Combined Experimental and Theoretical Survey of the Gas-Phase Reactions of Serine-Ca2+ Adducts.

ABSTRACT

The association of Ca2+ to serine and the subsequent gas-phase unimolecular reactivity of the [Ca(Ser)]2+ (Ser = Serine) adduct was investigated throughout the use of tandem mass spectrometry techniques and B3LYP/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) density functional theory calculations. In a first step, the structure and relative stability of all possible conformers of serine were obtained and analyzed, as well as the most stable [serine-Ca]2+ adducts. For the analysis of the different potential energy surfaces associated with the gas-phase unimolecular reactivity of these adducts, only those that differ by less than 100 kJ·mol-1 from the global minimum were taken into account. In agreement with previous studies, the serine-Ca2+ global minimum corresponds to a charge-solvated structure in which Ca is tricoordinated to neutral serine. The major peaks observed in the nanoelectrospray-MS/MS spectrum of [Ca(Ser)]2+ adduct correspond to both Coulomb explosions, yielding either CaOH+ + [C3,H6,N,O2]+ or [C2,H4,O,N]+ + [Ca(C,H3,O2)]+, and to the loss of neutrals, namely, CH2O and H2O. Our theoretical survey of the energy profile allow us to conclude that, although all the aforementioned fragmentation processes can have their origin at the global minimum, similar fragmentations involving low-lying conformers, both zwitterionic and nonzwitterionic, compete and should be considered to account for the observed reactivity. We have also found that in some specific cases post-transition state dynamics similar to the ones described before in the literature for formamide-Ca2+ reactions, may also play a role.