Modeling Interactions between an Amino Acid and a Metal Dication: Cysteine–Calcium(II) Reactions in the Gas Phase

The gas-phase interactions between Ca2+ and cysteine (Cys) have been investigated through the use of electrospray ionization/mass spectrometry techniques and B3LYP/6-311++G(3df,2p)//B3LYP/6-311+G(d,p) density functional theory computations. The unimolecular collision-activated decomposition of [Ca(Cys)]2+ is dominated by the loss of ammonia, a Coulomb explosion yielding NH4+ and [CaC3H3O2S]+, and the loss of H2S. The detection of lighter [C3H3OS]+ monocations indicates that the [CaC3H4O2S]2+ doubly charged species produced by the loss of ammonia undergo a subsequent Coulomb explosion yielding [C3H3OS]++CaOH+. This [C3H3OS]+ cation finally decomposes into [C2H3S]++CO. Alternatively, the aforementioned [CaC3H4O2S]2+ dications may also lead to lighter [CaCO2]2+ and [CaC2H4S]2+ dications by the loss of C2H4S and CO2, respectively. A detailed theoretical exploration of the Ca2+/Cys potential-energy surface indicates that the salt-bridge structures, in which the metal dication interacts with the carboxylate group of the zwitterionic form of cysteine, are at the origin of the different reaction pathways leading to the observed product ions, even though they lie higher in energy than the charge-solvated adduct in which the metal interacts simultaneously with the carbonyl oxygen, the amino, and the SH group of its canonical form. The interaction between the metal cation and the base is essentially electrostatic, with a calculated binding energy of 560 kJ mol−1