title: Structure and unimolecular chemistry of protonated sulfur betaines, (CH3)2S(+)(CH2)(n)CO2H (n = 1 and 2).
abstract: The fixed charge zwitterionic sulfur betaines dimethylsulfonioacetate (DMSA) (CH(3))(2)S(+)CH(2)CO(2)(-) and dimethylsulfoniopropionate (DMSP) (CH(3))(2)S(+)(CH(2))(2)CO(2)(-) have been synthesized and the structures of their protonated salts (CH(3))(2)S(+)CH(2)CO(2)H···Cl(-) [DMSA.HCl] and (CH(3))(2)S(+)(CH(2))(2)CO(2)H···Pcr(-) [DMSP.HPcr] (where Pcr = picrate) have been characterized using X-ray crystallography. The unimolecular chemistry of the [M+H](+) of these betaines was studied using two techniques; collision-induced dissociation (CID) and electron-induced dissociation (EID) in a hybrid linear ion trap Fourier transform ion cyclotron resonance mass spectrometer. Results from the CID study show a richer series of fragmentation reactions for the shorter chain betaine and contrasting main fragmentation pathways. Thus while (CH(3))(2)S(+)(CH(2))(2)CO(2)H fragments via a neighbouring group reaction to generate (CH(3))(2)S(+)H and the neutral lactone as the most abundant fragmentation channel, (CH(3))(2)S(+)CH(2)CO(2)H fragments via a 1,2 elimination reaction to generate CH(3)S(+)=CH(2) as the most abundant fragment ion. To gain insights into these fragmentation reactions, DFT calculations were carried out at the B3LYP/6-311++G(2d,p) level of theory. For (CH(3))(2)S(+)CH(2)CO(2)H, the lowest energy pathway yields CH(3)S(+)=CH(2)via a six-membered transition state. The two fragment ions observed in CID of (CH(3))(2)S(+)(CH(2))(2)CO(2)H are shown to share the same transition state and ion-molecule complex forming either (CH(3))(2)S(+)H or (CH(2))(2)CO(2)H(+). Finally, EID shows a rich and relatively similar fragmentation channels for both protonated betaines, with radical cleavages being observed, including loss of ˙CH(3).
authors.affiliation: School of Chemistry, The University of Melbourne, Victoria, 3010, Australia.
authors.forename: Ellie Jung-Hwa
authors.initials: EJ
authors.lastname: Yoo
authors.forename: Linda
authors.initials: L
authors.lastname: Feketeová
authors.forename: George N
authors.initials: GN
authors.lastname: Khairallah
authors.forename: Jonathan M
authors.initials: JM
authors.lastname: White
authors.forename: Richard A J
authors.initials: RA
authors.lastname: O'Hair
mesh_terms: D001622:Betaine / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D011522:Protons; D013455:Sulfur / Q000737:chemistry*
chemical_list: D011522:Protons; D001622:Betaine; D013455:Sulfur