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In practice, DP
n
values up to about 50 can be determined. Longer chains have
correspondingly smaller end group signals, the area of which cannot be
accurately determined.
Note that this value is calculated for the degraded sample analyzed by NMR,
not the parent sample (unless a low molecular weight alginate was made on
purpose).
1.2.12. Studying alginate
structure and epimerization by NMR
In this example the epimerization was carried out directly in the NMR tube,
and the process could be followed in real time (directly in the NMR tube while
recording spectra) as Ms were converted to Gs. The starting sample was
(almost pure) mannuronan (F
G
= 0.07, F
M
= 0.93), and the epimerase was
AlgE4.
Figure 16
AlgE4 introduces M-G-M-G-M-G-M-G-M-G- (alternating) sequences. We
observe (among other):
•
The G (H-1) peak increases
•
The MM peak (H-1) decrease (some Ms loose its neighbouring M)
•
The MG peak increases accordingly
A paradox is apparently observed: The peak corresponding to MGM does not
increase. This is due to the fact that we perform the reaction in D
2
O, and a