Pulsed Nano-Electrospray Ionization Applied to Solutions of Angiotensin II, Bradykinin, and Gramicidin S in Water/Acetonitrile (1/1) with the Addition of 1% Acetic Acid and 10 mM Ammonium Acetate.

In our previous work, pulsed nano-electrospray ionization was applied to aqueous mixtures of 5 × 10^-6 M angiotensin II (A), bradykinin (B), and gramicidin S (G). It was found that G was totally suppressed by the presence of A and B. In this work, mixtures of A, B, and G in water/acetonitrile (W/AcN) were investigated by pulsed nano-electrospray ionization. It was found that G and A were detected as major ions, but B was almost totally suppressed by the addition of 1% acetic acid in the W/AcN solution. In contrast, B was detected as one of the major ions for the solution with the addition of 10 mM ammonium acetate. These results were interpreted based on the solvent effect. While the hydration of ornithine -NH₃ ⁺ in aqueous solution makes the ion most hydrophilic, solvation of ornithine -NH₃ ⁺ by AcN in W/AcN makes the ion solvophobic and surface active.