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separating peptide bond isomers, especially if an aromatic amino acid is Nterminal bonded to the proline residue. These results are in agreement with other
findings which describe the high selectivity of (3-CD phases for analytes with an
aromatic ring system in the vicinity of the chiral, geometrical or structural center
(Ward and Armstrong 1988).
The high resolution power of (3-CD bonded silicas for the conformer separation of proline containing peptides was demonstrated for oligopeptides which
include two Xaa-Pro bonds. That means, theoretically four cis/trans isomers
could be detected. As shown in Fig. 14.1. we really succeeded in separating these
peptide isomers.
2.3
(alix[n]arenes
Calix[nJarenes are cyclic oligomeric condensates of phenols and formaldehyde.
As calixarenes have a cavity-shaped architecture, they are frequently used as
building blocks for host-guest receptors in supramolecular chemistry (Gutsche
1989, Vicens and Bohmer 1991, Bohmer 1995). Based on molecular recognition
the use of calixarenes as selectors in chromatography for the study of cis/trans
isomer separations of proline containing dipeptides seems to be possible (Friebe
et al. 1995). More recently, we introduced a series of new calix[nJarene bonded
stationary phases (n = 4, 5, 6, 8) for HPLC based on silica gel (Gebauer et al.
1998a, 1998b) and called these supports Si[nJArenes. Applying compounds of
p- tcrt - Bllty lc~ lixl"'larcllr
r -tcrt-Bu tylcalix l p-tert-Hu t}'IC:llix IXllire nc
n.J ,: 0. 76 11m
a
~ .J ' 1. 17 n lll:;]
Fig. 14.2. Molecular dimensions ofp-tert-butylcaIix[nlarenes (n = 4, 6, 8) (Vicens and Bohmer 1991)