Dear,

I am trying to build a spectral library in Skyline using an msp file based on predicted spectra. Since we work with histones, we derivatize our peptides using propionylation, so all unmodified lysines and N-terms are blocked with a propionyl group. Therefore, Propionyl (K) and Propionyl (N-term) are fixed modifications, unless another variable modification is present. However, Skyline does not seem to recognize the modification "Propionyl" from the msp, although I also defined it as a modification in the peptide settings. All other modifications (e.g. Acetyl, Dimethyl,..) that I defined, are used correctly in the library. After some research, we figured out that Skyline recalculates the precursor m/z for each peptide (based on its sequence) and doesn't use the one that is stated in the msp. So for a peptide that carries a propionyl group, the m/z is recalculated as if the peptide would be unmodified because Skyline does not recognize the propionyl.

In the attachment you can find a short example of a predicted msp and the Skyline document with the corresponding library I built with this msp. In the msp you can see that the Propionyl group in the header is stated in the exact same way other modifications are, so I don't understand where the problem is.

Thank you in advance for helping me out!

Best,

Laura