Amino acids under ionization conditions during electrospraying: use of nickel chloride to initiate cluster formation

Abstract

A systematic study of the ionization of amino acids by the method of electrospray ionization (ESI- MS) in the presence of nickel chloride was carried out. The influence of the mobile phase on the quality and composition of the mass spectra was studied. The composition of the acetonitrile-water mixture varied from 5 to 95% water. It was found that the change in the constituent part of the mass spectra, but not in the qualita- tive composition. The quality of mass spectra depends significantly on the purity of the acetonitrile used, which in large quantities increases chemical noise and makes identification difficult. The best ratio for work was 45% water and 55% acetonitrile. It has been shown that for aromatic amino acids (tyrosine, tryptophan) the most typical processes are both fragmentation (decarboxylation, deamination, decay in α-carbon) and, to some extent, cluster formation processes involving nickel atoms. It has also been shown that claster forma- tion occurs most intensively in simple non-aromatic amino acids. It can be seen that cluster formation in such cases proceeds in three different directions: the formation of molecular associates of amino acids with a charge carrying a proton, the formation of molecular associates of amino acids with a charge-carrying nickel atom, and the formation of polymolecular coordinated structures coordinated by two nickel ions.

It is shown that the results obtained in the study can be used not only for a more sensitive and infor- mative search for amino acids in solutions of complex mixtures with chromatographic separation and using direct input, but for more reliable identification of the target compounds due to the formation of clusters of the target compounds (in the case of non-aromatic compounds) and characteristic fragmentation (in the case of aromatic compounds). This study will be useful not only to expand the field of physico-chemical know- ledge, but also to improve the methods of applied research.