4.9.5 Acid–base properties of amino acids
The neutral forms of amino acids are zwitterions. This is why amino acids
are insoluble in apolar aprotic solvents, e.g. ether, but most nonprotonated
amines and unionized carboxylic acids dissolve in ether. For the same
reason, amino acids usually have high melting points, e.g. the m.p. of
glycine is 262
C, and large dipole moments. The high melting points and
greater water solubility than in ether are saltlike characteristics, not the
characteristics of uncharged organic molecules. This saltlike characteristic
is found in all zwitterionic compounds. Water is the best solvent for most
amino acids because it solvates ionic groups much as it solvates the ions of a
salt. A large dipole moment is characteristic of zwitterionic compounds that
contain great deal of separated charge. The pK a values for amino acids are
also typical of zwitterionic forms of neutral molecules. Peptides can also
exist as zwitterions; i.e., at pH values near 7, amino groups are protonated
and carboxylic acid groups are ionized.
4.9.6 Isoelectric points of amino acids and peptides
Isoelectric point (pI) or isoelectric pH is the pH at which a molecule carries
no net electrical charge, i.e. zero charge. It is an important measure of the
acidity or basicity of an amino acid. To have a sharp isoelectric point, a
molecule must be amphoteric, i.e. it must have both acidic and basic
functional groups, as found in amino acids. For an amino acid with only
Dietary proteins
Digestion
Amino acids in blood
Membrane
Amino acids
Proteins
N-containing
compounds
Nitrogen
Carbon
CO 2 + H 2 O
Urea and other
nitrogenous products
Excreted in the urine
4.9 AMINO ACIDS AND PEPTIDES
183
The neutral forms of amino acids are zwitterions. This is why amino acids
are insoluble in apolar aprotic solvents, e.g. ether, but most nonprotonated
amines and unionized carboxylic acids dissolve in ether. For the same
reason, amino acids usually have high melting points, e.g. the m.p. of
glycine is 262
C, and large dipole moments. The high melting points and
greater water solubility than in ether are saltlike characteristics, not the
characteristics of uncharged organic molecules. This saltlike characteristic
is found in all zwitterionic compounds. Water is the best solvent for most
amino acids because it solvates ionic groups much as it solvates the ions of a
salt. A large dipole moment is characteristic of zwitterionic compounds that
contain great deal of separated charge. The pK a values for amino acids are
also typical of zwitterionic forms of neutral molecules. Peptides can also
exist as zwitterions; i.e., at pH values near 7, amino groups are protonated
and carboxylic acid groups are ionized.
4.9.6 Isoelectric points of amino acids and peptides
Isoelectric point (pI) or isoelectric pH is the pH at which a molecule carries
no net electrical charge, i.e. zero charge. It is an important measure of the
acidity or basicity of an amino acid. To have a sharp isoelectric point, a
molecule must be amphoteric, i.e. it must have both acidic and basic
functional groups, as found in amino acids. For an amino acid with only
Dietary proteins
Digestion
Amino acids in blood
Membrane
Amino acids
Proteins
N-containing
compounds
Nitrogen
Carbon
CO 2 + H 2 O
Urea and other
nitrogenous products
Excreted in the urine
4.9 AMINO ACIDS AND PEPTIDES
183