Figure 6. Workflow for quantification of cross-linked peptides using pQuant.
For each identified cross-link spectrum, an extracted ion chromatogram (EIC) is constructed for each isotopic peak of the [d0]- and [d6]-labeled precursor. The [d6]/[d0] ratios can be calculated based on the monoisotopic peak, the most intense peak, or the least interfered peak of each isotopic cluster as specified by users. The accuracy of the ratio calculation was evaluated with the confidence score σ (range: 0–1, from the most to the least reliable). If a cross-link have ratios with σ < 0.5, the median of these ratios is assigned to this cross-link. The cross-link ratios of the proteins of interest are normalized to the median ratio of all BSA cross-links. For each cross-link, the median [state1]/[state2] ratio of three independent forward labeling experiments is plotted against the median ratio of three independent reverse labeling experiments. Cross-links that are only present in state1 or state2 due to a dramatic conformational change cannot be quantified as described above because the ratios would be zero or infinite and their σ values would be 1. Therefore, if a cross-link does not have a valid ratio after automatic quantification, the EICs were manually inspected to determine if it was an all-or-none change.