Permethylation fragmentation

This C-13 n.m.r. analytical technique has been referenced against 27 dextrans which had been studied by periodate-oxidation and permethylation fragmentation g.l.c.-m.s. techniques (14). 

In 1967 E. Lederer (Plate 28) and B.C. Das reported [32] that the volatility of acetyl-peptide esters can be increased by replacing all dissociable protons with methyl groups. This permethylation can be performed with methyl iodide in anhydrous dimethylsulfoxide in the presence of a strong base. In iV-methyl peptides the coherence caused by hydrogen bonds is overcome. Moreover, the fragmentation patterns are much simpler than the patterns found in N-acetyl peptide esters without permethylation. Fragmentation occurs almost exclusively at the peptide bonds and the mass spectra consist mainly of sequence peaks of high intensity (Fig. 14). 

The fact that only ethylene and tetramethylethylene are evolved from exp-[8]rotane 168 and permethyl-exp-[6]rotane 173 upon thermal decomposition leads to the conclusion that the spirocyclopropane moieties in these expanded [n]rotanes fragment only externally and leave carbene moieties behind. Indeed, the MALDI-TOF mass spectra of several exp-[ ]rotanes show fragment ions with M minus 28. Thus, if this fragmentation in an exp-[n]rotane were to continue n times, a cyclic C carbon cluster would be left over. So far, however, a fragment ion with m/z = 480 corresponding to 182 has not been recorded in the mass spectrum of exp-[8]rotane 168 and it remains to be seen whether a Cgo cluster 183 will be detected in the mass spectrum of exp-[12]rotane 171 (Scheme 35). 

Fig. 11.—Sequence of a 2S-Sugar Residue Glycosphingolipid Isolated from Rabbit Erythrocyte Membranes. (Cleavage points, and the masses of fragment ions of the permethylated derivative, are shown. No fragment-ions were observed above 4000, because of the poor sensitivity at high mass.)...

Fig. 13.—Predicted Sequence of the Smith-Degradation Product of the Glycosphingolipid Shown in Fig. 12. (Predicted Fragment-ions for the permethylated derivative are shown.)...

Fig. 14.—Schematic Representation of the Fragmentation Observed in the Positive F.a.b.-Mass Spectrum of a Permethylated Ganglioside Isolated from Granulocytes. [Other glyco-sphingolipids fragment in a similar way. Major cleavages are shown with solid lines, and minor cleavages with dotted lines. The masses of ions resulting from cleavages (a), (b), and (c) define the type of sphingosine and the type of fatty acid. In this example, (a) is 548, (b) is [M + H] minus 238, and (c) is [M + H] minus 533.]...

As expected, the spectrum of the 6-deoxyhexose (23) contains no peaks of the E series however, the B series appears (m/e 176, 131), the peaks of the F, G, H, and J series have the same intensities and positions as with permethylated methyl hexosides, and the peaks of fragments involving C-5 and C-6 are shifted 30 mass units to lower m/e values compared with the peaks of permethylated methyl hexosides (Di, m/e 119 C2, m/e 129, and so on). 

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