3.3.3. Example: Complexes with Ala

Let us focus on changes in hydroxy-group bond lengths in selected complexes. An exemplary case is shown for the complexes with Ala in Figure 3. For the *wc* and *al* cases, there is one O-H bond longer than 1 Å (for both *al*-CX and *al*-BCX, and for *wc*-CX) and in both cases, this O-H bond seems to be weakened by the formation of the H-bond with the NH2 group of amino acids. For the *al*-BCX there is also a second O-H bond longer than 1 Å, and again one can see that this elongation results from the involvement of the network of the H-bonds (this time with the second OH group). The O··· H distance, in this case, is 0.1 Å smaller than usual. No O-H bond is significantly elongated in the *wc*-BCX, although the OH-3 group does interact with the amino group of Ala. One can see, however, that the H··· N bond is longer than usual for this intermolecular H-bond. Interestingly, there is another intermolecular H-bond in this case which involves the hydroxy group of Ala and the O-6 atom of calixarene. For the *pc*-CX one has an H-bond with the amino group from Ala, too, but in this case, the H··· N binding is by about 0.2 Å longer than for the *al* or *wc* cases. One should keep in mind that for the pristine *pc* conformer there is a network of six H-bonds that virtually close up the top part of the calixarene, but the Ala amino acid is able to break this network for the CX case. Interestingly, on the other side of the calixarene cavity, there is one H-bond which becomes shorter by about 0.1 Å in comparison to the uncomplexed case (to 1.6 Å), and the involved O-H bond becomes somewhat elongated (to 1.0 Å). Finally, for the case of the *pc*-BCX the network of the H-bonds remains unharmed by the interaction with Ala. In this case, the amino group is not directly involved in the interaction with calixarene.

When considering all the cases, it turns out that for all the *wc*-CX complexes there is one hydroxy group that becomes significantly elongated after the complexation (e.g., to 1.02 Å for Ala or 1.04 Å for Asn). This length modification should be, in principle, detectable in the IR spectrum. The elongated O-H bond points to the nitrogen atom from the amino acid group, therefore it creates an intermolecular H-bond with the hydroxy group donating its hydrogen. The N··· H distance for this H-bond is quite small (1.6 Å) as for the intermolecular interaction, indicating its strong character.

**Figure 3.** Selected bond lengths for O-H and H-bonds for the case of complexes with Ala. Hydrogen atoms of calixarenes are removed for a better view.
