Figure 12: Correlation between the average charge state of protein ions generated by ESI under near-native conditions (10 mM ammonium acetate, pH adjusted to 7) and their surface areas in solution, whose calculation was based upon their crystal structures. The data are plotted in the logarithmic scale (a graph plotted in the normal scale is shown in the inset). A gray-shaded dot represents a pepsin data point. An open circle underneath represents the highest charge of pepsin if the extent of multiple charging was limited by the number of basic residues within the protein molecule. (Reprinted with permission from Analytical Chemistry [77], Copyright 2005, American Chemical Society).