Tryptophan receptors containing acridine-based thiourea

Four derivatives of acridine and acridinium compounds (L1, L2, L1H and L2H) comprised thiourea-binding sites were synthesised. The binding abilities of receptors L1, L2, L1H and L2H towards amino acids (L-Trp, L-Phe, L-Leu, L-Ala and L-Gly) were studied by 1H NMR spectroscopy, UV-vis and fluorescence spectrophotometry. Hydrogen bonding interactions between thiourea-binding site of the ligand and the carboxylate groups in zwitterionic amino acids were found to be the main interactions driving complexation to take place. The stoichiometry of 1:1 ligand to amino acid was observed in all cases. Neutral ligands L1 and L2 showed weak binding towards all studied amino acids. The cyclic ligand L1 showed better binding ability towards tryptophan (Trp) than the acyclic ligand L2 did (K for Trp is 307 and 266M(-1) for L1 and L2, respectively). Interestingly, binding abilities of the protonated ligands, L1H and L2H, towards studied amino acids, especially Trp (K for Trp is 3157 and 2873M(-1) for L1H and L2H, respectively), were increased due to R-COO- center dot center dot center dot H center dot center dot center dot N+-acridinium interactions. Calculated structures of L1H center dot Trp and L2H center dot Trp showed that the polyglycol moiety in L1H provided a hydrophobic cavity for binding Trp resulting in a stronger binding affinity of L1H over L2H.