Fluorescent properties of thiochrome in solvents of different polarity

Abstract

Spectral and fluorescent properties of thiochrome in solvents of different polarity were studied. It was found that the pK a value of the transition between the cationic and neutral forms of thiochrome in aqueous solutions increased from ~5.5 to 9.7 upon photoexcitation. It is supposed that protonation takes place in the excited state of the molecule resulting in fluorescence quenching of the thiochrome neutral form in aqueous solutions at neutral pH values. The fluorescence quantum yield of thiochrome increased by ~2.2 times upon the transition from aqueous solutions to alcohols or polar aprotic solvents. It was found that an increase of the solvent polarity led to an increase in the Stokes shift from 3200 to 4200 cm -1 for the thiochrome neutral form emission. The change in the dipole moment upon excitation into the S 1 -state was estimated to be less than 3D.Keywords: thiochrome, fluorescence quantum yield, fluorescence decay lifetime, hydrogen perodixe, oxidative transformation of thiamine.Introduction. Thiamine (vitamin B1) is a very important essential nutrient that is used in vivo as a structural component of thiaminediphosphate, which is a cofactor of very important energy metabolism enzymes such as pyruvatedehydrogenase, ketoglutaratedehydrogenase, and transketolase, a key enzyme of the pentose-phosphate cycle [1]. Thiamine is known to be unstable in basic solution and is readily oxidized to thiochrome (TChr) or thiaminedisulfide. TChr is one of the products of oxidative transformation of thiamine:

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“…By fitting the decay curves, three main lifetime components (τ 1 –τ 3 ) can be identified. The τ 1 is 1.26 ± 0.38 ns (31%), which is in good agreement with the previously published data on the charge transfer to the oxidized form of Vitamin B 1 (thiochrome) [ 30 , 31 ]. This component can form by metal reduction during the synthesis ( Scheme S1 ), and it is presumably on the surface via π···π stacking to take part in the stabilization of the final metal cores [ 32 ].…”

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“…Another avenue to increase the efficiency of thiochrome‐based detection involves including a miscible organic solvent such as ethanol or methanol in the aqueous alkaline media. This is believed to improve the yield of thiochrome versus thiamine disulfide, as well as increase the fluorescence intensity of formed thiochrome . Interferences with thiamine oxidation include ascorbic acid, polyphenols, and other substances with antioxidant capacity, which can consume ferricyanide, and polyphenols, which can additionally form complexes with thiamine .…”

Stability-Indicating Photochemical Method for the Assay of Thiamine by Spectrophotometry

“…e isobutanol layer was dried over Na 2 SO 4 and used for the determination of the concentration of TC in the solution at 370 nm using 20,400 M −1 ·cm −1 as the value of molar absorptivity [61] or 654 as the value of A (1% 1 cm) at that wavelength. e same procedure was used for the determination of the concentration of TC in diluted solutions of photooxidized vitamin preparations.…”

“…TC shows an absorption maximum at 370 nm with a molar absorptivity of 20,400 M −1 ·cm −1 [61] and an A (1% 1 cm) value of 654 (Figure 2). In the present study, a molar absorptivity value of 19,800 M −1 ·cm −1 and an A (1%, 1 cm) value of 635 for TC at 370 nm have been obtained (Table 2).…”

“…e formation of TC is pH-dependent, and its yield is maximum at pH 10 [52]. TC exhibits strong absorption in aqueous solution in the UV region [61].…”