Comparison of intensities between doubly charged ions [M + 2H]2+ and singly charged ions [M + H]+ of gramicidin S by electrospray mass spectrometry.

The reason why the intensity of doubly charged ions M + 2H]2+ of gramicidin S is higher than that of singly charged ions M + H]+ in electrospray is investigated by ion evaporation theory. As a result of comparison between the total free energies of extracting M + 2H]2+ and M + H]+ from a charged droplet to infinity, it is found that the total free energy of M + 2H]2+ is estimated to be lower than that of M + H]+. This clearly supports the experimental result. In addition, the importance of the electrostatic contribution in electrospray is demonstrated by showing the result that the total free energy of M + 2H]2+ without electrostatic contribution is higher than that of M + H]+.