The fragmentations of [M–H] − anions derived from underivatised peptides. The side‐chain loss of H 2 S from Cys. A joint experimental and theoretical study

Loss of H 2 S is the characteristic Cys side‐chain fragmentation of the [MH] − anions of Cys‐containing peptides. A combination of experiment and theory suggests that this reaction is initiated from the Cys enolate anion as follows: RNH‐ − C(CH 2 SH)CONHR′ Ø [RNHC(CH 2 )CONHR′ (HS − )] Ø [RNHC(CH 2 )CO‐HNR′‐H] − +H 2 S. This process is facile. Calculations at the HF/6‐31G(d)//AM1 level of theory indicate that the initial anion needs only ≥20.1 kcal mol −1 of excess energy to effect loss of H 2 S. Loss of CH 2 S is a minor process, RNHCH(CH 2 SH)CON − ‐R′ Ø RNHCH(CH 2 S − )CONHR′ Ø RNH  − CHCONHR+CH 2 S, requiring an excess energy of ≥50.2 kcal mol −1 . When Cys occupies the C‐terminal end of a peptide, the major fragmentation from the [M–H] − species involves loss of (H 2 S+CO 2 ). A deuterium‐labelling study suggests that this could either be a charge‐remote reaction (a process which occurs remote from and uninfluenced by the charged centre in the molecule), or an anionic reaction initiated from the C‐terminal CO 2 − group. These processes have barriers requiring the starting material to have an excess energy of ≥79.6 (charge‐remote) or ≥67.1 (anion‐directed) kcal mol −1 , respectively, at the HF/6‐31G(d)//AM1 level of theory. The corresponding losses of CH 2 O and H 2 O from the [MH] − anions of Ser‐containing peptides require ≥35.6 and ≥44.4 kcal mol −1 of excess energy (calculated at the AM1 level of theory), explaining why loss of CH 2 O is the characteristic side‐chain loss of Ser in the negative ion mode. Copyright © 2003 John Wiley & Sons, Ltd.