Positive and negative cluster ions of amino acids formed by electrospray droplet impact/secondary ion mass spectrometry (EDI/SIMS)

Abstract

Cluster ions of amino acids (AAs) have been measured by several ionization methods such as electrospray, sonic spray, thermal desorption, and gas cluster ion beam (Arn+) bombardment. Prior research has primarily focused on positively charged cluster ions, whereas the formation of negatively charged cluster ions remains largely unexplored. In this study, mass spectra of both positive and negative cluster ions for 20 amino acids were measured by electrospray droplet impact/secondary ion mass spectrometry (EDI/SIMS). Because EDI is capable of generating abundant negative ions as well as positive ions, it is best suited for the comparative study on the dependence of polarity for AA cluster ions. For AAs with apolar side chains such as Ala, Val, Leu, and Ile, while large cluster sizes of positive cluster ions [n(AA) + H]+ (n = 11–21) were observed, negative cluster ions [n(AA) − H]− were observed with only n up to 3. The AAs with polar side chains (Ser, Thr, Asn, Gln, Tyr and Cys) gave similar n up to 7–12 for both [n(AA) + H]+ and [n(AA) − H]−. For basic AAs (Lys and Arg), the maximum n values observed for [n(AA) + H]+ are larger than those for [n(AA) − H]− and vice versa for acidic AA (Asp and Glu). A sharp intensity drop with n→(n+1) was observed for [n(AA) + H]+ with n of 12, 8, 8 and 11 for Pro, Ser, Thr, and Tyr, respectively. The magic number 8 for positive and 6 for negative cluster ions of Ser and Thr are in agreement with previous work. No magic numbers were observed for [n(AA) + H]+ and [n(AA) − H]− for acidic amino acids (Asp and Glu). The ligands (n−1)AAs in the negative clusters [AA − H]−⋅⋅⋅⋅(n−1)AA are bound more loosely than those in the positive clusters [AA + H]+ ⋅⋅⋅⋅(n−1)AA. The stability of amino acid cluster ions is critically affected by the polarities and structures of amino acids.