Efficient Transport of Aromatic Amino Acids by Sapphyrin-Lasalocid Conjugates
摘要:
The synthesis and characterization of several sapphyrin-lasalocid conjugates is reported. This family of receptors is capable of acting as efficient and selective carriers for aromatic alpha-amino acids, as judged from both U-tube and W-tube through-model-membrane transport experiments. The first member of this family, system 6, was found to display an inherent preference for phenylalanine > tryptophan > tyrosine. Further, L-amino acids were shown to be transported with greater efficiency than the corresponding D-enantiomers by this particular carrier. The high level of amino acid carrier capability displayed by receptor 6 in dichloromethane solutions correlates well with the results of equilibrium binding studies carried out using visible-spectroscopic titrations. These latter studies revealed that system 6 does display significant affinity for zwitterionic amino acids in this organic solvent. These binding studies, as well as a number of control experiments involving, inter alia, porphyrin-lasalocid conjugate 7, showed the importance of having both the sapphyrin and lasalocid subunits contained within the same overall receptor framework. The four other second-generation sapphyrin-lasalocid conjugates reported here (11-14) were also tested as carriers for the transport of Phe, Trp, and Tyr. It was found that the esterified systems 11 and 12 functioned well as amino acid carriers, while the free-acid compounds 13 and 14 did not. These latter conjugates, containing both carboxylic acid and sapphyrin subunits, presumably undergo self-assembly in organic solutions, a process that hampers their ability to act as effective carriers. In the case of the functioning systems 11 and 12, the configuration of the stereogenic phenylalanine appendages could be varied such that either the L- or D-antipodes of the aromatic amino acid substrates being studied were transported al a greater rate.