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.