Features of the stable 18O isotope inclusion into peptide carboxyl groups in highly acidic media

Using ESI-MS and MALDI-MS the process of isotope exchange of 16О for 18О in peptide carboxyl groups was studied. It was shown, that carboxyl group protonization is a limiting stage in this exchange. Protonization depends on carboxyl group acidity as well as acidity of the medium. Optimum conditions for the production of 18O-containing peptides and amino acid derivatives which can be used as standards for the quantitative analysis are determined. Stability of the introduced label in the conditions approaching physiological, and in media commonly used in experimental work with specimen using HPLC and ESI-MS was evaluated.