Amino acids

Met dank aan Dr. Detke

Most amino acids are chiral All naturally occuring amino acids

except Gly are the L isomer L & D amino acids are stereoisomers

Most amino acids are zwitterions amino acids

are zwitterions at neutral pH (contains negative and positive charged groups)

Small aliphatic amino acids Glycine

non-chiral smallest most flexible in

polypeptides less hydrophobic

Alanine chiral next smallest less hydrophobic

Other aliphatic amino acids

Val, Leu, Ile Highly hydrophobic branched side-chains

Yet another aliphatic amino acid

Proline imino acid rigid ring structure puts kinks in

polypeptides

Aromatic amino acidsPhenylalanine•Absorbs UV•Very hydrophobic

Tyrosine•Absorbs UV•less hydrophobic•H-bonds•OH can ionize•OH can be modified

Tryptophan•Really absorbs UV!•less hydrophobic•H-bonds

Sulfur-containing amino acids

Methionine highly hydrophobic initiator amino acid

Cysteine less hydrophobic forms disulfide bonds SH group can ionize

Oxidation of cysteine to cystine

Disulfide bond formation is reversible

Disulfide bonds form in proteins and help stabilize their structures

Alcoholic amino acids

Ser, Thr less hydrophobic H-bonding potential can be modified Thr is an isostere of

Val

Acidic amino acids

Asp, Glu highly

hydrophilic negatively

charged at neutral pH

Basic amino acids Highly hydrophilic Lys and Arg are positively charged at neutral pH His is partially charged at neutral pH

Amide-containing amino acids

Asn, Gln highly hydrophilic H-bonding potential isosteres of Asp and

Glu

Peptide bond formation

The peptide bond is planar

Fig. 4.6

Aspartame – a dipeptide

200X Sweeter than Sucrose !!!

aspartylphenylalanine methyl ester