Zwitterionic Phospholipids Sphingomyelin, Phosphatidylcholine, and Phosphatidylethanolamine

These PLs represent the most abundant apolar constituents of the membranes of animal and human cells. Accordingly, the majority of MALDl-TOF investigations are aimed at these PL classes, with special emphasis on phosphatidylcholine (PC) that can be found in all cellular and tissue extracts in huge amounts. 

As noted above, PC is the by far most frequently investigated phospholipid [40, 82, 83], and consequently was one of the first PLs to be studied using MALDI-TOF-MS [46, 75]. As the pioneer in this field, Harvey [46] investigated, as early as 1995, a variety of PLs to determine which matrix would provide the best spectral quality. Among other matrices, a-cyano-4-hydroxycinnamic acid, 6,7-dihydroxy-coumarin and DH B gave the best results. Today, DHB is the most frequently used matrix for lipid analysis by MALDI-MS, and is regarded as the work horse of the field because essentially all Hpids-irrespective of their structure-are detectable with DH B. A comprehensive list of usefijl matrices for the lipid field is available in Refs [41] and [84]. 

It should be noted that, besides some DHB-derived cluster ions (marked with asterisks [86]), there are also peaks at higher m/z values. These are most pro-noimced in the negative-ion spectrum, and correspond to cluster ions of DHB with the analyte. As one selected example, the peak at m/z 897.5 may be explained by the addition of the sodium salt of DHB (176.0 amu) to the negatively charged PG (m/z 721.5). Misinterpretations due to the presence of matrix/analyte clusters can be easily avoided by either (i) using a different matrix in order to check if the peak is real or (ii) recording an MS/MS spectrum to check whether there is a 

Unfortunately, potential applications regarding mixture analysis are impaired by (i) the overlap between peaks resulting from differences in the fatty acyl composition and (ii) the superposition of H and Na adducts. For instance, the Na adduct of PC 16 0/18 1 and the H adduct of PC 16 0/20 4 result in the same m/z ratio. An initial attempt to overcome this problem involved digesting the PC with phospholipase C (PLC), an enzyme that generates the corresponding DAGs 

Recently, PNA was suggested as the matrix of choice for PL mixture analysis [42]. PNA has the advantage that PE becomes detectable as a negative ion, and since PC is not detectable in the presence of PNA as a negative ion, then problems arising from the suppression of PE by the presence of PC can be overcome [42]. 

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