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, PhCH2. for Phe, and O = C6H4 = CH2 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. © 1996 Elsevier Science B.V. All rights reserved.