Figure 3

Amino acid distribution for first position of the observed peptides. The amino acids are represented by the single amino acid code (A, C,..., Y), and the magnitude of the bar is sqrt(4*observed+2)-sqrt(4*expected+1), where "observed" refers to the observed numbers of the amino acids at the first amino acid position, and "expected" refers to the expected numbers based on the frequency of the amino acids in the data base. Bars that extend beyond the dashed-line limits indicate greater departures from the expected count than would occur simply by chance alone. For well-predicted ions, arginine (R) is more common than in the general data base; for under-predicted ions, it is less common. The given statistic is based on a transformation of an assumed Poisson count that tends to have a standard normal distribution; hence, departures that exceed three in magnitude (location of dashed lines) might be considered unexpected.