The Self-Assembly of Lipophilic Guanosine Derivatives - Molecular Self-Assembly: Advances and Applications - page 99

It should be noted that there is another stereochemical
consequence to the cation-templated self-assembly of guanosine
derivatives. The two faces of the G-quartets are diasterotopic and
can be labeled head (H) and tail (T) (see caption to Fig. 4.6 for a
definition of these descriptors). In principle, the two quartets in
the octamer can be arranged in three different orientations: H-to-T
( C 4 symmetry, homopolar stacking, see Fig. 4.6b), H-to-H and T-to-T

( symmetry, heteropolar stacking, see Fig. 4.6c, where the H-to-H

arrangement is shown).

D 4

Figure 4.6 (a) A G-quartet showing its head (H) diastereotopic face

(clockwise rotation on going from H-bond donors to
acceptors). Schematic drawing of (b) a

C 4 -symmetric and

(c) a -symmetric octamer, a

G-quartet with its tail side (lower face) stacks on the head
side (upper face) of the other G-quartet. In the

D 4 -symmetric octamer. In the C 4

-symmetric
octamer two G-quartets stack facing their head sides (H-to-H)
or tail sides (T-to-T, not shown). The head and tail sides of
each quartet in the octamers are colored in red and green,
respectively.

D 4

Therefore, considering these two stereochemical aspects ( syn /

anti glycosydic conformation and relative G-quartets orientation)

several diastereoisomers are possible for the octamer LG 8 .

+

.
Remarkably, NMR data indicated that this octamer was a single
diastereomer of

K

C 4 symmetry. In one G-quartet, all monomers had

a ”

conformation. Nuclear-Overhauser-effect (NOE) interactions
indicated a relative orientation with the head side of the “all-

syn conformation, while the other tetramer had an “all- anti

anti ”

G-quartet facing the tail side of the “all- syn ” G-quartet (Fig. 4.7).