Determination of acid dissociation constants of neamine by potentiometric and electrospray mass spectral techniques

The acid dissociation constants of neamine have been measured using potenti ometric titrations. The pK(a1), pK(a2), and pK(a3) values of neamine are 6. 35 +/- 0.2, 7.73 +/- 0.15, and 8.62 +/- 0.08, respectively. Neamine is read ily characterized using positive-ion electrospray ionization-mass spectrome try (ESI-MS). Various protonated species and their solvated ions are mainly observed. Nevertheless, the abundances of the observed speciation over the pH range 5.0-9.8 do not reflect the variation in the bulk solution. Hence, reaction quotients determined from the ESI-MS investigations vary signific antly from the pK(a)'s determined from potentiometric titrations, thereby i llustrating that care must be taken in determining thermodynamic properties using the former technique.