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Eur. J. Mass Spectrom. 8, 163 - 167 (2002)

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

Plamen A. Demirev* and Catherine Fenselau
Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA

ABSTRACT:
Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry has been applied for determining the 13C and 15N stable isotope content of a 10 kDa protein molecule. Purified samples of modified human ubiquitin, expressed in E. coli grown in a 13C and 13N 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 12C : 13C and 14N : 15N isotope enrichment. The isotope-enrichment ratios giving the best overlap between simulated and experimental data correspond to estimates of incorporation from the 13C and 15N isotopes present in the microorganism growth medium.

Keywords: FT-MS, multiply-charged ions, electrospray, ubiquitin, isotope distribution, 13C isotope, 15N isotope

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