Answer 11.4

The following MALDI spectrum has been obtained from a water-soluble biopolymer in onions. It has been acquired in the positive-ion mode and 2,5-DHB was used as the matrix. The isotopic pattern is not resolved.

MALDI spectrum of a biopolymer

a) What kind of biopolymer do you expect? Use the mass difference between peaks to identify the compound class.

There are two series of peaks, one starting at m/z 527.2 the other at 543.1. The peaks of each series are separated by 162 u which is typical for oligosaccharides and corresponds to the C6H10O5 monomer units.

b) Identify what kind of ionic species are occurring here.

Oligossaccharides preferably form [M+alkali]+ ions. Therefore, the difference of 16 u between peaks of either series is most probably due to the mass difference between [M+Na]+ and [M+K]+ ions (cf. Downloads area).

c) Now you can also try to identify the endgroups.

Stepwise subtraction of 162 u, e.g., three times in case of m/z 543.1, leads to a rest of 57.1 u, 39 u of which are due to the K+ ion. The remaining 18.1 u are in good accordance with the formula weight of H2O, i.e., we simply have H on one and OH on the other end of the carbohydrate chain.
The spectrum shows the fructans present in onions (cf. Anal. Biochem. 1997, 246, 195-204).