Fluorescence quenching enantioselective

In order to obtain independent evidence for the involvement of the cyclodextrin cavity, fluorescence measurements were carried out for copper(II) ternary complexes with L- or D-tryptophan. In fact, the fluorescence spectrum of tryptophan has already been shown to be sensitive to the polarity of the microenvironment in which it is located and has been used in many studies as a probe for the conformation of proteins and peptides [53]. As for many fluorophores, the indole fluorescence of Trp is quenched by the copper(II) ion this effect has been used as a measure of the stability constants of copper(II) complexes [54, 55]. In a recent work, it has been shown that the fluorescence of dansyl derivatives of amino acids undergo enantioselective fluorescence quenching by chiral copper(n) complexes and that fluorescence measurements can be used for the study of enatioselectivity in the formation of ternary complexes in solution [56]. Bearing this in mind, we performed the same type of experiments by adding increasing amounts of the [Cu(CDhm)] + complex to a solution of D- or L-tryptophan [36]. The fluorescence titration curve shows that the artificial receptor inhibits the indole... 

Corradini R, SartOT G, Dossena A et al (1992) Enantioselective fluorescence quenching by a chiral coppeifTI) complex in ligand exchange equilibria. J Chem Soc Perkin Trans II 1979-1983... 

Marchelli used the copper(II) complex of histamine-functionalized P-cy-clodextrin for chiral recognition and separation of amino acids [27]. The best results were obtained for aromatic amino acids (Trp). Enantioselective sensing of amino acids by copper(II) complexes of phenylalanine-based fluorescent P-cyclodextrin has been recently published by the same author [28, 29]. The host containing a metal-binding site and a dansyl fluorophore was shown to form copper(II) complexes with fluorescence quenching. Addition of d- or L-amino acids induced a switch on of the fluorescence, which was enantioselective for Pro, Phe, and Trp. This effect was used for the determination of the optical purity of proline. 

On the basis of the same principles, Pu et al. have synthesized a chiral dendritic compond (52) (Chart 10) which shows fluorescence quenching upon binding of amino alcohols. An enantioselective response was observed on addition of a number of amino alcohols the most significant effect was obtained with phenylalaninol.165... 

Pischel U, Abad S, Miranda MA (2003) Stereoselective fluorescence quenching by photoin-duced electron transfer in naphthalene-amine dyads. Chem Commun 1088-1089 Xu M H, Lin J, Hu Q et al (2002) Fluorescent sensors for the enantioselective recognition of mandeUc acid signal amplification by dendritic branching. J Am Chem Soc 124 14239-14246... 

In the presence of enantiomerically enriched sensitizer 36 (77% ee), the observed enantiomeric excesses increased from 1% ee at room temperature to 15% ee at -65°C2 With the enantiomerically enriched anthracene derivative 37 (91% ee), the enantioselectivity was enhanced to 23% ee at -65°C, favoring the opposite enantiomer. Based on fluorescence quenching experiments and the concentration dependence of the observed enantioselectivities, the authors suggest the formation of diastereomeric exciplexes by complexation of the different prochiral faces of the styrene to the chiral surface of the sensitizer. The... 

Stilbene photochemistry continues to provide a mine of riches for the dedicated photochemist, in particular Hochstrasser, Fleming, and their groups. Metcalf et al. have described chiral discrimination in electronic energy transfer processes enantioselective excited state quenching occurs. A study of singlet electronic energy transfer from cyclohexane to benzene appears to require revision of the benzene fluorescence efficiency in cyclohexane to 0.26 0.02 (Johnston and Lipsky). 

Another example is given by the interaction of chiral metal complexes with fluorescent chiral analyte bearing metal binding sites. Enantioselective quenching of fluorescence can be observed in such cases as a result of coordination or ligand... 

The low fluorescence in the FQ complex can be derived from non-radiative decay due to the occurrence of either energy or electron transfer. These processes are difficult to demraistrate, and even more difficult to design. Therefore in the recent literature this approach has not been extensively used for the design of enantioselective sensors, though combination of static and dynamic quenching has been documented in some cases. 

Fig. 3 (a) Fluorescent cyclodextrins used for enantioselective sensing of amino adds (b) enhancement of fluorescence by addition of proline to Cu((S)-l) (reprinted with permission from Elsevier from [87]) (c) quenching of fluorescence by addition of Cu(AA>2 to the free cyclodextrin (S)-l in box A the signal is strongly dependent on the concentration of Cu(AA)2 and in box B it is mainly dependent on the stereochemistry of the analyte, (reprinted from [90] with permission by the Royal Society of Chemistry)... 

Copper(ll) complexes of a glutamic acid derived diamido-diamino ligands 12 (Fig. 5c) containing anthracene units were also used as enantioselective sensors for a-hydroxy acids (in particular mandelate) and amino acids [68]. The ligand fluorescence was quenched by copper(ll) and recovered by addition of the analytes. A mechanism involving photoinduced electron transfer (PET) was proposed, and the conditional stability constants of the ternary complexes was found to be different for the two mandelate enantiomers (with KiJK = 15.2) in this study. 

The use of a disubstituted amino alcohol in cleft-Uke derivatives 32 (Fig. 9) was recently described as an efficient strategy for obtaining very high enantioselectivity (ef= 11.2 for mandelic acid enantiomers) [72]. This system was shown to act with different mechanisms according to its stereochemistry, since one of the two enantiomers induced recovery of fluorescence of the binaphthyl group, while the other was shown to induce exciplex formation, quenching the fluorescence of the monomeric fluorophore. 

The cholic acid derived sensors containing two pyrene units 47 (Fig. 14) showed excimer emission. The presence of two thiourea units allowed one to form stable complexes with dicarboxylic acids (with in the range of 10 -10 in acetonitrile), with a decrease of the fluorescence of the excimer band. This interaction was attributed to the enhancement of the PET effect and to a decrease of the n-n stacking interaction of the two pyrene units, which was found to be different for the two tetrabutylammonium salts of d- and L-glutamic acid [108]. Similarly, enantioselective quenching of bands due to both the monomeric fluorophore and the excimer emission was observed for the mandelate anion in a related sensor containing only one thiourea unit [109]. 

The appearance of a new fluorescence band was also observed in the case of the macrocyclic receptors containing naphthalene and vaUne 50 (Fig. 15), as a consequence of an unusual intramolecular exciplex with the amino groups [102]. Binding of protected amino acids in dichloromethane led to a quenching of the exciplex band (390 nm) and enhancement of the monomer band (340 run). This process was found to be enantioselective and dependent on the size of the macrocycle. Measurement of the ratio between the monomer and exciplex emission allowed one to measure differences in fluorescence with a higher precision [103]. A detailed study based on ESl-MS and NMR allowed these authors to propose a proton transfer mechanism from the acidic moiety of the amino acid to the amino group of the receptor, followed by the formation of an intimate ion pair stabilized by aromatic-aromatic interactions. This process competes with the exciplex formation, thus leading to enhancement of the naphthalene emission [104]. 

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