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Catalyzed and Electrocatalyzed Oxidation of L-Tyrosine and L-Phenylalanine to Dopachrome by Nanozymes Jianwen Hou, Margarita Vázquez-González, Michael Fadeev, Xia Liu, Ronit Lavi, and Itamar Willner Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b01522 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 10, 2018

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Catalyzed and Electrocatalyzed Oxidation of LTyrosine and L-Phenylalanine to Dopachrome by Nanozymes Jianwen Hou†§, Margarita Vázquez-González†§, Michael Fadeev†, Xia Liu†, Ronit Lavi‡ and Itamar Willner†* †

Institute of Chemistry, The Center for Nanoscience and Nanotechnology, The Hebrew

University of Jerusalem, Jerusalem 91904, Israel. ‡

Department of Chemistry, Bar–Ilan University, Ramat Gan 52 900, Israel.

§

These authors contributed equally to this work.

*E-mail: [email protected]. Tel: 972-2-6585272. Fax: 972-2-6527715.

ABSTRACT: Catalyzed oxygen insertion into C-H bonds represents a continuous challenge in chemistry. Particularly, driving this process at ambient temperatures and aqueous media represent a “holy grail” in catalysis. We report on the catalyzed cascade transformations of Ltyrosine or L-phenylalanine to dopachrome in the presence of L-ascorbic acid/H2O2 oxidizing mixture and CuFe-Prussian Blue-like nanoparticles, Fe3O4 nanoparticles or Au nanoparticles as catalysts. The process involves the primary transformation of L-tyrosine to L-DOPA that is

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further oxidized to dopachrome. The transformation of L-phenylalanine to dopachrome in the presence of CuFe-Prussian Blue-like nanoparticles and L-ascorbic acid/H2O2 involves, in the first step, the formation of L-tyrosine and, subsequently, the operation of the catalytic oxidation cascade of L-tyrosine to L-DOPA and dopachrome. ESR experiments demonstrate that ascorbate radicals,

-

·AA, and hydroxyl radicals, ·OH, play cooperative functions in driving the different

oxygen-insertion processes. In addition, the aerobic elecrocatalyzed oxidation of L-tyrosine to dopachrome in the presence of naphthoquinone-modified Fe3O4 nanoparticles and L-ascorbic acid is demonstrated. In this system magnetic-field attraction of the naphthoquinone-modified Fe3O4 nanoparticles onto the electrode allows the quinone-mediated electrocatalyzed reduction of O2 to H2O2 (bias potential -0.5 V vs. SCE). The electrogenerated H2O2 is, then, utilized to promote, in the presence of L-ascorbic acid and Fe3O4 catalyst, the transformation of L-tyrosine to dopachrome.

KEYWORDS: Prussian Blue, Fe3O4, iron oxide, catalysis, enzyme model, surface modification.

The use of inorganic nanoparticles as catalysts for diverse chemical transformations and, particularly, catalytic oxidation processes attracts substantial recent research efforts.1–3 For example, glucose is oxidized in the presence of Au nanoparticles4–9 to gluconic acid, dopamine is oxidized to aminochrome by Prussian Blue (or analogues) nanoparticles,10 Cu2+-modified g-C3N4 or graphene oxide11,12 and Cu2+-bipyridine metal organic-framework nanoparticles.13 In addition, Pt, Fe3O4, CeO2 or V2O5 nanoparticles were used as inorganic nanozymes that mimic peroxidases,14–24 e.g., horseradish peroxidase or NADH peroxidase. The room temperature catalyzed oxidation of C-H bonds to C-OH functionalities and, specially, the synthesis of


