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

The association of Ca 2+ o 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-Ca 2+ 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 + + [C 3 ,H 6 ,N,O 2 ] + or [C 2 ,H 4 ,O,N] + + [Ca(C,H 3 ,O 2 )] + , and to the loss of neutrals, namely, CH 2 O and H 2 O. 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-Ca 2+ reactions, may also play a role.