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