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catechol derivatives is, however, a challenging catalytic transformation, particularly in aqueous media. Different methods to promote oxygen insertion into C-H bonds were reported, yet most of these methods required elevated temperatures and harsh conditions, e.g. high pressure or highly acidic conditions.25–29 The oxygen source for the oxygen insertion process was O2,30,31 H2O in organic solvents32,33 or peroxides.34,35 In addition, oxygen insertion reactions of enhanced complexities were reported,36–39 e.g. the in situ hydrolysis of acyloxy functionalities introduced into the substrate were used as oxygen sources.40 Here we wish to report on the catalyzed oxidation of L-tyrosine or L-phenylalanine to dopachrome (the oxidation product of L-DOPA). We demonstrate that CuFe-Prussian Blue-like nanoparticles (CuFe NPs) and Fe3O4 nanoparticles catalyze the oxidation of L-tyrosine to dopachrome by H2O2 and L-ascorbic acid (Scheme 1). In addition, we report on the CuFe NPs-catalyzed oxidation of L-phenylalanine to dopachrome in the presence of L-ascorbic acid/H2O2. Furthermore, we demonstrate that naphthoquinonemodified Fe3O4 NPs act as electrocatalysts for the aerobic oxidation of L-tyrosine to dopachrome. This oxygen-insertion process proceeds at ambient temperature in aqueous solution, using H2O2 as the oxygen source (or O2 as the source for the generation of H2O2). Noteworthy is the fact that the use of L-ascorbic acid/H2O2 as oxidizing mixture plays a key role in the oxygen insertion processes. Aspects related to the mechanism of the reactions will be addressed. Prussian Blue nanoparticles (PB NPs) or analogues cyanometalate nanoparticles were previously reported to catalyze the oxidation of dopamine or L-DOPA (cathechol derivatives) by H2O2 to yield aminochrome or dopachrome, respectively.10 We find that the CuFe NPs catalyze, in the presence of L-ascorbic acid and H2O2, the hydroxylation of L-tyrosine to L-DOPA, and the subsequent oxidation of L-DOPA to dopachrome. The CuFe NPs also catalyze the oxidation


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of L-phenylalanine to dopachrome. The CuFe cyanometalate NPs were prepared according to previous reported methods.10,41 Figure 1A and Figure S1 depict the SEM (I) and TEM (II) images of the CuFe NPs. The nanoparticles exhibit a cubic shape, with average dimension of 150-200 nm. The X-ray diffraction patterns of the CuFe NPs indicate a face-centered cubic structure, space group Fm3m, same structure as the original PB NPs (Figure 1B). The absorbance spectra of the CuFe NPs as compared with the PB NPs are shown in Figure 1C. Figure 2A depicts the time-dependent absorbance spectra of dopachrome upon the oxidation of L-tyrosine, 1 mM, in the presence of 10 µg mL-1 CuFe NPs, H2O2, 15 mM and L-ascorbic acid, 5 mM. The absorption spectra are intensified and reach a saturation value after ca. 90 min. The time-dependent absorbance changes of dopachrome using the same amount of CuFe NPs, H2O2 and L-ascorbic acid but different concentrations of L-tyrosine are provided in Figure 2B. For all the systems we observe an induction time-interval for the oxidation of tyrosine to dopachrome. This induction time-interval is attributed to the mechanism involved in the oxidation of Ltyrosine to L-DOPA and dopachrome (vide infra). As we will demonstrate, the intermediary formation of ·OH is a key step for driving the oxidation process. The time-interval required to accumulate a steady state concentration of this intermediate represents the induction time for the oxidation. Furthermore, we noted that for all the concentrations of L-tyrosine a saturation value for the formation of dopachrome is observed, the saturation value is higher as the concentration of L-tyrosine increases. From the saturation value and knowing the extinction coefficient (ε) of dopachrome we estimated a limited conversion yield of ca. 10 % of L-tyrosine to dopachrome. This relatively low conversion yield is attributed to the CuFe NPs-catalyzed depletion of H2O2 and L-ascorbic acid by a competitive path, during the oxidation of L-tyrosine (vide infra). In fact, the addition of new portions of H2O2 and L-ascorbic acid reactivates the oxidation of L-tyrosine


