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