Lecture 7:

Amino Acids as acids and bases and peptides

Campbell and Farrell: Ch 3.3-3.5

Amino acids can exist in different ‘charge states’

NEUTRAL AMINO ACIDS

ZWITTERION:

Ions with more than one charged group, but a net charge of zero

ALANINE

CHARGED AMINO ACIDS

e.g. HISTIDINE

http://www.pdfonline.com/easypdf/?gad=CLjUiqcCEgjbNejkqKEugRjG27j-AyCw_-AP

http://www.pdfonline.com/easypdf/?gad=CLjUiqcCEgjbNejkqKEugRjG27j-AyCw_-AP

Isoelectric pH (point):

pH at which an amino acid is in the zwitterionic form (charge = 0)

p. 75

CALCULATION OF pI

(neutral amino acids)

CALCULATION OF pI

(ionizable amino acids)

Formation of covalent bond between amino acids

http://www.pdfonline.com/easypdf/?gad=CLjUiqcCEgjbNejkqKEugRjG27j-AyCw_-AP

Peptide bond:

the special name given to the amide bond between the -carboxyl group of one amino acid and the -amino group of another amino acid

• dipeptide: a molecule containing twoamino acids joined by a peptide bond

• tripeptide: a molecule containing threeamino acids joined by peptide bonds

• polypeptide: a macromolecule containing many amino acids joined by peptide bonds

Peptide: 2-100 amino acids

Protein: > 100 amino acids

Fig. 3-9, p. 78

N-terminal

(free amino group)

C-terminal

(free carboxyl group)

Resonance structures of the peptide bond

Peptide bonds have partial double bond character

The peptide bond is ‘planar’(the atoms indicated will lie flat on a ‘table’)

Biologically important peptides

(structures not important for test/exam)

p. 80

Aspartame: 200 x sweeter than sugar

(dipeptide)

Phenylketonuria

Accumulates in cells

Y F

Accumulation leads to BRAIN DAMAGE

Glutathione (tripeptide)

‘Anti-oxidant’-carboxyl -carboxyl

Glutathione (tripeptide)

Peptide hormones

(cyclic peptides)

Amide group