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)

Can act as acid (proton donor) and base (proton acceptor) The structure is dependent on pH – due to presence -COOH and -NH2 R – COOH  R – COO- + H+ R – NH3+  R – NH2 + H+ acid conjugate base conjugate acid base 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+

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– 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). H+ OH– + H3N–CH2–COOH + H3N–CH2–COO– H2N–CH2–COO– Positive ion zwitterion Negative ion Low pH neutral pH High pH

By rearranging the above equation we arrive at the Henderson-Hasselbalch equation: pH = pKa + log[A-]/[HA]

The Henderson-Hasselbalch Equation 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.

pKa of –NH2 [9.1-12.5] pKa of –COOH [1.8-4.3] 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: pI = (pKa1 + pKa2)/2 Leucine:

pH and Ionization (2) The pI for aspartic acid occurs at a pH of 2.8 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

Learning Check 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

Peptides and Proteins Oligopeptide :a few amino acids Carboxyl terminal- C-terminal Amino terminal- N-terminal- Oligopeptide :a few amino acids Polypeptide : many amino acids

Tetrapeptide 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 of C-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.