Quinoline-2-thiol Derivatives as Fluorescent Sensors for Metals , pH and HNO

Article history: Received January 22, 2014 Received in revised form February 02, 2014 Accepted 6 March 2014 Available online 7 March 2014 A tautomeric equilibrium exists for quinoline-2-thiol and quinoline-2(1H)-thione. Quantum mechanical calculations predict the thione is the major tautomer and this is confirmed by the absorption spectra. The utility of quinolone-2-thiol/quinoline-2(1H)-thione as a chromophore for developing fluorescent sensors is explored. No fluorescence is observed when excited at absorption maxima, however a fluorescence increase is observed when exposed to HNO, a molecule of import as a cardiovascular therapeutic. Alkylated quinoline-2-thiol derivatives are found to be fluorescent and show a reduction in fluorescence when exposed to metals and changes in pH. © 2014 Growing Science Ltd. All rights reserved.


Introduction
Thiols are uniquely versatile, displaying a wide variety of chemistry including being nucleophilic, acidic, metal coordinative and redox reactive.As such they are uniquely positioned to be the platform from which versatile fluorescent sensors can be developed.Sulfur based functional groups have been utilized as fluorescent sensors, particularly as metal sensors.2][3][4] These metal sensors take advantage of fluorescence quenching by sulfur via photoinduced electron transfer (PET).
Thiols are also sensitive to oxidizing agents and electrophilic species and therefore offer versatility for developing fluorescent probes.Nitrogen oxide (NO˙) and its derivatives have recently come under scrutiny because of their importance in biological function and recent connections with diseases.NO˙ is important for signaling and immune response in humans. 5,6Over-production of NO˙ has been linked to cancer, chronic inflammatory diseases and neurodegenerative diseases.The nitroxyl anion NO -resulting from the reduction of NO˙ has received recent attention for its biological function and for its potential therapeutic application against cardiovascular diseases. 7At physiological pH the protonated species HNO predominates.HNO can be detected indirectly by measuring N 2 O, the reductive nitrosylation of oxidized metal complexes or observing the reaction products of thiols. 8Now a molecule of great import, fluorescent sensors for HNO have only been recently developed.One strategy relies on the reduction of a Cu 2+ complex by HNO. 9,10Another recent strategy has been to utilize a nonfluorescent TEMPO derivative that reacts with HNO and becomes fluorescent. 11Thiols react with HNO to produce sulfinamides and disulfides and are well situated to produce fluorescent sensors for the important molecule.
Pyridine-2-thiol is of interest as a sensing functionality as it is well known to form metal complexes with transition metals including mercury 12 , silver 13 and copper 14,15 .Additionally the deprotonation of the thiol and protonation of the pyridine functionalities can allow it to act as a pH probe.However a tautomeric equilibrium exists between pyridine-2-thiol and pyridine-2(1H)-thione, and though studied heavily, there are conflicting reports about the identity of the major tautomer. 16he effect a pyridyl-2-thiol subunit and its tautomerization on the fluorescence properties of a larger conjugated system is unknown.To study the effect of the pyridyl-2-thiol/thione functionality on fluorescence, quinolone-2-thiol/thione 1a-b is examined.Evidence suggests the major tautomer is the thione 1b and that it is non-fluorescent.The utility of the pyridine-2-thiol functionality as a sensing unit is displayed by using quinoline-2-thiol and derivatives 2-3 as fluorescent probes for heavy metals, pH and HNO.

