Tandem collision-induced dissociation (CID) mass spectra of doubly protonated peptide (a) exendin-4(15–39), (b) exendin-4(16–39), and (c) exendin-4(12–39).

<p>Precursor ions were generated by electrospray ionization (ESI) using the on-line liquid chromatography (LC)-ESI/tandem mass spectrometry (MS-MS) procedure described in the Methods section.</p>