Amino acids are grouped based upon the properties and structures of side chains.

1) aliphatic (R groups consist of carbons and hydrogens)
glycine - R=H smallest a.a. with no chiral center
alanine - R=CH3 methyl group
valine R = branched; hydrophobic; important in protein folding
leucine R= 4 carbon branched side chain
isoleucine R = 2 chiral centers
proline R = ring; puts bends or kinks in proteins; contains a secondary amino
group
2) aromatic (R groups have phenyl ring)
phenylalanine - very hydrophobic
tyrosine - hydrophobic, but not as much because of polar groups
tryptophan - “
Absorb UV light at 280 nm --> used to estimate [protein]
3) sulfur-containing R groups
methionine - sulfur is internal (hydrophobic)
cysteine - sulfur is terminal --> highly reactive; can form disulfide bonds
4) side chains with alcohols
serine - -hydroxyl groups --> hydrophilic
threonine - “
5) basic R groups
histidine - hydrophilic side chains - + charged at neutral pH
lysine - “
arginine - strong base
6) acidic R groups and amide derivatives
aspartate - carboxyl group - confer - charges on proteins
glutamate - carboxyl group
asparagine - amide of aspartate - side groups uncharged --> polar
glutamine - amide of glutamate - “
Amide groups can form H bonds with atoms of other polar amino acids.
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