Results and Discussion
A tautomeric equilibration exists for quinoline-2-thiol/quinoline-2(1H)-thione (1a-b) and the favoured tautomer is responsible for the shape of the absorption spectrum.The identity of the major tautomer in the quinoline-2-thiol/quinoline-2(1H)-thione equilibrium was confirmed by a B3LYP DFT computation of the relative populations that showed the thione favoured at 99.9% with ΔE = 5.83 kcal/mol.This was confirmed by a TD-DFT computation of the absorption spectra of the tautomers.The thione 1b calculated spectrum shows strong bands at 358.8 nm (S 2 ) and 269.8 nm (S 5 ) and matches well with the experimentally obtained spectrum with strong bands at 372 nm and 273 nm in H 2 O (Figure 2).The calculated thiol 1a has strong bands at 308.0 nm (S 1 ) and 242.9 nm (S 4 ) and occurs in regions of low thione absorption.When excited at the UV/vis absorption maxima for both transitions, no fluorescence is observed and it is confirmed that the thione 1b is non-fluorescent.Unlike quinolone-2-thiol/thione, derivatives 2-3 are fluorescent and show a response to changes in pH and metals.Derivatives 2 and 3 showed decreases in fluorescence between pH 3 and 4 when titrated from basic to acidic pH (Figure 4a).This fluorescence decrease coincides with the protonation of the quinoline nitrogen.A >5 fold change in fluorescence was seen for 2 and a >10 fold was seen for 3. Heavy metals are known to form complexes with pyridine-2-thiol and therefore compounds 1-3 were titrated with metal ions to examine their effect on fluorescence.Thiol/thione 1 shows a fluorescence response to metals but this was found to be dependent on the metal's counterion.For example, 1 produces a fluorescent metal complex in the presence of CuBr 2 and CuCl 2 (λ em 425, 685 nm respectively) but not in the presence of CuSO 4 and Cu(OAc) 2 .Reaction of pyridine-2thiol and Cu 2+ ions results in the formation of various copper complexes and some oxidation to the disulfide. 18,19Analysis of the transformation of 1 on introduction of Cu 2+ showed the oxidation product disulfide, which was confirmed by NMR and direct oxidation to form the disulfide with NaIO 4 .Therefore the pyridyl-2-thiol functionality is not practical for fluorescent metal probes as their response is metal complex specific.Efforts to isolate the fluorescent species have proven difficult as the reaction results in a complex mixture.Alkylated quinolone-2-thiol derivatives 2 and 3 were in contrast observed to be fluorescent and titration with various heavy metals resulted in the quenching of fluorescence.Quinoline-2-thiol derivatives 2 and 3 have a λ em of 380 nm and diminished fluorescence is observed in the presence of CrCl 3 , CuSO 4 , FeCl 2 and FeCl 3 (Figure 4b).The turning off of fluorescence is immediate and steady, indicating quenching is due to chelation and not resultant from downstream reactions as in the case of quinoline-2-thiol.To support this, the decrease in fluorescence was observed to be reversible by adding the strong chelator EDTA.Changing the sulphide appendage from a pyridyl to a phenyl shows no changes, so all selectivity is determined by the quinoline sulphide functional group.

Conclusions
The pyridyl-2-thiol functionality displays a broad array of chemistry and therefore can be utilized to develop fluorescent sensors for metals, pH and HNO.The reaction of quinoline-2-thiol/thione with the HNO to create a fluorescent species shows a simple approach to develop sensors for this molecule of growing import.Alkylation of quinoline-2-thiol results in fluorescent molecules that are respondent to pH and metals.

Computational Studies.
The method used in this study was based on ab-initio techniques within the density functional approximation.Specifically, the nonlocal Becke exchange coupled to the Lee-Yang-Parr gradient corrected correlation functional (B3LYP) was used with the cc-pVQZ basis set (B3LYP/cc-pVQZ level).For both systems a full geometry optimization was performed using Gaussian 03 software package 22 .Harmonic vibrational analysis was performed on both systems to ensure that a minimum in the potential energy surface was obtained.Vibrational and rotational temperatures were also computed and used in the calculation of the molecular partitions functions.
The fraction of tautomer i, X i , at T = 300 K was computed from RT , where Q i and Q j are the partition functions for tautomers i and j, respectively, R is the ideal gas constant, and ΔE is the difference in energy between tautomers i and j.The absorption spectra for each tautomer was computed using time-dependent density functional theory, i.e.B3LYP-TD/cc-pVQZ level, coupled to the polarizable continuum model using the integral equation formalism variant (IEFPCM).