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24
F
G
= 0.57 simply corresponds to 57% G and 43% M. Hence, F
G
= 1 – F
M
. M
and G are sometimes referred to as monads. The reason will soon become
apparent.
However, F
M
and F
G
say nothing about the sequence, as illustrated by the
three different alginate fragments shown below. They all contain the same
number of G residues (13) but are distributed in different ways.
MMM
G
M
GGGGGG
MMMMMM
G
M
G
M
GGG
M
G
MMMM
F
G
= 13/30
G
MMM
G
MMM
G
M
GGGGG
M
G
MMMMM
G
MM
GG
MM
G
F
G
= 13/30
M
G
MM
G
M
G
M
G
M
G
M
G
M
G
MMM
G
M
G
M
G
M
G
M
G
M
G
M
F
G
= 13/30
However, sequence information can be obtained – to a certain extent – by
means of frequencies (or fractions) of
diads
. Alginates contain 4 diads:
•
MM (M followed by another M)
•
MG (M followed by a G)
•
GM (etc)
•
GG (etc)
Since all M and G residues are either followed by an M or a G, and we
necessarily (by definition) have:
ž
F
M
=
F
MM
+
F
MG
F
G
=
F
GM
+
F
GG
Moreover, F
MG
= F
GM
for long chains. Can you see why? (Hint: Use figure
above, top sequence).
Diads can be easily be determined by
1
H-NMR. With a 300 MHz instrument
(or better) we also obtain information about triads.
There are 8
triads
in alginates:
GGG, GGM, GMG, GMM, MMM, MMG, MGM, MGG
With their corresponding triad frequencies given by: