Figure 6.

Mass-scale expanded spectra of two isotopic peaks of ¹³C, ¹⁵N doubly depleted p16 tumor suppressor protein, obtained by coadding nine frequency-domain ESI FT-ICR spectra. The theoretical isotopic distribution is shown at upper right, in which ★ denote the monisotopic peak (Left), and isotopic peak (Right) ∼2 Da above the monoisotopic mass. From the abundance ratio of the (resolved) ³⁴S to the monoisotopic species, we determined the number of sulfur atoms in the protein to be 5.1 ± 0.3 without use of any other information about the protein.