Acid-base fluorescent guests

Boronic acid-based fluorescent PET sensors developed for the recognition of simple monosaccharides have been extended to include ditopic recognition sites and so introduce selectivity for a diverse range of guest species. 

Zhao and co-workers describe boronic acid based systems for enantioselec-tive fluorescent recognition. The scaffold used for the chiral sensors employs a fluorophore, arylboronic acid binding sites (two are best) and a linker (of appropriate size for the guest) between the two receptors. 

As very different systems, Ueno et al. prepared a-helix peptides bearing y-cyclodextrin and one (16,17) or two (18) naphthalene units in their side chains (Fig. 2) [38], They are alanine-based peptides composed of 17 amino acid residues. The peptides 16 and 17, which have y-CD and one naphthalene unit, display a simple monomer emission and the fluorescence intensity slightly decreases with increasing guest concentration. On the other hand, the peptide 18, which has y-CD and two naphthalene units on both sides of the y-CD unit, exhibits considerable excimer emission in addition to the predominant monomer emission and the excimer emission decreases with increasing guest concentration. This indicates that 18 excludes the two naphthalene units from the y-CD cavity to the opposite directions upon guest accommodation. 

Amide- and Urea-Based Anion Receptors, p. 51 Amino Acids Applications, p. 42 Chiral Guest Recognition, p. 236 Deoxycholic, Cholic, and Apocholic Acids, p. 441 Fluorescence Sensing of Anions, p. 566 Guanidinium-Based Anion Receptors, p. 615 Hydrogen Bonding, p. 658 lonophores, p. 760... 

For the community of supramolecular chemists, however, the sensor-analyte assemblies that display defined complex stoichiometry are more important. An excellent example of important electroneutral guests successfully addressed by fluorescence-based PET sensors are sugars, and saccharide sensing by boronic acids is a major success story of supramolecular chemists. Figure 17 shows two examples of PET-based saccharide sensors. 

The Schiff bases obtained by reaction of 4-methyl-2,6-diformylphenol with l- or D-phenylglycinol 52 (Fig. 16) were foimd to interact enantioselectively with the two enantiomers of mandelic acid on account of the appearance of a fluorescent component with a longer lifetime. A job plot of the complex formation, however, revealed a 2 3 guest host stoichiometry of the complex formed, thus suggesting an oligomeric form of the type reported in Fig. 16b. The good enantioselectivity observed (Ks/K = 4 for the (R,R)-sensor) also allowed these authors to calibrate the fluorescence response vs the enantiomeric compositions of mandelic acid in benzene solutions containing 10 " M of the sensor [81]. 

Fig. 17 Fluorescent sensors based on displacement of fluorophores. (a) Chiral boronate 56 containing a fluorophore which is quenched upon displacement (b) CdSe/ZnS quantum dot (QD) modified with a spacer and a boronic acid unit bound to a fluorophore-containing P-cyclodextrin 57, which, upon interaction with the analyte, releases the fluorophOTe, thus pievtaiting energy transfer (ET) from QD (c) guest-induced phosphorescence of CD-l-hromonaphthalene complex...

Bruns et al. pioneered self-reporting materials based on the mechanical perturbation of proteins within polymeric matrices/ " The protein cage thermosome (THS), a chaperonin from the archaea Thermoplasma acid-ophilum, is composed of sixteen protein subunits that form two hemispheres with the ability to enclose macromolecular guests (Figure 11.13)/ THS is approx. 16 nm in diameter. The hemispheres of chaperonins are connected through a mechanically weak plane, so that the two halves of these protein cages can be separated from each other by mechanical forces, as demonstrated in AFM experiments.THS was turned into a sensor for mechanical deformation by covalently linking two proteins, enhanced cyan fluorescent protein (eCFP) and enhanced yellow fluorescent protein (eYFP), into the... 

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