Fig. S1. Collected isotopic distributions of native peptides Cal12a (A) and Cal12b (B). Note that the actual data are displayed with +4 ions, and the calculated spectra shows +1 ions. Collected spectra exhibit isotopic profiles consistent with incorporation of multiple bromine atoms.

Fig. S2. Matching of the primary structure of peptides predicted by cDNAs Cal12.1.1a (A) and Cal12.1.1b (B) to the actual masses of the mature, processed toxins cal12a and cal12b as observed with MS analysis. This was accomplished by informative addition and subtraction of masses associated with specific PTMs. W, bromotryptophan; O, hydroxyproline; C, non-specifically disulfide-bonded cysteine; γ, Gla (gamma-carboxyglutamic acid).

Fig. S3. NSI fragmentation spectra of ion 1415⁺⁴ derived from reduced and alkylated cal12.b. Excellent coverage of the peptide is exhibited with the b and y ion series. The inset shows the y₆ ion showing the bromine isotopic distribution. C, cysteine with carbamidomethyl group; W, bromotryptophan; O, hydroxyproline.