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to dopachrome (Figure 2C), and a conversion yield of ca 40 % was achieved. These results indicate that the conversion yields of L-tyrosine to dopachrome can be increased by the cyclic additions of H2O2/L-ascorbic acid, and that the catalytic features of the nanoparticles are not affected within the oxidation process. It should be noted that an initial increase of the H2O2/Lascorbic acid concentration does not significantly increase the conversion yield of L-tyrosine to dopachrome, implying that the competitive depletion of H2O2/L-ascorbic acid oxidizing mixture is substantially higher than the oxidation rate of L-tyrosine to dopachrome. Control experiments (Figure 2D, curves (a), (b) and (c)) indicate that the addition of L-ascorbic acid to the oxidizing mixture is essential to drive the oxidation of L-tyrosine to dopachrome. The addition of Lascorbic acid is indispensable to stimulate the first step of oxidation of L-tyrosine to L-DOPA by H2O2 (insertion of oxygen into the C-H bond of the phenyl ring). In addition, it should be noted that the oxidation of L-DOPA to dopachrome proceeds in the presence of H2O2 and the CuFe NPs in the absence of L-ascorbic acid. In the absence of H2O2, no aerobic oxidation of L-tyrosine occurs in the presence or absence of L-ascorbic acid. A second class of nanoparticles that demonstrated catalytic activities toward the oxidation (hydroxylation) of L-tyrosine to L-DOPA in the presence of H2O2 and L-ascorbic acid and, subsequently, the H2O2-stimulated catalyzed oxidation of L-DOPA to dopachrome include Fe3O4 NPs. The Fe3O4 NPs (Figure S2) were prepared according to previous reported methods.42 The X-ray diffraction patterns show the typical bands for magnetite, with a cubic spinel structure, space group Fd3m (227) (Figure S3). We find that the Fe3O4 NPs catalyze the oxidation of LDOPA to dopachrome by H2O2 (Figure S4). Nonetheless, as for the CuFe NPs, subjecting Ltyrosine to H2O2 in the presence of the Fe3O4 NPs does not lead to the formation of dopachrome, implying that the Fe3O4 NPs do not catalyze the intermediate oxidation of L-tyrosine to the


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intermediate L-DOPA product. Treatment of L-tyrosine with H2O2 and L-ascorbic acid resulted, however, in the oxidation (hydroxylation) of L-tyrosine to L-DOPA, and the subsequent oxidation of L-DOPA to dopachrome. Figure 3A shows the time-dependent oxidation of different concentrations of L-tyrosine in the presence of Fe3O4 NPs, 100 µg mL-1, H2O2, 50 mM, and L-ascorbic acid, 5 mM. Evidently, as the concentration of L-tyrosine is higher, the rate of formation of dopachrome is enhanced, and a higher saturation value of the generated dopachrome is observed. The yield of conversion of L-tyrosine to dopachrome is controlled by the concentration of the Fe3O4 NPs, and as the concentration of the NPs increases the oxidation rate is higher (Figure 3B). The conversion yield of L-tyrosine to dopachrome is low (ca 4 %). This low conversion yield is attributed to a competitive path that depletes the oxidizing mixture composed of H2O2/L-ascorbic acid. Further addition of the H2O2/L-ascorbic acid oxidizing mixture reactivates the Fe3O4 NPs-catalyzed oxidation of L-tyrosine to dopachrome (Figure S5). By applying 5 cycles of portion wise addition of the H2O2/L-ascorbic acid oxidizing mixture, the conversion yield of L-tyrosine to dopachrome increases to ca 30 %. This indicates that the Fe3O4 NPs retain their catalytic activities toward the oxidation of L-tyrosine to dopachrome upon the cyclic additions of H2O2/L-ascorbic acid. Evidently, the results demonstrate that the Fe3O4 NPs catalyze the challenging transformation of oxidation of L-tyrosine to dopachrome in aqueous media, at ambient temperature. Furthermore, the results reveal that the co-added L-ascorbic acid plays a key function in the catalytic oxidation of L-tyrosine to dopachrome (Figure 3C, curves (a), (b) and (c)) (for further discussion vide infra). It should be noted that besides the successful CuFe or Fe3O4 NPs-catalyzed oxidation of L-tyrosine to dopachrome, we find that other nanoparticles, e.g. Au NPs (10 nm) catalyze the oxidation of L-tyrosine to dopachrome by H2O2 and co-added L-ascorbic acid (see Figure S6). Although the conversion yields are lower, the


