The isotopic composition of each component, including
    the % labeling level, if known. For example:
    1. Uniform (random) labeling with 15N: U-15N
    2. Uniform (random) labeling with 13C, 15N at known labeling
     levels: U-95% 13C;U-98% 15N
    3. Residue selective labeling: U-95% 15N-Thymine
    4. Site specific labeling: 95% 13C-Ala18,
    5. Natural abundance labeling in an otherwise uniformly labled
      biomolecule is designated by NA: U-13C; NA-K,H