On the origin of the abundance distribution of apomyoglobin multiply charged ions in electrospray mass spectrometry.

The abundance distribution of multiply charged ions produced during the electrospray process is thought to depend on the initial conformational state of the analyte in solution and on solution chemistry, but with some proteins, including apomyoglobin, the correlation between solution and mass spectrometry data is unclear. In this study, we compare our results obtained by mass spectrometry with available data describing conformations and average number of charges of apomyoglobin in solution. A good correlation between average number of charges in solution and in the mass spectrum is only found for the pH 4.2 solution. We propose that the discrepancies between solution and electrospray ionization mass spectrometry observed with apomyoglobin under other solution conditions can be mainly explained by (1) a preferential fragmentation of the highly charged ions in the source and (2) a variation of pH in the droplets during the electrospray process.