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results suggest that other diverse NP systems could catalyze the L-tyrosine to L-DOPA transformation. The characterization of the CuFe, Fe3O4 and Au NPs-catalyzed oxidation of L-tyrosine to dopachrome by H2O2 reveal common features: (i) For all catalytic NPs systems the H2O2/Lascorbic acid oxidizing mixture is essential to drive the oxidation of L-tyrosine to yield dopachrome. In the absence of co-added L-ascorbic acid the process does not take place. (ii) The oxidation of L-DOPA to dopachrome by H2O2 proceeds effectively without the addition of Lascorbic acid, in the presence of all catalysts. This suggests that the oxidation of L-tyrosine to dopachrome involves the primary H2O2/L-ascorbic acid-dependent oxidation of L-tyrosine to LDOPA (hydroxylation step), followed by the L-ascorbic acid independent oxidation of L-DOPA to dopachrome by H2O2 (we find that L-ascorbic acid has no effect on the oxidation of L-DOPA to dopachrome by different catalysts). (iii) The conversion yields of L-tyrosine to dopachrome are low but they can be increased by the repeated addition of the H2O2/L-ascorbic acid oxidizing mixture. This implies that the catalytic oxidation process suffers from a competitive path that depletes the oxidizing reagents, H2O2/L-ascorbic acid. On the time scales of the experiments no changes in the concentrations of H2O2 or L-ascorbic acid could be detected in the absence of the different catalytic NPs, implying that the depletion of H2O2/L-ascorbic acid is associated with the interactions of the reagents with the catalysts. To further understand the mechanism of formation of L-DOPA and dopachrome in the presence of CuFe or Fe3O4 NPs, we performed a series of ESR experiments and correlated these results with the catalytic properties of the two catalysts upon oxidation of L-tyrosine and/or LDOPA to dopachrome. We find that the CuFe NPs do not generate hydroxyl radicals (·OH)43 in the presence of H2O2 (Figure 4A) but they oxidize, in the presence of H2O2, L-DOPA to


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dopachrome, thus ·OH are not involved in the oxidation of L-DOPA to dopachrome. In contrary, we find that the Fe3O4 NPs generate effectively ·OH in the presence of H2O2 (Figure 4B). The bulk catalytic properties of the Fe3O4 system presented in our report emphasized, however, that Fe3O4 NPs (in the absence of L-ascorbic acid) do not lead to the transformation of L-tyrosine to L-DOPA (and dopachrome). These results imply that ·OH by themselves do not lead to the transformation of L-tyrosine to L-DOPA. ESR experiments probing the radicals generated by CuFe NPs in the presence of L-ascorbic acid, in the absence or presence of H2O2 reveal interesting results: (i) The CuFe NPs do not generate ·OH in the presence of H2O2. (ii) In the absence of H2O2, but in the presence of L-ascorbic acid a low concentration of ascorbate radicals -

( ·AA)44 is detected (Figure 4C). (iii) In the presence of L-ascorbic acid and H2O2 a significant increase in the concentration of

-

·AA is observed, and this is accompanied by the detection of

·OH in the system (Figure 4D). These results imply that the CuFe NPs stimulate, in the presence of L-ascorbic acid and H2O2, an inter-communicated amplification path that increases the concentration of

-

·AA that provide the key species for the transformation of L-tyrosine to

L-

-

DOPA. A possible mechanism for the amplified formation of ·AA in the presence of CuFe NPs and H2O2 is suggested in Figure 5A. (iv) The results clearly indicate that ·OH radicals do not stimulate, by themselves, the formation of L-DOPA, and thus the

-

·AA

seems to be the key

promoter for the formation of the tyrosil radical. Nonetheless, albeit CuFe NPs catalyze inefficiently, in water, the formation of

