The negative ion mass spectra of deprotonated 2,5-diketopiperazines

Abstract Deprotonation of 2,5-diketopiperazine (with HO − ) can occur either on N (position 1(4)) or on carbon (position 3(6)). The two depotonated forms are interconvertible on collisional activation. The major collision-induced fragmentations of (M  H) − ions of substituted 2,5-diketopiperazines are (i) characteristic side-chain losses (e.g. Me . for Ala, PhCH 2 . for Phe, and O  C 6 H 4  CH 2 for Tyr), which identify the particular 2,5-diketopiperazine, and (ii) an unusual loss of RCHO (R is the substituent, e.g. Me for Ala), which involves initial 1,2 migration of R . to the carbon of the adjacent carbonyl group.