Shanshan Yu

Simultaneous Determination of Both the Enantiomeric Composition and Concentration of a Chiral Substrate with One Fluorescent Sensor

A fluorescent sensor is discovered to exhibit high sensitivity at one emission wavelength and high enantioselectivity at another when treated with a chiral diamine. By using this fluorescent sensor, it is demonstrated for the first time that both the concentration and enantiomeric composition of a chiral substrate can be determined simultaneously with one fluorescence measurement.

Zn(II) promoted dramatic enhancement in the enantioselective fluorescent recognition of functional chiral amines by a chiral aldehyde

The addition of Zn2+ dramatically enhanced the enantioselective fluorescent responses of 3,3′-diformyl-1,1′-bi-2-naphthol toward chiral functional amines in methanol. One enantiomer of the chiral substrates, including diamines, amino alcohols and amino acids, was found to turn on the emission of this molecular probe at λ > 500 nm much more than the other enantiomer. This emission signal is greatly red-shifted from most of the other BINOL-based enantioselective fluorescent sensors whose fluorescent signals are generally at 400 ± 50 nm. Thus, a new window is opened for the use of BINOLs to observe chiral recognition events. The fluorescent responses of the new probe in the presence of Zn2+ toward a chiral diamine have also allowed a visual discrimination of these two enantiomers because of their different emitting color and intensity. The mass spectroscopic analyses for the reaction of the probe plus Zn2+ with the two enantiomers of a chiral diamine have revealed that the chirality match and mismatch between the probe and the substrate have produced different reaction products, generating very different fluorescent responses.

Study of the Fluorescent Properties of Partially Hydrogenated 1,1′-Bi-2-naphthol-amine Molecules and Their Use for Enantioselective Fluorescent Recognition

The fluorescent properties of a series of H(8)BINOL-amine compounds are investigated. It is revealed that the intramolecular hydrogen bonds of these compounds contribute to the shift of the emission of their H(8)BINOL unit to a much longer wavelength. That is, the emission of H(8)BINOL is at λ = 323 nm, but that of the H(8)BINOL-amino alcohol (S)-5 is at λ = 390 nm. Binding of (S)-5 with mandelic acid suppresses its intramolecular hydrogen bonding and restores the short wavelength emission of the H(8)BINOL unit. When (S)-5 (1.0 × 10(-4) in CH(2)Cl(2)) was treated with (R)-mandelic acid (4.0 × 10(-3)), a large fluorescence enhancement at the short wavelength (λ(emi) = 330 nm) was observed with I(R)/I(0) = 11.7. When (S)-MA was used under the same conditions, the enhancement at the short wavelength emission was much smaller. Thus, a good enantioselective fluorescent response was observed with ef =3.5 [ef: enantioselective fluorescence enhancement ratio = (I(R) - I(0))/(I(S) - I(0))]. This study demonstrates that the H(8)BINOL-based molecules are promising as a new class of enantioselective fluorescent sensors.

Greatly Enhanced Fluorescence by Increasing the Structural Rigidity of an Imine: Enantioselective Recognition of 1,2-Cyclohexanediamine by a Chiral Aldehyde.

An aldehyde that is not fluorescent responsive toward a chiral diamine has been converted to a sensitive fluorescence enhancement sensor through incorporation of an additional hydrogen bonding unit to increase the structural rigidity of the reaction product of the aldehyde with the diamine. This new chiral aldehyde is synthesized in one step from the reaction of (S)-3-formylBINOL with salicyl chloride. When treated with trans-1,2-cyclohexanediamine in ethanol, it shows greatly enhanced fluorescence at λ=410 nm with good enantioselectivity. NMR and mass spectroscopic methods are used to investigate the reaction of the chiral aldehyde with the diamine. This study has revealed a two-stage reaction mechanism including a fast imine formation and a slow ester cleavage.

