Figure 5

Amino acid distributions for the third position of the observed peptides. Frequencies of amino acids in the third position are plotted as described in Figure 3. For well-predicted ions, proline (P) is much more common than in the general data base; for under-predicted ions, it is highly less common. The displayed statistic (see Figure 3 label) is based on a transformation of an assumed Poisson count that tends to have a distribution that is standard normal; hence, departures that exceed three in magnitude (location of dashed lines) might be considered unexpected.