Amino acids as amphoteric compounds

• Acidity

• Basicity

• pKa

• Electronic and structural features that influence acidity and basicity

General Structure of Amino Acid• Building blocks of proteins

• Carboxylic acid group• Amino group• Side group R gives unique characteristics

Amino acids are polar

• Due to presence – polar covalent bonds– N, O and H atoms - are capable to form

hydrogen bonds with water– Carry charges COO- and NH3

+

The water solubility of amino acids vary to some extend, depending of side chain

Carries positive charge when pH<6

Learning Check• Classify the following amino acids as hydrophobic

(nonpolar), hydrophilic (polar, neutral), acidic, or basic:

A. Lysine (polar basic)

B. Leucine (nonpolar)

C. Serine (polar neutral) D. Aspartate (polar acidic)

The structure is dependent on pH – due to presence -COOH and -NH2

Can act as acid (proton donor) and base (proton acceptor)

• R – COOH R – COO- + H+

• R – NH3+ R – NH2 + H+

pKa of –COOH [1.8-4.3], therefore at pH 7 is COO-

pKa of –NH2 [9.1-12.5], therefore at pH 7 is NH3+

conjugate base

conjugate acid

acid

base

Zwitterion• At a particular pH, the amino acid carries no

net charge and is called a zwitterion.

• Zwitterion …. dipolar ion – has 1 positive and 1 negative charge

• Amphoteric (ampholytes)

• pH, at which the amino acid has a net charge of zero is called the isoelectric point (pI),

• At the isoelectric point (pI), the + and – charges are equal.

pH and ionization (1)

H+ OH–

+ H3N–CH2–COOH + H3N–CH2–COO– H2N–CH2–COO–

Positive ion zwitterion Negative ion

Low pH neutral pH High pH

In solutions more basic than the pI, the —NH3+ in the amino acid donates a proton and become (-NH2) . In solution more acidic than the pI, the COO- in the amino acid accepts a proton and become (-COOH).

By rearranging the above equation we arrive at the Henderson-Hasselbalch equation:

pH = pKa + log[A-]/[HA]

At the point of the dissociation where the concentration of the conjugate base [A-] = to that of the acid [HA]:

pH = pKa + log[1]

The log of 1 = 0. Thus, at the mid-point of a titration of a weak acid:

pKa = pH

The term pKa is that pH at which an equivalent

distribution of acid and conjugate base (or base and conjugate acid) exists in solution.

The Henderson-Hasselbalch Equation

• For an amino acid with only one amine and one carboxyl group, the pI can be calculated from the mean of the pKa of this molecule:

KCOOH=[R - COO-] [H+]

[R - COOH]; K =

[R - NH2] [H+]

[R - NH3+]NH3

+

pI = 2

=2,36 + 9,6

= 5,982

pKCOOH + pK NH3+

pI = 2

=2,36 + 9,6

= 5,982

pKCOOH + pK NH3+Leucine:

pKa of –COOH [1.8-4.3] pKa of –NH2 [9.1-12.5]

pI = (pKa1 + pKa2)/2

pH and Ionization (2)• Acidic amino acids such as aspartic acid have a second

carboxyl group that can donate and accept protons. • If there were three titratable groups or other dissociating

side chain groups, the pI equation would involve all three pKa's and the denominator would be "3“

• The pI for aspartic acid occurs at a pH of 2.8

pI = (pKa1 + pKa2 + pKa3)/3

Glu ionization in water. • Indicate ionizable

groups.– Predict ionization of this

amino acid at pH=1.0– Predict ionization of this

amino acid at pH=10.0– Predict ionization of this

amino acid at pH=7.0

pKa(COOH) pKa(NH2) pKa(R)

Glutamic Acid

Glu 2.19 9.67 4.25

Learning Check

Peptides and Proteins

Oligopeptide :a few amino acids

Polypeptide : many amino acids

Amino terminal-

N-terminal-

Carboxyl terminal-

C-terminal

Tetrapeptide

1. Acid-base behavior of a peptide:

N-terminal, C-terminal, R-groups

2. Peptides have a characteristic titration curve and a characteristic pI value

Acidity of organic compounds

• Proton can be formed during break ofC-H, N-H, O-H or S-H bonds.

• Acidity of organic compounds increases in the following way:

C-H acids < N-H acids < O-H acids < S-H acids

Acidic properties

• Strength of an acid depends on the stability of the formed anion.

• If the formed anion is stable, it does not form the stable undissociated acid molecule and therefore there are H+ in the medium.

Stability of acid anions depends on• Electronegativity of

the atom to which hydrogen is attached.

• Radius of the atom to which hydrogen is attached.

• Delocalization of negative electric charge.

Acidity and electronegativity• The more electronegative an

element is, the more it helps to stabilize the negative charge of the conjugate base.

• Acidity increases as the atom to which hydrogen is attached becomes more electronegative.

Thus, acidity increases:

CH4 < NH3 < H2O < HF

(pKa values are 48, 38, 16 and 3 respectively)

Basicity and and pKa values

• Basicity is related to the ability of a compound to use its nonbonding electrons to combine with a proton.

• A strong base has a large pKa.

Basicity and electronegativity

• Basicity will decrease as an atom becomes more electronegative.

• Oxygen is more electronegative than nitrogen, therefore its electrons are less likely to be donated to a proton.

Basicity and electronic properties

• Proton can attach to the free electron pair.

• Basicity increases where electrons are not delocalizated.

• Basic properties increase in the row:

S-H < O-H < N-H

.. .. ..

Delocalization effects

• Delocalization of charge in the conjugate base anion through resonance is a stabilizing factor and will be reflected by an increase in acidity.