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 [CaC₃ H₃ O₂ S]⁺ , and the loss of H₂ S. The detection of lighter [C₃ H₃ OS]⁺ monocations indicates that the [CaC₃ H₄ O₂ S]²⁺ doubly charged species produced by the loss of ammonia undergo a subsequent Coulomb explosion yielding [C₃ H₃ OS]⁺ +CaOH⁺ . This [C₃ H₃ OS]⁺ cation finally decomposes into [C₂ H₃ S]⁺ +CO. Alternatively, the aforementioned [CaC₃ H₄ O₂ S]²⁺ dications may also lead to lighter [CaCO₂ ]²⁺ and [CaC₂ H₄ S]²⁺ dications by the loss of C₂ H₄ S 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 .