In situ SERS monitored photoactive yellow protein (PYP) chromophore model elimination, nano-catalyzed phenyl redox and I₂ addition reactions

Abstract

S-Phenyl-(E)-3-(4-hydroxyphenyl)prop-2-enethioate elimination, phenyl nitration, nitro reduction, amino oxidation and I₂ addition to C=C reactions were confirmed with surface-enhanced Raman scattering (SERS).

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Photoactive Yellow Protein (PYP) is a photoactive pigment found in Ectothiorhodospira halophila.^1–4 As a PYP chromophore model, S-phenyl-(E)-3-(4-hydroxyphenyl)prop-2-enethioate (PhSCAOH, CAOH = 4-hydroxycinnamic aldehyde) has been widely studied in C–H activation reactions, where the phenylthio (PhS–) group acts as a leaving group for sulfur atom attack,^5–9 and C=C double bond [2 + 2] photodimerization under UV irradiation.¹⁰ In addition, it is well known that the olefin group’s C=C bond undergoes addition with group VII elements/acids, such as Cl₂,¹¹ Br₂,^12–14 I₂ and HCl/HBr. However, the SERS/Raman spectra peak assignments and mechanistic explanations have been contradictory.¹ Our group’s work on (a) laser induced photochemistry^15–17 and (b) visible light induced PhS– elimination reactions,⁷ inspired us to believe that the PhS– elimination reaction could be induced using Ag/Au NPs’ surface energy.¹⁷ Furthermore, it might be possible to laser-induce (413.1 nm)¹ the PhS– elimination reaction of PhSCAOH, rather than its deprotonated form.¹

Surface-enhanced Raman Scattering (SERS) has utilized silver/gold nanoparticles’ (Ag/Au NPs) surface plasmon (SP) effects¹⁸ to detect heterocycle and thiol compounds. Recently, SERS was applied in a mechanistic study of chemical reactions on Ag/Au NPs’ surfaces, e.g. the phenyl nitration reaction,¹⁸ 4-nitrothiophenol (4-NTP)/4-aminothiophenol (4-ATP) dimerization to p,p′-dimercaptoazobenzene (DMAB)¹⁹ and DMAB reduction to 4-ATP.^20–22 Furthermore, noble metallic Ag/Au NPs can catalyze many chemical reactions, for example nitro-group (–NO₂) reduction²³ and cyclobutane cleavage.¹⁷

Thin layer chromatography (TLC) combined with SERS^15–17 as a powerful and time-efficient tool has been used to study Ag/Au NPs’ surface reactions. Here, it is demonstrated that the versatile Ag/Au sol can be a SERS platform for detecting the 4-hydroxycinnamic aldehyde (CAOH) of PhSCAOH elimination reaction, phenyl group (PhS–Ag/Au) nitration reaction (NO₂–PhS–Ag/Au), nitro-group (–NO₂) reduction (NH₂–PhS–Ag/Au), amino-group (–NH₂) oxidation to DMAB and even I₂ addition to the C=C double bond of CAOH. In particular, Ag/Au NPs are able to catalyze CAOH group elimination and –NO₂ reduction.

PhSCAOH and Ag/Au NPs were synthesized according to the literature method and characterized (for ¹H, ¹³C NMR, synthesis procedure, PhSCAOH single crystal structure, PhSCAOH cif file, Ag/Au NPs’ TEM and UV-vis spectra see the Experimental section and Fig. 1–6 in the ESI†).^10,17,24 SERS & solid-state Raman spectra of PhSCAOH were recorded using a Renishaw inVia Raman Spectroscopy system. A schematic illustration of the PhSCAOH reaction routes on Ag/Au NPs is shown in Fig. 1.

Fig. 1 Schematic illustration of the PhSCAOH reaction routes on Ag/Au NPs.

PhSCAOH was developed on TLC plates and Ag sol was sprayed on (Fig. 2(a)). SERS spectra readily proved the hypothesis that Ag NPs induced the CAOH elimination reaction and the PhS– had been adsorbed on the Ag NPs’ surface (for PhSCAOH SERS spectra on Au NPs see Fig. 8 in the ESI†).²⁵ The peaks at ∼1570 cm⁻¹, ∼1070 cm⁻¹, ∼1020 cm⁻¹ and ∼999 cm⁻¹ were assigned to the ν_8a, ν₁, ν_18a and ν₁₂ characteristic vibration modes of the PhS– functional group on the Ag NPs’ surface.^18,26 Other proof includes the disappearance of the peaks attributed to the C=C double bond at ∼1622 cm⁻¹ and the CAOH phenyl group vibration at ∼1585 cm⁻¹. It can be concluded from this that PhS– is adsorbed onto the Ag/Au NPs’ surface through a strong coordination bond,^6–8 which induced the elimination of stable conjugated CAOH.

Fig. 2 PhSCAOH elimination reaction promoted by Ag NPs. (a) Schematic illustration of PhSCAOH elimination; (b) SERS spectra of PhSCAOH; (c) SERS spectra after the NaBH₄ solution was added; (d) standard SERS spectra of PhSH.

A NaBH₄ water solution (0.1 M) was sprayed onto the TLC plate (shown in Fig. 2(a)) to complete the elimination reaction, with silver Ag NPs as the catalyst. The obtained SERS spectra are illustrated in Fig. 2(c). The uniformity of the elimination reaction was recorded and certified with SERS mapping (see Video 1 in ESI†). Comparison of Fig. 2(b) and (c) with Fig. 2(d), shows that the reduction treatment completed the elimination reaction. Specifically, the disappearance of the peak at ∼1585 cm⁻¹ indicated that pure PhS– had been absorbed on the Ag NPs’ surface, without solid-state PhSCAOH.

