THE NEGATIVE-ION MASS-SPECTRA OF DEPROTONATED 2,5-DIKETOPIPERAZINES

Deprotonation of 2,5-diketopiperazine (with HO-) can occur either on N (position 1(4)) or on carbon (position 3(6)). The two depotonated for ms are interconvertible on collisional activation. The major collision -induced fragmentations of (M - H)(-) ions of substituted 2,5-diketopi perazines are (i) characteristic side-chain losses (e.g. Me. for Ala, PhCH(2)(.) for Phe, and O = C6H4 = CH2 for Tyr), which identify the pa rticular 2,5-diketopiperazine, and (ii) an unusual loss of RCHO (R is the substituent, e.g. Me for Ala), which involves initial 1,2 migratio n of R. to the carbon of the adjacent carbonyl group.