Amino acids

General
Amino acids are of the general form (R represents any group):

Residue masses
See Peptide Mass Spec Data

The 20 common amino acids are (with 3 letter and 1 letter abbreviations):

Asx, B ("B" near "D")
Either Aspartic acid or Asparagine, uncertainty due to hydrolysis.

Xle, J ("J" between "I" and "L")
Either Isoleucine or Leucine, ambiguity from Mass spec data

Xaa, X
Unknown amino acid

Glx, Z ("Z" near "X")
Either Glutamic acid or Glutamine, uncertainty due to hydrolysis

Sec, U
Selenocysteine (Uniprot uses C and a feature rather than U)

Pyl, O ("pyrrOlysine")
Pyrrolysine (Uniprot uses K and a feature rather than O)