The multistage mass spectrometric (MS/MS and MS 3 ) gas-phase fragmentation reactions of methionine side-chain sulfonium ion containing peptides formed by reaction with a series of parasubstituted phenacyl bromide (XBr where X ¼ CH 2 COC 6 H 4 R, and R ¼ -COOH, -COOCH 3 , -H, -CH 3 and -CH 2 CH 3 ) alkylating reagents have been examined in a linear quadrupole ion trap mass spectrometer. MS/MS of the singly (M R ) and multiply) charged precursor ions results in exclusive dissociation at the fixed charge containing side chain, independently of the amino acid composition and precursor ion charge state (i.e., proton mobility). However, loss of the methylphenacyl sulfide side-chain fragment as a neutral versus charged (protonated) species was observed to be highly dependent on the proton mobility of the precursor ion, and the identity of the phenacyl group para-substituent. Molecular orbital calculations were performed at the B3LYP/ 6-31RGÃÃ level of theory to calculate the theoretical proton affinities of the neutral side-chain fragments. The log of the ratio of neutral versus protonated side-chain fragment losses from the derivatized side chain were found to exhibit a linear dependence on the proton affinity of the side-chain fragmentation product, as well as the proton affinities of the peptide product ions. R2H]3R precursor ions, respectively, from the peptide GAILM(X)GAILK revealed significant differences in the abundances of the resultant product ions. These results suggest that the protonated peptide product ions formed by gas-phase fragmentation of sulfonium ion containing precursors in an ion trap mass spectrometer do not necessarily undergo intramolecular proton 'scrambling' prior to their further dissociation, in contrast to that previously demonstrated for peptide ions introduced by external ionization sources.

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A 29-month-old boy presented to a pediatric emergency department with complaints of trouble sleeping for more than a week. History consisted of episodes of screaming while asleep from which he could not be awakened. A detailed physical examination revealed left arm dystonia and left plantar reflex to be upgoing. Upon admission, all imaging and an electroencephalogram were normal. Extensive laboratory work was done showing positive anti-N-methyl-D-Aspartate (NMDA) antibody in the cerebrospinal fluid. Inpatient care included intravenous immunoglobulin (IVIG) and Solumedrol. Cellcept was started after definitive diagnosis and continued on discharge. The patient was discharged with residual defects that will need long-term therapy. The varied presenting symptoms are easily misinterpreted as common clinical entities. Pediatric emergency physicians need to be aware of the wide spectrum of presenting symptoms for this clinical entity because earlier diagnosis and treatment have been shown to improve long-term morbidity.