Decomposition of ESR spectra using MALDI-TOF mass spectrometry

ESR (or EPR) spectroscopy on spin-labeled site-directed cysteine mutants is ideally suited for structural studies of membrane proteins due to its high sensitivity and its low demands with respect to sample purity and preparation. Many features can be inferred from the spectral line shape of an ESR spectrum, but the analysis of ESR spectra is complicated when multiple sites with different line shapes are present. Here, we present a method to decompose the spectrum of a doubly labeled peptide that is composed of a singly labeled, noninteracting component and a doubly labeled, dipolar-broadened component using a combination of optical and matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry. The effect on the interspin distance calculation based on the dipolar broadening is quantified and discussed.