FIG. 6.

Mass spectral analysis of individual peptide peaks from the reverse-phase chromatogram of PV VP1 (Fig. 5A). The ion signals of two representative peptides for which 50% of the molecules have incorporated ¹⁸O are shown ([M−H]⁺, 456.2 [A] and 1,032.5 [B]). The isotope distribution in panels A and B indicates the mixture of two peptide species, one containing ¹⁶O (peaks of 456.2 and 1,032.5); the other species has exchanged one ¹⁶O for one ¹⁸O resulting in a net mass increase by two mass units for 50% of the molecules (peaks of 458.2 and 1,034.5). In contrast, the isotope distribution of the [M−H]⁺ ion at m/z 612.1 reveals no ¹⁸O incorporation (C) and therefore is the carboxy-terminal peptide.