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Figure 2.10
Surfaces of (a) a second generation tube and (b) a third
generation tube. The available room which can be occupied
by the LC is clearly visible (empty space). Each new
generation is geometrically similar to the previous one. (c)
Microscopic hollow tube observed by SEM after removal of
theLC(pictureis10
µ
wide).
provedasflexibleascyclohexaglycine,
sinceitcouldadoptnumerousdiferentconformations.Wethen
wanted to conceive a ring monomer for which stacking and packing
processes are completely controlled and limited to only one possibility.
The existence of numerous conformations is obviously related to the
flexibilityofthemacrocycle.Themobilityofthebackboneincyclo-
(Gly//Gly)
Thus, cyclo-(Gly//Gly)
3
β
-turn
that induces rectangular and hexagonal geometries, respectively. A
logical way to prevent the
originates from the presence or absence of the
3
β
-turn from appearing is simply to reduce
the size of the ring as in cyclo-(Gly//Gly)
)(Fig.2.11)[65].
In this new 12-membered ring, isosteric to the cyclotetrapeptide
cyclo-Gly
(
2
2
β
-turn is impossible.
The molecule looks like a square whose sides are distorted by
theextremetensionexperiencedbythetransamides(21°from
planarity)aswellasbythealkenes(18°fromplanarity).Another
way to restrict the conformation whilst keeping the same ring
size of 18 atoms in macrolactam (
, strain becomes so intense that a
4
1
) consists in relocating the
E
-alkenes to positions directly adjacent to the three carbonyls.
Thus, carbonyls and
E
-alkenes which were originally independent
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