   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 