Cannonical chemical sequence expressed as string of
               one-letter amino acid codes. Modifications are coded
               as the parent amino acid where possible.
A  for alanine or adenine
B  for ambiguous asparagine/aspartic-acid
R  for arginine
N  for asparagine
D  for aspartic-acid
C  for cysteine or cystine or cytosine
Q  for glutamine
E  for glutamic-acid
Z  for ambiguous glutamine/glutamic acid
G  for glycine or guanine
H  for histidine
I  for isoleucine
L  for leucine
K  for lysine
M  for methionine
F  for phenylalanine
P  for proline
S  for serine
T  for threonine or thymine
W  for tryptophan
Y  for tyrosine
V  for valine
U  for uracil