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' circle divide [RNHC(=CH2)CONHR' (HS-)] circle divide [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 greater than or equal to20.1 kcal mol(-1) of excess energy to effect loss of H2S. Loss of CH2S is a minor process, RNHCH(CH2SH)CON-R-' circle divide RNHCH (CH2S-)CONHR' circle divide RNH-CHCONHR+CH2S, requiring an excess energy of greater than or equal to50.2kcal 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 greater than or equal to79.6 (charge-remote) or greater than or equal to67.1 (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 greater than or equal to35.6 and greater than or equal to44.4 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 (C) 2003 John Wiley Sons, Ltd.