The above results led us to believe that Ag NPs catalyzed the –NO₂ to –NH₂ reduction reaction with NaBH₄. Furthermore, the reverse process for –NH₂ to –NO₂ could also be realized through an oxidation reaction. Thus, it was established that a redox cycle between –NO₂ and –NH₂ exists (see Fig. 3(a)). Phenyl group nitration was carried out according to our lab’s established method with modifications to give 4-NTP.¹⁸ In detail, a drop of HNO₃ solution (∼0.5%) was sprayed and was simply dried with a hair drier (Fig. 3(b)). Ag NPs catalyzed the 4-NTP reduction reaction with NaBH₄ as reductant, and a weak –NO₂ peak at 1330 cm⁻¹ was observed (Fig. 3(c)).

Fig. 3 PhS–Ag underwent nitration and redox reactions. (a) Schematic illustration of phenyl thiol reactions on the Ag NPs’ surface; (b) phenyl nitration at 1332 cm⁻¹, confirmed by SERS; (c) NaBH₄ reduced 4-NTP to 4-ATP using Ag NPs as a catalyst; (d) HNO₃ oxidized 4-ATP to 4-NTP.

Inconveniently, there were no characteristic Raman peaks assigned to the –NH₂ functional group of the reduction product, 4-ATP. However, the laser heat effect induced coupling reaction product, DMAB, has characteristic N=N vibrations located at ∼1430 cm⁻¹ and ∼1380 cm⁻¹ which were picked out, after careful examination of the SERS mapping spectra (Fig. 4(a)–(c)).

Fig. 4 4-ATP dimerization to DMAB under the laser heat effect. (a) Schematic illustration of 4-ATP dimerization to DMAB; (b) SERS spectra of 4-ATP; (c) SERS spectra of DMAB induced by the laser heat effect.

When sprayed with HNO₃ solution (∼0.5%) and placed in open air for 24 hours, 4-ATP was gradually oxidized to 4-NTP as illustrated in Fig. 3(d). Oxidation of –NH₂ to –NO₂ was proved by the presence of a peak at ∼1333 cm⁻¹, assigned to an –NO₂ vibration. The uniformity of oxidation was confirmed by mapping a randomly selected area (see Video 2 in the ESI†). It was concluded that –NH₂ was converted to –NO₂ readily with HNO₃ water solution.

As shown in Fig. 5(a), iodonium salt coordination with the Ag NPs’ surface was taken advantage of to detect CAOH. A TLC plate was exposed to I₂ vapor in an I₂ cylinder, where I₂ addition to C=C, formed a iodonium salt. The phenyl vibration of CAOH at ∼1593 cm⁻¹ was different to that of the phenyl group of PhS– at ∼1571 cm⁻¹. A transient four membered ring formed with I₂ was identified, with peaks at ∼1170 cm⁻¹ and ∼856 cm⁻¹ (see Fig. 5(b) and (c)).^15–17 The peak at ∼1297 cm⁻¹ (CH-I in-plane twist) indicated that the C=C chemical environment changes after I₂ addition. Excessive I₂ deposition on the TLC plate depressed the SERS peak intensities (Fig. 7 in ESI†).

Fig. 5 I₂ addition to the CAOH C=C double bond. (a) Schematic illustration of the I₂ addition reaction; (b) SERS spectra of the I₂ addition reaction; (c) SERS spectra of the pseudo-cyclobutane structure.

In summary, in situ SERS spectra confirmed that the strong Ag–S coordination bond formation between PhSCAOH and Ag/Au NPs induced CAOH elimination, and the process could be promoted with NaBH₄. PhS– nitration to 4-NTP was achieved simply, and Ag/Au NPs catalyzed 4-NTP reduction to 4-ATP with NaBH₄. An oxidation reaction of 4-ATP to 4-NTP was realized with HNO₃. The laser heat effect induced a 4-ATP coupling reaction to DMAB. I₂ addition to the C=C bond was also observed in the SERS spectra. The PhSCAOH elimination reaction could be seen as a C–H activation process which is utilized in many visible-light induced photochemical reactions, and it could serve as an explanation for the neurotoxicity of heavy metal ions, for example, in the well-known case of cis-platinum complexes. The range of reaction mechanism studies carried out via SERS combined with TLC was expanded to include visible light photochemistry, phenyl nitration, amino oxidation, nitro reduction and double bond I₂ addition reactions.

Acknowledgements

We are grateful for the financial support from the NSFC (No. 21471039, 21203045), Fundamental Research Funds for the Central Universities (Grant No. HIT. NSRIF. 2010065 and 2011017, PIRS of HIT A201502 and HIT. BRETIII. 201223), China Postdoctoral Science Foundation (2014M560253), Postdoctoral Scientific Research Fund of Heilongjiang Province (LBH-Q14062, LBHZ14076), Natural Science Foundation of Heilongjiang Province (B2015001), Open Project Program of the Key Laboratory for Photonic and Electric Bandgap Materials, Ministry of Education, Harbin Normal University, China (PEBM 201306), Open Project of the State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No. ES201411), and Open Foundation of the State Key Laboratory of Electronic Thin Films and Integrated Devices (KFJJ201401).

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Footnotes

† Electronic supplementary information (ESI) available: Experimental section, Fig. 1–11, PhSCAOH CIF file, Videos 1–2. CCDC 1485881. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra16823j

‡ These authors contributed equally to this work.

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