The fragmentations of [M-H]− anions derived from underivatised peptides. The side-chain loss of H2S from Cys. A joint experimental and theoretical study

Loss of H2S is the characteristic Cys side‐chain fragmentation of the [MH]− 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(CH2SH)CONHR′ Ø [RNHC(CH2)CONHR′ (HS−)] Ø [RNHC(CH2)CO‐HNR′‐H]−+H2S. 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 H2S. Loss of CH2S is a minor process, RNHCH(CH2SH)CON−‐R′ Ø RNHCH(CH2S−)CONHR′ Ø RNH −CHCONHR+CH2S, 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 (H2S+CO2). 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 CO2− 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 CH2O and H2O from the [MH]− 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 CH2O is the characteristic side‐chain loss of Ser in the negative ion mode. Copyright © 2003 John Wiley & Sons, Ltd.