1 O O' "OCH3

Spectroscopic investigations confirmed that intramolecular hydrogen bonds between the oxygens of the aromatic substituents in 4a and 4b and the neighboring NH protons prevent amide rotation and cause the peptide NH groups to diverge [18]. Since the geometry of the cavity required for cation complexation is not significantly affected by this, both peptides are able to bind cations. The strong increase in BTMA+ complex stability observed for 4a and 4b in comparison with 3 demonstrates the favorable effect of the reduced cyclopeptide flexibility on binding properties (Table 2.2.2). As predicted, the conformational control imposed by the aromatic subunits not only improves cation affinity, it also eliminates the ability of 4a and 4b to interact with anions. This loss is reflected, for example, in the de-

Tab. 2.2.2. Stability constants Ka and maximum chemical shifts A<5max of the BTMA+ complex of 4a and 4b in the presence of different anions (0.2% d6-DMSO/CDCl3; T = 298 K; Ka in M"1; A(5max maximum chemical shift of the BTMA+ N-methyl protons in ppm; error limits for Ka < 20%).
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