Hydrophobic, Aliphatic Amino Acids

Glycine (Gly or G) ,the smallest amino acid with the easiest structure, has a hydrogen atom in the side-chain position, and therefore does not exist as a pair of stereoisomers  as there are 2 identical groups (hydrogen atoms) associated to the C_α atom. Aliphatic side chains of alanine (Ala or A), leucine (Leu or L), valine (Val or V), methionine (Met or M) and Fisoleucine  (Ile or I) are chemically  unreactive,  but in nature hydrophobic  . Proline (Pro or P) is

 (a)

 Glycine                        Alanine                                Valine                                  Leucine

(Gly, G)                       (Ala, A )                                 (Val, V)                                  (Leu, L)

 Isoleucine                                Methionine                             Proline                        Cysteine

(Ile, I)                                        (Met, M)                                (Pro, P)                            (Cys, C)

(b)

        Phenylalanine                        Tyrosine                              Tryptophan

          (Phe, F)                                (Tyr, Y)                                      (Trp, W)

 The standard amino acids (a) Hydrophobic, aliphatic R groups, (b) hydrophobic, aromatic R groups

also hydrophobic  but, on to the amino group ,with its aliphatic side-chain bonded back, it is conformationally inflexible. The sulfur containing side chain of cysteine (C or Cys) is hydrophobic also and is greatly reactive, capable of reacting along another cysteine to form a disulfide bond.