1. ©CMBI 2001 A Ala Alanine Alanine is a small, hydrophobic residue. Its side chain, R, is just a methyl group. Alanine likes to sit in an alpha helix,it doesn’t like beta strand very much, but it hates beta-turns. If you want to mutate a residue, but you don’t have a good plan about what to replace it with, take alanine.

2. ©CMBI 2001 C Cys Cysteine Cysteine is a small till intermediately large hydrophobic residue. It doesn’t like the alpha helix, but doesn’t mind strand and turn. It can form Cys-Cys bridges with other cysteines. It can bind metals (especially Zn and Cu). The S-H group is very reactive.

3. ©CMBI 2001 D Asp Aspartic acid Aspartic acid, or aspartate, is an intermediately large, hydrophilic, negatively charged residue. Its side chain normally titrates at pH 4.5. It likes to sit near the N-terminus of a helix, and in turns. It hates strands.It often occurs in active sites. It can bind ions (mainly Ca).

4. ©CMBI 2001 E Glu Glutamic acid Glutamic acid, or glutamate, is a large, hydrophilic, negatively charged residue. Its side chain titrates at pH 4.6. It loves the helix, doesn’t mind being in a strand, but is not so good for a turn.

5. ©CMBI 2001 F Phe Phenylalanine Phenylalanine is a large, hydrophobic, aromatic residue. It is good for a strand, it doesn’t mind sitting in a helix, but it hates the turn.

6. ©CMBI 2001 G Gly Glycine Glycine is the smallest residue. It doesn’t have a side chain, so its hydrophobicity is a bit undetermined, but you can call it hydrophobic or assign it an intermediate hydrophobicity. The fact that it doesn’t have a side chain means that its backbone is very flexible so that it can make backbone turns that other residues cannot make. It is very bad for helix, bad for strand, but it is the star of the turns.

7. ©CMBI 2001 H His Histidine Histidine is very special. It is hydrophilic due to the two nitrogens in its side chain. Both nitrogens can titrate (the first one at pH 6.2). It is a little bit aromatic. It is often seen in active sites.It is neutral at physiological pH, but it can easily become positive, and occasionally even negative. It can bind metal ions (mainly Zn, Ni, Cu). It is not particularly picky about its secondary structure.

8. ©CMBI 2001 I Ile Isoleucine Isoleucine is an intermediately large, hydrophobic residue. It is beta branched which means that it likes to sit in a strand. It doesn’t mind sitting in a helix either, but it cries its eyes out in a turn.

9. ©CMBI 2001 K Lys Lysine Lysine is a large, hydrophilic, positively charged residue. It is not a good strand residue, but it doesn’t mind sitting in a helix or in a turn. Its side chain is very long and flexible.

10. ©CMBI 2001 L Leu Leucine Leucine is an intermediately large, hydrophobic residue that really loves to sit in a helix. It is also good for a strand, but it hates turns.

11. ©CMBI 2001 M Met Methionine Methionine is a large, sulphur containing, hydrophobic residue. It loves helices, doesn’t mind sitting in a strand, but it hates turns. Methionine can bind metals with its sulphur, but this sulphur is much less reactive than the sulphur in cysteine. It is often the first residue of a molecule. The N-terminus is mostly positive and thus mostly at the surface. Therefore, methionine, despite being hydrophobic, is often at the surface. We call this a forced marriage.

12. ©CMBI 2001 N Asn Asparagine Asparagine is an intermediately large, polar residue. It hates the helix, is mildly un-amused in a strand, but it loves the turn. It can bind metal ions (Ca), but doesn’t do that as well as its isosteric partner aspartic acid.

13. ©CMBI 2001 P Pro Proline In proline, the side chain is connected to the backbone at two places: the C  and the N. Therefore, it is not an amino acid, but an imino acid. Unless it is the N-terminal residue, proline does not have a backbone proton, and thus is not good for helices and strands. Due to the extra covalent bond, proline is already pre-bend, and thus good for turns. Even though it is very hydrophobic, it often sits at the surface

14. ©CMBI 2001 Q Gln Glutamine Glutamine is a large, polar residue. It is not very picky about its secondary structure. It is isosteric with glutamic acid.

15. ©CMBI 2001 R Arg Arginine Arginine is a big, hydrophilic, positively charged residue. Its side chain contains a so-called guadinium group that is rigid. It is not picky about its secondary structure.

16. ©CMBI 2001 S Ser Serine Serine is a small, alcoholic residue of intermediate hydrophobicity. It is not too happy in helices and strands, but it loves to sit in turns. It often forms the active site of an enzyme together with histidine and aspartic acid. It is occasionally involved in metal (Ca) binding.

17. ©CMBI 2001 T Thr Threonine Threonine is a small, alcoholic residue of intermediate hydrophobicity. It is beta- branched and thus good for beta strands. It doesn’t care about helices or turns. It is occasionally involved in metal (Ca) binding.

18. ©CMBI 2001 V Val Valine Valine is a small hydrophobic residue. It is beta-branched and thus good for beta strands. It is isosteric with threonine. Valine doesn’t care about helices, but it hates turns.

19. ©CMBI 2001 W Trp Tryptophan Tryptophan is the biggest residue. It is aromatic. Despite that the nitrogen in the five-ring is donor for hydrogen bonds, it is very hydrophobic. It doesn’t care about helices or turns, but it loves strands.

20. ©CMBI 2001 Y Tyr Tyrosine Tyrosine is a large, aromatic, alcoholic residue of intermediate hydrophobicity. It is not so happy in a helix, indifferent about turns, and it loves a strand.