Acridine- and steroid-based amino acid receptors and sensors

Abstract

Four derivertized acridine and acridinium compounds (L1, L2, L1H and L2H) comprised of thiourea binding site were synthesized, Binding abilities of receptors L1, L2, L1H and L2H toward amino acids (Trp. Phe, Leu, Ala and Gly) were studied by 1H-NMR, UV-vis and fluorescence spectrophotometry. Hydrogen bonding interactions between thiourea binding site of the ligand and carboxylate groups in zwitterionic amino acids were found to be main interactions driving complexation to take place. Neutral ligands L1 and L2 showed weak binding (K for Trp = 266 M-1 and 307 M-1 for L1 and L2, respectively) due to repulsion between the host and the guest. The cyclic ligand L2 showed selective binding with aromatic amino acids while the acyclic ligand L1 did not. The stiochiometry of 1:1 complex was observed. In case of L1H and L2H (the protonated ligands), all results revealed the movement of the NH*-proton toward the carboxylate group of the amino acids. A large enhancement of emission band upon amino acid addition, especially in Trp suggested that the protonation of N-acridine could improve the binding ability (K for Trp = 2873 M-1 and 3157 M-1 for L1H and L2H, respectively) of the ligand toward amino acid due to a complementary from electrostatic forces. A steroidal compound L3 comprised of urea binding site at C3, C7 and C12 positions processing an intrinsic chiral structure was synthesized to investigate binding abilities for amino acids (Trp, Phe, Leu, Ala and Gly). Binding studies of L3 toward both L-and D-forms of amino acids were carried out using ‘H-NMR spectroscopy, UV-vis spectrophotometry and modeling studies. Changes in ‘H-NMR spectra of L3 revealed that the complexes of L3 and amino acids occurred via hydrogen bond interactions (between urea groups and carboxylate unit) and π-π stacking interactions (between phenyl rings of a receptor and amino acid side chains). Results from all techniques exhibited the preference of L3 toward L-Trp (log K=4.54) over D-Trp (log K=2.08) and other amino acids. The preference over L-form amino acids, therefore, diminished in cases of the less bulky-side-chain amino acids (Phe, Leu and Ala).