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Figure 2.24 (a) Boc-L-Phe-D-Oxd-OBn after slow evaporation of a 1 : 1 mixture of cyclohex-
ane/ ethyl acetate. (b) SEM image of filament bundles of Boc-L-Phe-D-Oxd-OBn. Scale bar:
20 mm. Reprinted with permission from Ref. [71b]. Copyright 2003 WILEY-VCH Verlag
GmbH & Co. KGaA, Weinheim.
A solid fiberlike material (Figure 2.24) was obtained after slow solvent evaporation of a
20-25mM solution of Boc- L -Phe- D -Oxd-OBn in a 1 : 1 mixture of cyclohexane/ethyl ace-
tate [71]. This material shows strong birefringence and has defined edges. It is made of bun-
dles of crystalline-shaped filaments clustered and aligned along the main bundle direction.
Crystals suitable for X-ray diffraction study were grown by slow evaporation of a solu-
tion of Boc- L -Phe- D -Oxd-OBn in diethyl ether at room temperature. Interestingly, linear
chains are formed in the crystal packing of Boc- L -Phe- D -Oxd-OBn by only one inter-
molecular hydrogen bond between the units; thus the solid-state structure of Boc- L -Phe-
D -Oxd-OBn can be considered as a borderline case of a parallel b-sheet structure, as this
fiberlike material is stabilized only by single hydrogen bonds between dipeptide units.
Another interesting outcome in the formation of supramolecular materials is the self-
assembly of the two epimers, Boc- L -Phe- D -Oxd-( S )-b
3 -hPhg-OBn and Boc- L -Phe- D -Oxd-
3 -hPhg-OBn [72]. Both of these molecules contain the scaffold L -Phe- D -Oxd, but
differ by the introduction of the two enantiomers of b
( R )-b
3 -homophenylglycine (Figure 2.25).
The reversal of the stereogenic center contained in this moiety has important effects on
the preferred molecular conformation and, as a result, on the crystal morphology.
A different aggregation in the solid state is expected for the two epimers, because one
tends to form a g-turn structure in solution, while the other does not, as demonstrated by
IR and 1 H NMR spectroscopy. Therefore an analysis of the two compounds has been car-
ried out by means of microscopy and X-ray diffraction. The two compounds show a dif-
ferent morphology by optical and electronic microscopy, as reported in Figure 2.26.
By means of X-ray diffraction analysis, we could demonstrate that, in Boc- L -Phe- D -
Oxd-( S )-b
3 - h Phg-OBn crystals, each molecule is engaged in two intermolecular hydro-
gen bonds with the two neighbors, but the two interactions are not equivalent since one is
O
Ph
O
(b)
O
Ph
O
O
O
(a)
N
N
O
O
H
OBn
H
OBn
N
N
O
O
O
O
O
O
Ph
Ph
3
- h Phg-OBn and (b) Boc- L -Phe-
Figure 2.25 Chemical structure of: (a) Boc- L -Phe- D -Oxd-( S )-b
3
D -Oxd-( R )-b
- h Phg-OBn.
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