|Chem. Rev. 2011, 111, 7941-7980 Chemical Reviews REVIEW water-soluble macrocyclic hosts they have been used in combination, how they have been utilized as solvatochromic probes to obtain information about the hydrophobicity, polarity, and polarizability of the inner supramolecular concave phases, how their UVÀvis absorption and emission properties are affected, and how their complex formation can be applied for sensing, imaging, lasing, stabilizing, and other purposes. An emphasis will be given to examples in which binding constants, spectral shifts, Scheme 1. Acridine and Xanthene Dyes Scheme 2. Quinone-imine, Arylmethane, and Azo Dyes dx.doi.org/10.1021/cr200213s |Chem. Rev. 2011, 111, 7941-7980 Chemical Reviews REVIEW photophysical effects accompanying host/guest complexation, and conclude with diverse applications.

We conceptualize a novel approach towards enzyme assays based on the reversible and competitive binding of a fluorescent dye and the substrate as well as product of an enzymatic reaction to a macrocyclic host. This method was termed "supramolecular tandem assay", and has been applied to inhibitor and activator screening, sensor array development, and enantiomeric excess determination of amino acids. The simple and rapid read-out by fluorescence allows their straightforward implementation into high-throughput screening.

The rates of acid hydrolysis of N-benzoyl-cadaverine (1), mono-N-(tert-butoxy)carbonyl cadaverine (2), and benzaldoxime (3) with binding motifs for cucurbit[6]uril (1,2) and cucurbit[7]uril (1,3) were investigated in the absence and presence of these hosts. Significant rate enhancements (up to a factor of ca. 300 for the hydrolysis of 3) were observed. Competitive inhibition due to encapsulation of added cadaverine and the successful use of sub-stoichiometric amounts of macrocycle confirmed the function of cucurbiturils in promoting acid hydrolysis.

The structural and dynamic properties of a flexible peptidic chain codetermine its biological activity. These properties are imprinted in intrachain site-to-site distances as well as in diffusion coefficients of mutual site-to-site motion. Both distance distribution and diffusion determine the extent of Förster resonance energy transfer (FRET) between two chain sites labeled with a FRET donor and acceptor. Both could be obtained from time-resolved FRET measurements if their individual contributions to the FRET efficiency could be systematically varied. Because the FRET diffusion enhancement (FDE) depends on the donor-fluorescence lifetime, it has been proposed that the FDE can be reduced by shortening the donor lifetime through an external quencher. Benefiting from the high diffusion sensitivity of short-distance FRET, we tested this concept experimentally on a (Gly-Ser)(6) segment labeled with the donor/acceptor pair naphthylalanine/2,3-diazabicyclo[2.2.2]oct-2-ene (NAla/Dbo). Surprisingly, the very effective quencher potassium iodide (KI) had no effect at all on the average donor-acceptor distance, although the donor lifetime was shortened from ca. 36 ns in the absence of KI to ca. 3 ns in the presence of 30 mM KI. We show that the proposed approach had to fail because it is not the experimentally observed but the radiative donor lifetime that controls the FDE. Because of that, any FRET ensemble measurement can easily underestimate diffusion and might be misleading even if it employs the Haas-Steinberg diffusion equation (HSE). An extension of traditional FRET analysis allowed us to evaluate HSE simulations and to corroborate as well as generalize the experimental results. We demonstrate that diffusion-enhanced FRET depends on the radiative donor lifetime as it depends on the diffusion coefficient, a useful symmetry that can directly be applied to distinguish dynamic and structural effects of viscous cosolvents on the polymer chain. We demonstrate that the effective FRET rate and the recovered donor-acceptor distance depend on the quantum yield, most strongly in the absence of diffusion, which has to be accounted for in the interpretation of distance trends monitored by FRET.

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A new approach towards the rapid identification of quality binders to cucurbiturils--those that combine high affinity with high selectivity for a particular homologue--was developed. The assay exploits macrocycle-specific optical fingerprints (colorimetric or fluorimetric) of carefully selected indicators dyes. The screening of a guest library revealed known (e.g., adamantane derivatives) and new (e.g., terpenes) quality binders. The predictive power of the assay was underpinned by the modeling of the involved thermodynamic equilibria.

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