Figure 1 of
Robinson, Mol Vis 2006;
Charge state and isotopic distributions for βB2-crystallin in one of the four 2-day-old human lenses. The charge state distribution (B) shows that essentially all of the βB2-crystallin molecules received between 13 and 30 positive charges during passage through the nanospray mass spectrometer source. These quantities have been divided by the molecular charge in order to correct for the increased ion image current detection signal with increase in charge. Each of these charges, caused by the addition of single protons to the molecules, increases the mass of the protein by 1 Da. The absence from the distribution function shown in (B) of protein molecules with 10 or fewer charges verifies that there are essentially no molecules with zero charge, so all have been ionized. The isotopic distribution of the 22+ charge βB2-crystallin ions (A) shows that greater than 98% of the isotopic species are being measured and quantitatively summed, since only the smallest abundance isotopic peaks approaching zero relative abundance are lost in background noise on each side of the distribution. Taken together, (A) and (B) demonstrate that virtually all of the βB2-molecules are accounted for and measured by this ICR FTMS method.