-

·AA,

we cannot detect, even in trace amounts, the


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formation of L-DOPA in the absence of H2O2. This implies that the coupled CuFe NPs-catalyzed formation of

-

·AA and ·OH lead to the transformation of L-tyrosine to L-DOPA as summarized

in Figure 5. That is, the catalyzed generation of ascorbate radical presumably leads to ·OH radicals that autocatalyze the generation of

-

·AA

and ·OH. The

-

·AA

reaction with H2O2

provides a competing path for the depletion of H2O2 and, concomitantly, the dimerization of -

·AA acts as a competitive route for the depletion of L-ascorbic acid. Furthermore, we note that

the CuFe NPs are stabilized by citrate, whereas the Fe3O4 NPs are not stabilized by this ligand. Thus, we do not expect that the capping ligands play a significant role in the catalytic transformations. The aerobic (oxygen-induced) oxidation of L-tyrosine to dopachrome is, however, still a challenge. In fact, the insertion of oxygen into C-H bonds, particularly at ambient temperature and aqueous media, is one of the “holy grails” in chemistry. In nature, the enzyme tyrosinase catalyzes the aerobic oxidation of L-tyrosine to L-DOPA and the subsequent oxidation of LDOPA to dopachrome,45 suggesting that such transformation could be achieved by synthetic nanozymes. As a first step to accomplish the aerobic oxidation of L-tyrosine to dopachrome, we made use of the paramagnetic properties of the Fe3O4 NPs and the possibility to chemically modify the NPs, and particularly to modify the NPs with redox-active groups and electrochemically activate these molecular units by magnetic-field attraction of the NPs to electrode surfaces,46 as outlined in Figure 6. The Fe3O4 NPs were modified with a layer of polyethyleneimine (PEI) and subsequently, the polymer coating was reacted with 1, 2-dichloronaphthoquinone to yield the quinone-modified NPs. The quinone-modified Fe3O4 NPs were attracted by an external magnet, positioned below an Au-electrode (Figure 6A) to allow electrical


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communication between the quinone units and the electrode. Figure 6B, inset, depicts the cyclic voltammogram of the naphthoquinone units associated with the Fe3O4 NPs. Coulometric analysis of the reduction wave of the quinones, knowing the number of NPs in the weighted amounts of NPs in the electrochemical cell and assuming that all units communicate with the electrode, we estimate that the average coverage of the NPs corresponds to 80 quinone units per particle. Figure 6B shows the cyclic voltammogram of the quinone-modified Fe3O4 NPs attracted to the Au electrode by means of the external magnet under an inert atmosphere, curve (a), and under oxygen, curve (b). Clearly, in the presence of oxygen an electrocatalytic cathodic current is observed implying that the electrogenerated hydroquinone electrocatalyzes the reduction of O2 to H2O2. The formation of H2O2 by the electrocatalytic process was confirmed by using the electrogenerated H2O2 for the HRP-catalyzed oxidation of dopamine to aminochrome. Using an appropriate calibration curve, we estimated that the electrogenerated quinone-functionalized Fe3O4 NPs yields H2O2 at a rate corresponding to 12.5 µmol minute-1 (see Figure S7). The electrocatalyzed generation of H2O2 by the quinone-modified Fe3O4 NPs and their catalytic activity toward the H2O2-catalyzed oxidation of L-tyrosine to dopachrome was, then, applied to develop an integrated electrocatalytic system for the aerobic oxidation of L-tyrosine to dopachrome by the Fe3O4 NPs. The electrode, that was functionalized with the magneticallyattracted quinone-modified NPs was subjected to a bias potential of -0.5 V vs. SCE in the presence of L-tyrosine and L-ascorbic acid. This resulted in the electrocatalyzed generation of H2O2 and the subsequent Fe3O4 NPs-catalyzed oxidation of L-tyrosine to dopachrome. The absorbance spectra of the dopachrome generated upon biasing the electrode potential for different time-intervals are shown in Figure S8A. Figure S8B shows the time-dependent evolution of dopachrome upon biasing the electrode potential at E = -0.5 V vs. SCE. Switching


