neutral solution since the acid group and the basic amine group on the root amino acid neutralize each other in the zwitterion. If the amino acid structure contains two acid groups and one amine group, there is a net acid producing effect. The two acidic amino acids are aspartic and glutamic.

Basic Side Chains:

If the side chain contains an amine functional group, the amino acid produces a basic solution because the extra amine group is not neutralized by the acid group. Amino acids which have basic side chains include: lysine, arginine, and histidine.

Amino acids with an amide on the side chain do not produce basic solutions i.e. asparagine and glutamine.

Neutral Side Chains:

Since an amino acid has both an amine and acid group which have been neutralized in the zwitterion, the amino acid is neutral unless there is an extra acid or base on the side chain. If neither is present then then the whole amino acid is neutral.

Amino acids with an amide on the side chain do not produce basic solutions i.e. asparagine and glutamine. You need to look at the functional groups carefully because an amide starts out looking like an amine, but has the carbon double bond oxygen which changes the property. Amides are not basic.

Even though tryptophan has an amine group as part of a five member ring, the electron withdrawing effects of the two ring systems do not allow nitrogen to act as a base by attracting hydrogen ions.