Protein kinases, which is one of the largest families of enzymes and structurally diverse, catalyze and the transfer the γ-phosphoryl group of ATP to the amino acid residues in their target proteins. The phosphorylation of protein is one of the major posttranslational modifications required for regulation of cellular activities, so the study of detailed chemical mechanism for the phosphoryl-transfer step is of profound importance. It is well known that electrospray ionization mass spectrometry (ESI-MS) is a powerful method to observe the intact non-cavalent complexes between biomolecules and small molecular. In this paper, the interactions of ATP with N-DIPP-AA, N-Boc-AA and AA were studied by ESI-MS. Shown as Fig.1 (Ala was as the example), the complex of ATP and Boc-Ala or DIPP-Ala was clearly observed, while that of ATP and Ala could not be found in the spectrum. The parameter of capillary exit on which the complex disappeared gives a proof that the interaction between ATP and DIPP-Ala was stronger than that between ATP and Boc-Ala. The binding energy of ATP with Ala, Boc-Ala and DIPP-Ala was computed using the software Sybyl 7.1. From all the data, it can be concluded that DIPP-Ala is the winner in the competition for ATP.
