Biomimetic taste receptors with chiral recognition by photoluminescent metal-organic frameworks chelated with polyaniline helices

The adsorption of phenylaniline (Phe) enantiomers on (+)-polyaniline (PAN)-chelated [In(OH)(bdc)]_n microcrystals was carefully designed and studied by using the Job titration, circular dichroism, X-ray photoelectron spectroscopy, and photoluminescence to mimic heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors in selective, but not specific, ligand binding with chiral recognition and signal transduction. Six essential working principles across different length scales are unraveled: 1) a chiral (+)-PAN (host), 2) specific sites for Phe-(+)/PAN (guest-host) binding, 3) a conformational change of (+)-PAN after binding with Phe enantiomers, 4) different degrees of packing for (+)-PAN, 5) interactions between (+)-PAN and the underlying signal-generating framework (i.e., [In(OH)(bdc)]_n microcrystals), and 6) a systematic photoluminescent signal combination by using principal-component analysis from the other three polymer-chelated metal-organic frameworkds (MOFs), such as poly(acrylic acid) (PAA), sodium alginate (SA), and polyvinylpyrrolidone (PVP) to enhance the selectivity and discrimination capabilities. A question of taste: Helices of (+)-polyaniline (PAN) with five to six nitrogen atoms per dopant acid (+)-CSA per turn are chelated on [In(OH)bdc]_n needlelike microcrystals. Detailed studies based on Job titrations and CD and XPS analysis reveal essential factors that are needed to transform ligand discrimination and chiral recognition into photoluminescent signals for the (+)-PAN-chelated [In(OH)(bdc)]_n system, thus simulating the functions of guanine nucleotide-binding protein (G protein)-coupled receptors (see figure).