Biology Reference
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Two structural aspects should be noted. First, the stereochemistry
of AS1 arrangement is similar to that reported for
C 4
-symmetric
8 .
+
octamer LG
, suggesting that the octamer presumably represents
the first step of aggregation. Second, the symmetry of this polymeric
assembly is different from that observed for the polymeric
supramolecular structure of 5
K
-GMP in water [17]. In the latter case,
owing to the head-to-tail orientation of all the contiguous G-quartets,
the polymer has
symmetry. The stereochemical regularity of
these columnar polymeric G-aggregates is truly amazing as far as
the rotamers around the glycosidic bond within each G-quartet, the
repeated sequence of the G-quartets along the columns, and their
relative stacking orientation are concerned.
C 4
+
While derivatives of type
1a
and
1c
usually form the K
-
templated
-symmetric octameric structure and, in case, the
polymeric assembly (LG
C 4
4 .
+
M
)
described above in solution, lipophilic
n
guanosines of type
1b
prefer to give a different stereoregular
octamer with a
symmetry [18,34,36]. In each octamer, all
eight guanosine molecules are in a
D 4
conformation and the two
G-quartets are stacked in a tail-to-tail or head-to-head fashion (Fig.
4.6c). Once confirmed that the assembly has an octameric structure
(e.g., by UV-vis, DOSY, or
syn
1
H-NMR spectroscopy) it is relatively easy
to assign its symmetry to the
point group: in fact, while
in the former case two sets of signals are observed in the
C 4
or
D 4
1
H-NMR
spectrum (the two G-quartets are diastereotopic), in the latter
case only a single set of signals is observed for the two homotopic
G-quartets. Furthermore, CD is diagnostic of the stacking polarity
of two contiguous G-quartets [19]. Similar to base pairs in DNA, the
tetramers do not stack in register but are rotated with respect to
each other to give, in the 230-300 nm region characteristic of the
π
π
* transitions of guanine chromophore, a double-signed exciton-
like CD signal. This couplet, the sign of which allows the assignment
of the stacking helicity (handedness), exhibits opposite signed
bands at about 260 and 240 nm for the head-to-tail (
-
-symmetric)
stacking, while both bands are blue-shifted by 20-30 nm in the
C 4
D 4
-
symmetric stacking (Fig. 4.9).
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