Investigation of disulfide bond cleavage, water-cluster cations and PO bonds in phosphates using AB initio electronic structure calculations

The chemical bonds that cleave in proteins and peptides during electron capture dissociation (ECD) and electron transfer dissociation (ETD) mass spectrometry are N-C! and disulfide bonds. These types of bonds cleave at a significantly higher rate in ECD and ETD as compared to other types of mass spectrometry, such as collisionally activated dissociation (CAD) mass spectrometry. The mechanism of bond cleavage in ECD and ETD has been a subject of debate. The original mechanism proposed involves a hydrogen atom transfer from the positive site of electron capture to either an SS or N-C! bond. This mechanism came under scrutiny after further investigation. Described herein, is the use of Hartree-Fock and second order Møller-Plesset perturbation calculations to investigate the mechanism of bond cleavage in proteins and peptides during ECD and ETD. The behavior of water-cluster cations of the form [M(H2O)n]2+ were studied in ECD experiments and the outcomes of these experiments were investigated using ab initio electronic structure calculations. These calculations were done in order to understand ECD experiments performed by the Williams group. These experiments were performed with different alkaline earth metals and various numbers of water molecules. Mass-selected doubly charged cations containing alkaline earth metals were subjected to ECD and the mass-to-charge ratios and abundances of the fragmentation ions were determined. For the Ca2+-containing clusters, ECD mass spectrometry was carried out on clusters with 4 to 47 water molecules. It was shown that two different outcomes were possible for these metal-cluster cations, depending on the number of water molecules that were in the cluster. Described herein, is the investigation of the mechanism involved in the two possible reaction pathways. The nature of phosphorus oxygen bonds in phosphates is also described. Hartree-Fock and Møller-Plesset perturbation calculations were employed to examine several systems.