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the bias potential to 0 V vs. SCE resulted in the formation of the quinone-modified NPs that prohibit the formation of H2O2 and the switching-off of the catalyzed oxidation of L-tyrosine. By the reversible application of bias potential corresponding to -0.5 V and 0 V vs. SCE the electrocatalyzed formation of H2O2 can be switched to “ON” and “OFF” states, resulting in the cycling ON/OFF switching of the oxidation of L-tyrosine to dopachrome (Figure 6C). In the previous systems the CuFe, Fe3O4 or the Au NPs catalyze the oxidation of L-tyrosine to dopachrome by H2O2 and L-ascorbic acid. Preliminary results reveal, however, that the CuFe NPs catalyze the challenging cascade oxidation of L-phenylalanine to dopachrome by H2O2 in the presence of L-ascorbic acid (Figure 7A). In this system L-phenylalanine is hydroxylated by the CuFe NPs/H2O2/L-ascorbic acid to yield L-tyrosine in the first step. Subsequently, the resulting L-tyrosine is oxidized to L-DOPA and dopachrome as described earlier. Figure 7B depicts the time-dependent absorbance changes corresponding to the formation of dopachrome in the presence of the CuFe NPs and H2O2/L-ascorbic acid. Using an appropriate calibration curve, the conversion yield of L-phenylalanine to dopachrome corresponds to 4 %. As before the low conversion yield is attributed to the depletion of H2O2 and L-ascorbic acid, and re-addition of H2O2/L-ascorbic acid reactivated the formation of dopachrome. Control experiments indicated that all of the ingredients are essential to stimulate the transformation of L-phenylalanine to dopachrome (Figure 7C). Based in the mechanism that was discussed for the transformation of L-tyrosine to dopachrome, we suggest the mechanism outlined in Figure S9 as the route to transform L-phenylalanine to dopachrome. In this scheme, the primary step involves the ascorbate radical-induced cleavage of the C-H bond associated with the phenyl ring, resulting in the phenyl-radical species. The subsequent reaction of the phenyl radical with ·OH (or with H2O2) yields L-tyrosine. The subsequent CuFe NPs-catalyzed oxidation of the L-tyrosine product


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by the H2O2/L-ascorbic acid oxidizing mixture yields, then, the stepwise formation of L-DOPA and dopachrome. In conclusion, the present study has introduced a series of catalytic nanoparticles (nanozymes) composed of CuFe-Prussian Blue-like NPs, Fe3O4 NPs or Au NPs for the catalyzed hydroxylation of L-tyrosine to form L-DOPA, and its subsequent oxidation to dopachrome using a mixture of L-ascorbic acid/H2O2. The integration of L-ascorbic acid in the oxidizing mixture is essential to drive the hydroxylation of L-tyrosine. Mechanistic studies revealed the CuFe NPscatalyzed formation of ascorbate radicals and hydroxyl radicals and the participation of two species was found to be essential to transform L-tyrosine to L-DOPA. We found that the conversion yields are low due to the competitive depletion of the L-ascorbic acid/H2O2 oxidizing mixture, presumably by the dimerization of the reactive radical species. Nonetheless, the conversion yields can be increased by the re-addition of the L-ascorbic acid/H2O2 oxidizing mixture. In addition, the aerobic oxidation of L-tyrosine to dopachrome was demonstrated by the electrocatalyzed generation of H2O2 by quinone-modified Fe3O4 NPs attracted by an external magnet to an electrode surface biased at -0.5 V vs. SCE. The resulting electrochemicallygenerated H2O2 was then used to catalyze the transformation of L-tyrosine to dopachrome in the presence of the Fe3O4 nanozyme NPs. These results suggest that other aerobic catalytic transformations that yield H2O2, e.g Au NPs-catalyzed aerobic oxidation of glucose could be used to drive the transformations of L-tyrosine to dopachrome. Finally, the CuFe NPs-catalyzed hydroxylation of L-phenylalanine to L-tyrosine, and the subsequent transition of L-tyrosine to dopachrome represent a major chemical challenge addressing catalytic means to transform C-H to C-OH bonds, in aqueous environments at ambient temperature. The application of this


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approach to insert oxygen into the C-H bonds of water insoluble organic substrates, by applying the nanozymes in protic polar organic solvents, e.g. ethanol, are underway in our laboratory.


