EFFICIENT TRANSPORT OF AROMATIC-AMINO-ACIDS BY SAPPHYRIN-LASALOCID CONJUGATES

The synthesis and characterization of several sapphyrin-lasalocid conj ugates is reported. This family of receptors is capable of acting as e fficient and selective carriers for aromatic alpha-amino acids, as jud ged from both U-tube and W-tube through-model-membrane transport exper iments. The first member of this family, system 6, was found to displa y an inherent preference for phenylalanine > tryptophan > tyrosine. Fu rther, L-amino acids were shown to be transported with greater efficie ncy than the corresponding D-enantiomers by this particular carrier. T he high level of amino acid carrier capability displayed by receptor 6 in dichloromethane solutions correlates well with the results of equi librium binding studies carried out using visible-spectroscopic titrat ions. These latter studies revealed that system 6 does display signifi cant affinity for zwitterionic amino acids in this organic solvent. Th ese binding studies, as well as a number of control experiments involv ing, inter alia, porphyrin-lasalocid conjugate 7, showed the importanc e 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 carrie rs, while the free-acid compounds 13 and 14 did not. These latter conj ugates, containing both carboxylic acid and sapphyrin subunits, presum ably undergo self-assembly in organic solutions, a process that hamper s their ability to act as effective carriers. In the case of the funct ioning systems 11 and 12, the configuration of the stereogenic phenyla lanine appendages could be varied such that either the L- or D-antipod es of the aromatic amino acid substrates being studied were transporte d al a greater rate.