Determination of isotope-enrichment ratios in proteins by high-resolution Fourier transform ion cyclotron resonance mass spectrometry

Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry has been applied for determining the C-13 and N-15 stable isotope content of a 10 kDa protein molecule. Purified samples of modified human ubiquitin, expressed in E. coli grown in a C-13 and N-13 isotope-enriched medium, are electrosprayed in a 4.7 T FT-ICR instrument. The intensity profiles of the isotopically-resolved envelope of the molecular ions in different charge states are averaged. The experimental data are then compared to theoretically-generated isotopic profiles for the same molecule at varying ratios of C-12 : C-13 and N-14 : N-15 isotope enrichment. The isotope-enrichment ratios giving the best overlap between simulated and experimental data correspond to estimates of incorporation from the C-13 and N-15 isotopes present in the microorganism growth medium.