Highly selective ratiometric fluorescent recognition of histidine by tetraphenylethene–terpyridine–Zn( ii ) complexes

TPE–monoTpy and TPE–diTpy compounds (TPE = tetraphenylethene, Tpy = 2,2′:6′,2′′-terpyridine) were prepared and showed significant red shifts in fluorescence upon coordination to Zn(NO3)2 in THF : HEPES (1 : 4) solutions. These in situ prepared Zn(II) complexes have achieved highly selective ratiometric fluorescent recognition of histidine even in the presence of other natural amino acids and metal cations. This fluorescent recognition of histidine is visually observable with distinctive color changes from yellow to blue under UV irradiation. The mechanism for the interaction of the Zn(II) complexes with histidine was studied by UV-Vis absorption, NMR and MS.

Spectroscopic studies on the interaction of terpyridine-CuCl2 with cysteine

Interaction of the classical coordination complex TpyCuCl2 (Tpy = terpyridine) with cysteine was studied. Addition of cysteine to the solution of TpyCuCl2 was found to give greatly enhanced fluorescence. UV-vis absorption and mass spectroscopic analyses were conducted to investigate this reaction under various conditions. These studies indicate that in a pure water solution (pH = 6.10–6.55), cysteine can reduce Cu(II) to generate TpyCuSCH2CH(NH2)CO2H with greatly enhanced fluorescence. In a HEPES buffer solution (pH = 7.40), the Tpy ligand can be further displaced off the copper center, contributing to the enhanced fluorescence.

One enantiomeric fluorescent sensor pair to discriminate four stereoisomers of threonines

The BINOL-amino alcohol enantiomeric pair (S)-1 and (R)-1 are discovered to conduct both enantioselective and diastereoselective fluorescent discrimination of the four stereoisomers of threonine derivatives. This study utilizes different fluorescence responses of one sensor at two emission wavelengths toward the stereoisomeric substrates which expands the capability of the sensor in chiral recognition. In addition, the sensor pair also allows visual recognition of the N-protected l-allo-threonine and d-allo-threonine by enantioselective precipitation.

1,1′-Bi-2-naphthol-fluoroacetyl compounds in fluorescent recognition of amines

Several 1,1′-bi-2-naphthol (BINOL) derivatives containing fluoroacetyl functions were synthesized, which included (S)-4 with two difluoroacetyl groups at the 3,3′-positions of BINOL, (R)-7 with two trifluoroacetyl groups at the 3,3′-positions of a partially hydrogenated BINOL, and (S)-12 containing two trifluoroacetyl groups at the 6,6′-positions. Compounds (S)-4 and (R)-7 were nonfluorescent but (S)-12 gave intense fluorescence. This indicates that intramolecular hydrogen bondings between the BINOL hydroxyl groups and the 3,3′-dicarbonyl groups of (S)-4 and (R)-7 could have effectively quenched their fluorescence. The fluorescence responses of these compounds towards a variety of amines were studied. It was found that (S)-4 showed greatly enhanced fluorescence in the presence of diamines and also highly enantioselective fluorescent response toward a chiral diamine. However, monoamines could not turn on the fluorescence of this compound. Compound (S)-12 showed greatly increased fluorescence quenching by amines in comparison with BINOL.

Chiral Recognition by Fluorescence: One Measurement for Two Parameters

This outlook describes two strategies to simultaneously determine the enantiomeric composition and concentration of a chiral substrate by a single fluorescent measurement. One strategy utilizes a pseudoenantiomeric sensor pair that is composed of a 1,1′-bi-2-naphthol-based amino alcohol and a partially hydrogenated 1,1′-bi-2-naphthol-based amino alcohol. These two molecules have the opposite chiral configuration with fluorescent enhancement at two different emitting wavelengths when treated with the enantiomers of mandelic acid. Using the sum and difference of the fluorescent intensity at the two wavelengths allows simultaneous determination of both concentration and enantiomeric composition of the chiral acid. The other strategy employs a 1,1′-bi-2-naphthol-based trifluoromethyl ketone that exhibits fluorescent enhancement at two emission wavelengths upon interaction with a chiral diamine. One emission responds mostly to the concentration of the chiral diamine and the ratio of the two emissions depends on the chiral configuration of the enantiomer but independent of the concentration, allowing both the concentration and enantiomeric composition of the chiral diamine to be simultaneously determined. These strategies would significantly simplify the practical application of the enantioselective fluorescent sensors in high-throughput chiral assay.