ELECTROSPRAY MASS-SPECTRA OF PROTEIN CATIONS FORMED IN BASIC SOLUTIONS

The electrospray mass spectra of gramicidin S cations that originated from 0.2 M solutions of 18 nitrogen-containing bases were examined. The relative abundances of the [M + 2H](2+) to the [M + H](+) ion were found to correlate not with the solution pH but with the proton affinities of the bases. It is postulated that some of the [M + 2H](2+) and the [M + H](+) ions exist as adducts with the nitrogen bases in solution, these adducts being desorbed into the gas phase during electrospray and dissociated in the lens region via collision-induced dissociations to yield apparent proton attachment spectra. Some of these adducts were observed under nominally zero collision energy conditions.