Files in this Data Supplement:

• Supplemental Figure S1 -

  Fig. S1. Peptide fragmentation for one of the factor XIII A1 peptides that was identified in barnacle cement. The corresponding tandem mass spectra, and the b and y ions that were identified, are shown in Fig. 5B. The precursor ion is denoted by an arrow in Fig.^.5B (inset). Since the charge of the detected precursor ion (1164.67 m/z) is +2, the atomic mass of the ion is 2329.34 a.m.u. Collision-induced dissociation of the precursor ion produces fragments of the ion with a combined mass of 2329.34 a.m.u. Although fragmentation of the precursor ion is a random process, the collision energy is tuned to favor fragmentation at the (rigid) peptide bond, producing b and y ions across the sequence. Detection of these b and y ions is used for the database search, since the mass of these fragments is unique to the corresponding peptide sequence. For example, detection of the b₁₀ ion fragment represents the sequence EEFYVLNDIGV and detection of the y₁₁ ion fragment represents the sequence VIFYGEVNDIK of the identified peptide. These two ion fragments alone confirm the sequence of the identified peptide. Additional b and y ions provide further sequence confirmation of the identified peptide. Tandem mass spectrometry cannot be confused with peptide sequencing methods. In tandem mass spectrometry not all b and y ions are expected to be detected because of the nature of the technique.