ACID–BASE PROPERTIES OF AMINO

ACIDS

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D-AMINO ACIDS AND L-AMINO ACIDS

Every amino acid (except glycine) can occur in two isomeric forms, because of the possibility of forming two different enantiomers (stereoisomers) around the central carbon atom. By convention, these are called L- and D- forms, analogous to left-handed and right-handed configurations.

Only L-amino acids are manufactured in cells and incorporated into proteins. Some D-amino acids are found in the cell walls of bacteria, but not in bacterial proteins.

An amino acid is a molecule that contains two functional groups, a basic amine group and an acidic carboxylic acid group.

AMPHOTERIC COMPOUNDS Amino acids are amphoteric (or

amphiprotic); they can react either as an acid or as a base.

An amino acid can have several forms depending on the pH of the system. At low ph or acid conditions, the amino group (-NH2) is protonated by the addition of a proton (H+) from the acid.

At high pH or basic conditions, the carboyxlic acid (-COOH) is deprotonated by the removal of a proton.

ZWITTERIONSThere is an internal transfer of a

hydrogen ion from the -COOH group to the -NH2 group to leave an ion with both a negative charge and a positive charge.

A zwitterion is a compound with no overall electrical charge, but which contains separate parts which are positively and negatively charged.

Zwitterion– dipolar ions In an aqueous solution at a

certain compound-specific pH, this structure may change so that a proton from the COOH, carboxylic acid group, transfers to the NH2, amino group, leaving an ion with both a negative charge and a positive charge.

The transfer of electrons or ions results in a net neutral charge because the number of protonated ammonium groups with a positive charge and deprotonated carboxylate groups with a negative charge are equal.

In the zwitterion form, function groups have different charges yet doesn’t have a net charge

When can we say that an amino acid is an acid or a

base?

BASIC Α-AMINO ACID

ACIDIC Α-AMINO ACID

ISOELECTRIC POINT (PI) The pH at which essentially all amino acid is

in the zwitterions form, with very low and equal concentrations of the positive and negative ions.

In more acidic media (pH<pI), the concentration of ions increases while the concentration of the zwitterion decreases.

In a more basic (pH>pI), the concentration of negative ions increases while the concentration of the zwitterion decreases.

More than 98% of the amino acid is in zwitterion from over a pH range of +,- 2 of pI.

At pH=pI, the amino acid present as the zwitterion with one amine or carboxyl group inuncharged form.

Isoelectric points found at the values ranging from 7.8 to 10.8 (basic)

Isoelectric points found at the values ranging from 4.8 6 to 6. 3 (neutral)

Isoelectric points found at the values ranging from 2.8 to 3.3 (acidic)

ISOELECTRIC POINTS OF SOME AMINO ACIDS

Amino Acid Isoelectric point (pI)Arganine (Arg) 10.8

Lysine (Lys) 9.7

Alanine (Ala) 6.0

Glycine (Gly) 6.0

Serine (Ser) 5.7

Glutamic acid (Glu) 3.2

Aspartic acid (Asp) 2.9

ELECTROPHORESIS An analytical method for identifying amino acids

by observing their migration as a function of pH under an applied electric field gradient.

At its pI, the amino acid is present in the zwitterion form with no net charge and will not migrate in electrophoresis.

At pH < pI, the amino acid carries a positive charge and will migrate to the negative electrode.

At pH,pI, the amino acid carries a negative charge and will migrate to the positive electrode .

A mixture of amino acids can be separated by electrophoresis on the basis of their pI values

Ninhydrin is used to detect the individual amino acids

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