Initial letters of the names of the amino acids were chosen where
there was no ambiguity. There are six such cases: cysteine, histidine.
isoleucine, methionine, serine and valine. All the other amino acids
share the initial letters A, G, L, P or T, so arbitrary assignments
were made. These letters were assigned to the most frequently
occurring and structurally most simple of the amino acids with these
initials, alanine (A), glycine (G), leucine (L), proline (P) and
Other assignments were made on the basis of associations that might be
helpful in remembering the code, e.g. the phonetic associations of F
for phenylalanine and R for arginine. For tryptophan the double ring
of the molecule is associated with the bulky letter W. The letters N
and Q were assigned to asparagine and glutamine respectively; D and E
to aspartic and glutamic acids respectively. K and Y were chosen for
the two remaining amino acids, lysine and tyrosine, because, of the
few remaining letters, they were close alphabetically to the initial
letters of the names. U and O were avoided because U is easily
confused with V in handwritten material, and O with G, Q, C and D in
imperfect computer print-outs, and also with zero. J was avoided
because it is absent from several languages.
Two other symbols are often necessary in partly determined sequences,
so B was assigned to aspartic acid or asparagine when these have not
been distinguished; Z was similarly assigned to glutamic acid or
glutamine. X means that the identity of an amino acid is undetermined,
or that the amino acid is atypical. "Nomenclature and Symbolism for Amino Acids and Peptides"