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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