calculated from its sequence as 1295.69. The experimentally derived values are,
for the singly charged ion, [(MþH)/1]¼1296.65 and for the doubly charged ion,
[(Mþ2H)/2]¼648.82.
For elements such as chlorine, the isotopic abundance is approximately 3 : 1,^35 Cl :(^37) Cl. If a compound contains a single chlorine atom, two ion species will be observed,
with peak intensities in an approximate ratio of 3 : 1. If a compound contains two
chlorine atoms then three peaks will be seen.
The technique is particularly useful for determining which are the high-energy
doubly charged tryptic peptides, for tandem MS.
9.5.5 Post-translational modification of proteins
Many chemically distinct types of post-translational modification of proteins are
known to occur. These include the wide variety of acylations at the N-terminus of
proteins (mentioned above) as well as acylations at the C-terminus and at internal
sites. In this section, examples of the application of MS techniques employed for
analysis of glycosylation, phosphorylation and disulphide bonds are given.
An up-to-date list of the broad chemical diversity of known modifications and
the side chains of the amino acids to which they are attached is on the website
Example 5PEPTIDE SEQUENCING (II)
Question Determine the primary structure of the oligopeptide that gave the following, positive
mode, MS–MS data:
m/z 149 305 442 529 617
Use the amino acid residual mass values in Table 9.2.
Answer m/z¼ 617 (MþH)þ
m/z 149 305 442 529 616
D 156 137 87 87
aa Arg His Ser Ser
Conventional order for the sequence would be:
Ser-Ser-His-Arg-?
It is important to note that no assignment has been given for the remainingm/z¼149. It
may not in fact be a sequence ion and moreinformation would be required, such as an
accurate molecular mass of the oligopeptide, in order to proceed further. It is, however,
possible to speculate as to the nature of this ion. If them/z¼149 ion is the C-terminal
amino acid then it would end in -OH and be17 mass units greater than the corresponding
residue mass. The difference between 149and 17 is 132, which is extremely close to
methionine, so this amino acid remains a possibility to end the chain.
386 Mass spectrometric techniques