Acylation events preferentially occur at basic sites, such as the amino (N terminus, Lys) or guanidino (Arg) functions. Myristoylation usually occurs at the N terminus of proteins and is thought to support their localization at membranes [65]; for example, a variety G-proteins and kinases [66] are covalently modified at their N terminus, which then starts with a myrG unit. N-terminal acylation results in the formation of b1 ions, which cannot normally be formed, since b ion formation proceeds mainly by a cyclic mechanism requiring a carbonyl function on the N-terminal side of the pep-tide bond to be cleaved. With an N-terminal myrG residue, the b1 ion is found at m/z 268, and this characteristic ion is accompanied by two additional fragments at m/z 240 (a1) and m/z 211 (myristoyl acyl ion). By this characteristic triplet of marker ions, an N-terminal myrG structure can be reliably recognized, as shown in Figure 6.15 for the N-terminal peptide of a Golgi-associated protein [67].

Recording the MS/MS data at high resolution, as performed for the analysis shown in Figure 6.15, facilitates discrimination of the myrG-specific fragment ions

Fig. 6.15 NanoESI-MS/MS spectrum of the C-terminal peptide of a Golgi-associated protein. The myristoylation of this peptide is recognized according to the marker ion triplet at m/z 211, 240, and 268 [67].

from other peptide fragment ions in this mass region, since their accurate mass values show a significant high-mass shift due to the unusually high number of hydrogen atoms present.

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