Figure 2.

ESI mass spectrum of lac permease collected upon elution of the 600-sec UV peak. ESI generates multiply charged ions, each with different mass/charge (m/z) ratios through the addition of variable numbers of protons. In the case of lactose permease, it was necessary to generate ions with at least 20 protons such that their m/z was in the scanning range of the mass spectrometer. Spectra from several scans across the peak were added to generate the spectrum shown. Typically ions carrying from 20 to 50 positive charges were detected, appearing to fall into at least three distributions, suggestive of different tertiary structures in the gas phase. The measured mass is calculated from the m/z of these ions after assigning the charge states. A computer-generated reconstruction of the zero-charged protein (Hypermass, PE Sciex, Toronto) is shown (Inset).