Molecular Recognition of Aromatic and Aliphatic Amino Acids

Molecular Recognition of Aromatic and Aliphatic Amino Acids 

About one half of the twenty common amino acids were selected in this molecular recognition study, and several criteria were considered in the selection process (1) solubility in H2O (2) representativeness and (3) stability of the hosts in the presence of the amino acids. According to these criteria, tyrosine, cysteine, and methionine were excluded, since the first example is not soluble in H2O, and the latter two apparently caused slight decomposition of the hosts. The structures of the key aromatic and aliphatic amino acids, aromatic and aliphatic carboxylic acids, and di- and tripeptides are shown in Chart 10.1, along with the relevant proton designations that were affected by host 1 [4a, b]. The pfQ values of these amino acids are indicative of the zwitterion forms being the predominant species at pH 7 [4a, b, 7]. 

Chart 10.1 Guests used with host 1 in molecular recognition studies. 

The molecular recognition process of these different amino acid guests with host 1 was studied using H NMR spectroscopy at ambient temperature [4a,b]. 

For nucleoside host 1, the role of the hydroxyl groups on the ribose may be understood by comparing their interactions with guests, L-Phe and L-Trp. Host 1 has one OH group per each ribose unit and therefore, at the opening of this host cavity, the hydrophobicity increases a delicate balance of hydrophobicity and hydrophilicity exists. More importantly, the steric hindrance on the sugar of the nucleotide can decrease the non ovalent interactions with guests. For example. 

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Recently, Fish et al. (4) reported on the molecular recognition of aromatic and aliphatic amino acid and aromatic and aliphatic carboxylic acid guests with supramolecular Cp Rh-nucleotide cyclic trimer hosts, such as 1, in aqueous solution at pH 7 (4a-d). 

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