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Scheme 1. Schematic depiction of the catalyzed oxidation of L-tyrosine to dopachrome using CuFe-Prussian Blue-like NPs or Fe3O4 NPs as catalysts, in the presence of L-ascorbic acid/H2O2.


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Figure 1. (A) SEM image (I) and TEM image (II) of the CuFe-Prussian Blue-like NPs. (B) XRD spectra corresponding to: (a) Prussian Blue NPs, (b) CuFe-Prussian Blue-like NPs. (C) Absorption spectra corresponding to: (a) Prussian Blue NPs, (b) CuFe-Prussian Blue-like NPs.


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Figure 2. (A) Time-dependent absorption spectra of dopachrome generated upon the CuFe NPscatalyzed oxidation of L-tyrosine, 1 mM, in the presence of L-ascorbic acid, 5 mM, H2O2, 15 mM and CuFe NPs, 10 µg mL-1. (B) Time-dependent absorption changes upon the CuFe NPscatalyzed oxidation of different concentrations of L-tyrosine to dopachrome, in the presence of L-ascorbic acid, 5 mM and H2O2, 15 mM, CuFe NPs concentration 10 µg mL-1: (a) 0, (b) 0.1, (c) 0.25, (d) 0.5, (e) 0.75, (f) 1, (g) 1.5, (h) 2 mM. (C) Dopachrome production in the presence of the CuFe NPs, 10 µg mL-1, upon re-addition of L-ascorbic acid (5 mM)/H2O2(15 mM) oxidizing mixture, L-tyrosine, 0.5 mM (arrow indicates the time of re-added oxidizing mixture). (D) Control experiments that correspond to the CuFe NPs-catalyzed oxidation of L-tyrosine to dopachrome: (a) System includes CuFe NPs, 10 µg mL-1, L-tyrosine, 1 mM and L-ascorbic acid, 5 mM (no H2O2). (b) System includes CuFe NPs, 10 µg mL-1, L-tyrosine, 1 mM and H2O2, 15 mM (no L-ascorbic acid). (c) System includes L-tyrosine, 1 mM, L-ascorbic acid, 5 mM and H2O2, 15 mM (no catalytic NPs). (d) System includes CuFe NPs, 10 µg mL-1, L-tyrosine, 1 mM, L-ascorbic acid, 5 mM and H2O2, 15 mM.


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Figure 3. (A) Time-dependent absorption changes upon the Fe3O4-catalyzed oxidation of different concentrations of L-tyrosine to dopachrome, in the presence of L-ascorbic acid, 5 mM and H2O2, 50 mM, Fe3O4 NPs concentration 100 µg mL-1: (a) 0, (b) 0.1, (c) 0.25, (d) 0.5, (e) 1, (f) 2 mM. (B) Absorption spectra of dopachrome generated upon the Fe3O4-catalyzed oxidation of L-tyrosine, 2 mM, in the presence of L-ascorbic acid, 5 mM, H2O2, 50 mM and different concentrations of Fe3O4 NPs: (a) 0, (b) 5, (c) 10, (d) 20, (e) 50, (f) 100, (g) 200, (h) 500, (i) 1000 µg mL-1. (C) Control experiments that correspond to the Fe3O4 NPs-catalyzed oxidation of Ltyrosine to dopachrome: (a) System includes Fe3O4 NPs, 100 µg mL-1, L-tyrosine, 0.5 mM and L-ascorbic acid, 5 mM (no H2O2). (b) System includes Fe3O4 NPs, 100 µg mL-1, L-tyrosine, 0.5 mM and H2O2, 50 mM (no L-ascorbic acid). (c) System includes L-tyrosine, 0.5 mM, L-ascorbic acid, 5 mM and H2O2, 50 mM (no catalytic NPs). (d) System includes Fe3O4 NPs, 100 µg mL-1, L-tyrosine, 0.5 mM, L-ascorbic acid, 5 mM and H2O2, 50 mM.


