MALDI-induced fragmentation of leucine enkephalin, nitro-Tyr-leucine enkaphalin, and d5-Phe-nitro-Tyr-leucine enkephalin

The long-term objective of this study is to use MALDI MS and MS/MS to study the fragmentation pattern of in vitro nitrotyrosine-containing peptides in order to assist the interpretation of MS-identification of endogenous nitroproteins in human tissues and fluids. The short-term objective is to study synthetic leucine enkephalin, nitro-Tyr-leucine enkephalin, and d5-Phe-nitro-Tyr-leucine enkephalin with a vacuum matrix-assisted laser desorption/ionization linear ion-trap mass spectrometer (vMALDI-LTQ). The results demonstrated the UV laser-induced photochemical decomposition of the nitro group. Although photochemical decomposition decreased the ion intensity and complicated the MS spectrum, the recognition of that unique decomposition pattern unambiguously identified a nitrotyrosine. The a4- and b4-ions were the most intense fragment ions found in the MS/MS spectra for those three synthetic peptides. Compared to the unmodified peptides, more collision energy optimized the fragmentation of the nitropeptide, increased the intensity of the a4-ion, and decreased the intensity of the b4-ion. Optimized laser fluence maximized the fragmentation of the nitropeptide. MS3 analysis confirmed the MS2-derived amino acid sequence, but required much more sample. To detect a nitropeptide, the sensitivity of vMALDI-LTQ is 1 fmol for MS detection and 10 fmol for MS2 detection; the S/N ratio was ca. 50:1 in those studies. Those data are important for an analysis of low-abundance endogenous nitroproteins, where preferential enrichment of nitroproteins and optimized mass spectrometry parameters are used.