Uncoiling collagen: a multidimensional mass spectrometry study

Abstract

Mass spectrometry can be used to determine structural information about ions by activating precursors and analysing the resulting series of fragments. Two-dimensional Fourier transform ion cyclotron resonance mass spectrometry (2D FT-ICR MS) is a technique that correlates the mass-to-charge (m/z) ratio of fragment and precursor ions in a single spectrum. 2D FT-ICR MS records the fragmentation of all ions in a sample without the need for isolation. To analyse specific precursors, horizontal cross-sections of the spectrum (fragment ion scans) are taken, providing an alternative to conventional tandem mass spectrometry (MS/MS) experiments. In this work, 2D FT-ICR MS has been used to study the tryptic digest of type I collagen, a large protein. Fragment ion scans have been extracted from the 2D FT-ICR MS spectrum for precursor m/z ratios: 951.81, 850.41, 634.34, and 659.34, and 2D FT-ICR MS spectra are compared with a set of 1D MS/MS spectra using different fragmentation methods. The results show that two-dimensional mass spectrometry excells at MS/MS of complex mixtures, simplifying spectra by eliminating contaminant peaks, and aiding the identification of species in the sample. Currently, with desktop computers, 2D FT-ICR MS is limited by data processing power, a limitation which should be alleviated using cluster parallel computing. In order to explore 2D FT-ICR MS for collagen, with reasonable computing time, the resolution in the fragment ion dimension is limited to 256k data points (compared to 4M data points in 1D MS/MS spectra), but the vertical precursor ion dimension has 4096 lines, so the total data set is 1G data points (4 Gbytes). The fragment ion coverage obtained with a blind, unoptimized 2D FT-ICR MS experiment was lower than conventional MS/MS, but MS/MS information is obtained for all ions in the sample regardless of selection and isolation. Finally, although all 2D FT-ICR MS peak assignments were made with the aid of 1D FT-ICR MS data, these results demonstrate the promise of 2D FT-ICR MS as a technique for studying complex protein digest mixtures.