unimod
Managing amino acid modifications for mass spectrometry in R.
Unimod is a public domain database, distributed under a copyleft licence: "a copyright notice that permits unrestricted redistribution and modification, provided that all copies and derivatives retain the same permissions."
The aim is to create a community supported, comprehensive database of protein modifications for mass spectrometry applications. That is, accurate and verifiable values, derived from elemental compositions, for the mass differences introduced by all types of natural and artificial modifications. Other important information includes any mass change, (neutral loss), that occurs during MS/MS analysis, and site specificity, (which residues are susceptible to modification and any constraints on the position of the modification within the protein or peptide).