Analysis of tryptic peptides derived from e.coli lysate by SCX-RP (red) and RP-RP (blue) fractionation. RP-RP provided for higher numbers of identified (A) peptides and (B) proteins as compared to SCX-RP for independent experiments spanning 10, 20, 30, and 40 first dimension fractions, respectively. The number of peptides identified per protein was slightly higher for RP-RP, while peptide length was on average shorter by one amino acid (B, bottom). A permutation test (C), revealed that peptides identified by SCX-RP exhibited a statistical bias for histidine (-log[p-val]>2) residues as compared to those identified by RP-RP.