Laser-Induced Acoustic Desorption/Electron Ionization of Amino Acids and Small Peptides

Laser-induced acoustic desorption (LIAD) allows for desorption of neutral nonvolatile compounds independent of their volatility or thermal stability. Many different ionization methods have been coupled with LIAD. Hence, this setup provides a better control over the types of ions formed than other mass spectrometry evaporation/ionization methods commonly used to characterize biomolecules, such as ESI or MALDI. In this study, the utility of LIAD coupled with electron ionization (EI) was tested for the analysis of common amino acids with no derivatization. The results compared favorably with previously reported EI mass spectra obtained using thermal desorption/EI. Further, LIAD/EI mass spectra collected for hydrochloride salts of two amino acids were found to be similar to those measured for the neutral amino acids with the exception of the appearance of an HCl+au ion. However, the hydrochloride salt of arginine showed a distinctly different LIAD/EI mass spectrum than the previously published literature EI mass spectrum, likely due to its highly basic side chain that makes a specific zwitterionic form particularly favorable. Finally, EI mass spectra were measured for seven small peptides, including di-, tri-, and tetrapeptides. These mass spectra show a variety of ion types. However, a(n) type ions are prevalent. Also, electron-induced dissociation (EID) of protonated peptides has been reported to form primarily a(n) type ions. In addition, the loss of small neutral molecules and side-chain cleavages were observed that are reminiscent of other high-energy fragmentation methods, such as EID. Finally, the isomeric dipeptides LG and IG were found to produce drastically different EI mass spectra, thus allowing differentiation of the leucine and isoleucine amino acids in these dipeptides.