Leaving group and gas phase neighboring group effects in the side chain losses from protonated serine and its derivatives

The gas phase fragmentation reactions of protonated serine and its YNHCH(CH 2X)CO2H derivatives, beta -chloroalanine, S-methyl cysteine, O-methyl serin e, and O-phosphoserine, as well as the corresponding N-acetyl model peptide s have been examined via electrospray ionization tandem mass spectrometry ( MS/MS). In particular, the competition between losses from the side chain a nd the combined loss of H2O and CO from the C-terminal carboxyl group of th e amino adds or H2O or CH2CO from the N-acetyl model peptides are compared. In this manner the effect of the leaving group (Y = H or CH3CO, vary X) or of the neighboring group can be examined. It was found that the amount of HX lost from the side chain increases with the proton affinity of X [OP(O)( OH)(2) > OCH3 approximate to OH > Cl]. The ion due to the side chain loss o f H2O from the model peptide N-acetyl serine is more abundant than that fro m protonated serine, suggesting that the N-acetyl group is a better neighbo ring group than the amino group. Ab initio calculations at the MP2(FC)/6-31 G*//HF/6-31G* level of theory suggest that this effect is due to the transi tion state barrier for water loss from protonated N-acetyl serine being low er than that for protonated serine. The mechanism for side chain loss has b een examined using MS3 tandem mass spectrometry, independent synthesis of p roposed product ion structures combined with MS/MS, and hydrogen/deuterium exchange. Neighboring group rather than cis 1,2 elimination processes domin ate in all cases. In particular, the loss of H3PO4 from O-phosphoserine and N-acetyl O-phosphoserine is shown to yield a 3-membered aziridine ring and 5-membered oxazoline ring, respectively, and not the dehydroalanine moiety . This is in contrast to results presented by DeGnore and Qin (J. Am. Sec. Mass Spectrom. 1998, 9, 1175-1188) for the loss of H3PO4 from larger peptid es, where dehydroalanine was observed. Alternate mechanisms to cis 1,2 elim ination, for the formation of dehydroalanine in larger phosphoserine or pho sphothreonine containing peptides, are proposed. (J Am Soc Mass Spectrom 20 00, 11, 1047-1060) (C) 2000 American Society for Mass Spectrometry.