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69
Chair forms: All linkages are diequatorial:
D-Gal:
4
C
1
just like glucose (axial OH(4) does not destabilise sufficiently to flip
over to
1
C
4
)
3,6-anhydro-D-Gal: The
4
C
1
turns out to be sterically impossible, so in this
case
1
C
4
is the only alternative
3,6-anhydro-L-Gal: L-Gal would obviously be
1
C
4
, but again the internal 3,6
ring forces the sugar over to
4
C
1
.
These ring geometries, combined with the
α
/
β
configurations lead to the fact
that all linkages in carrageenans and agar are diequatorial.
O
O
H
2
C
O
OH
H
O
OSO
3
HOH
2
C
OH
κ−
carrageenan
O
O
H
2
C
O
OSO
3
H
O
OSO
3
HOH
2
C
OH
ι−
carrageenan
O
O
OH
HOH
2
C
OH
O
O
O
CH
2
OH
Agarose
1
C
4
4
C
1
4
C
1
4
C
1
4
C
1
1
C
4
3,6-anhydro-
α
-D-Gal
β
-D-Gal-4 sulphate
3,6-anhydro-
α
-D-Gal-2-sulphate
β
-D-Gal-4 sulphate
3,6-anhydro-
α
-L-Gal
β
-D-Gal