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Figure 4. ESR signals corresponding to the reactive intermediates generated by the catalytic CuFe NPs and Fe3O4 NPs systems: (A) CuFe NPs and added H2O2. (B) Fe3O4 NPs and added H2O2. (C) CuFe NPs and added L-ascorbic acid. (D) CuFe NPs and added L-ascorbic acid/H2O2 (inset arrows indicate the ·OH radical).


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Figure 5. The participation of ESR to detect the reactive species in the catalyzed oxidation of Ltyrosine to dopachrome: (A) CuFe NPs stimulated generation of H2O2 with

-

·AA

-

·AA, the catalyzed reaction of

to form ·OH and the autocatalytic amplified formation of

-

·AA

and ·OH

radicals. (B) The stepwise formation of L-DOPA via the primary generation of the tyrosyl radical by the

-

·AA removal of H from the tyrosine, and the subsequent recombination of the

tyrosyl radical with ·OH.


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Figure 6. (A) Schematic electrocatalyzed oxidation of L-tyrosine to dopachrome in the presence of naphthoquinone-functionalized Fe3O4 NPs. The modified NPs are attracted to the electrode surface by positioning an external magnet below the electrode. (B) Cyclic voltammograms corresponding to the electrocatalyzed oxidation of O2 to H2O2 by the napthoquinonefunctionalized Fe3O4 NPs: (a) under N2, scan-rate 100 mVs-1. (b) under O2, scan-rate 20 mVs-1. Inset: Enlarged cyclic voltammogram of the naphthoquinone-functionalized Fe3O4 NPs under N2, scan-rate 100 mVs-1. (C) Switchable elecrocatalyzed aerobic oxidation of L-tyrosine to


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dopachrome, in the presence of L-ascorbic acid, 5 mM. State ON is generated by biasing the electrode at -0.5 V vs. SCE. Stage OFF is formed by the removal of the potential bias.


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Figure 7. (A) Oxygen-insertion cascade of the CuFe NPs-catalyzed transformation of Lphenylalanine into dopachrome. (B) Time-dependent absorbance changes upon the CuFe NPs, 25 µg mL-1, catalyzed transformation of different concentrations of L-phenylalanine in the presence of L-ascorbic acid (10 mM)/H2O2 (50 mM): (a) 0, (b) 0.25, (c) 0.5, (d) 0.75, (e) 1, (f) 1.5, (g) 2 mM. (C) Control experiments that correspond to the CuFe NPs-catalyzed oxidation of L-phenylalanine to dopachrome: (a) System includes CuFe NPs, 25 µg mL-1, L-phenylalanine, 1 mM and L-ascorbic acid, 10 mM (no H2O2). (b) System includes CuFe NPs, 25 µg mL-1, Lphenylalanine, 1 mM and H2O2, 50 mM (no L-ascorbic acid). (c) System includes Lphenylalanine, 1 mM, L-ascorbic acid, 10 mM and H2O2, 50 mM (no catalytic NPs). (d) System includes CuFe NPs, 25 µg mL-1, L-phenylalanine, 1 mM, L-ascorbic acid, 10 mM and H2O2, 50 mM.


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ASSOCIATED CONTENT Supporting Information. A list of the materials, synthetic procedures and some figures discussed in the text are provided in the supporting information. This material is available free of charge on ACS Publications website. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: 972-2-6585272. Fax: 972-2-6527715. Author Contributions †

J. Hou, M. Vázquez-González contributed equally to this work.

All authors participated in the formulation of the paper and all have given their approval to this form of the manuscript. Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENTS This study is supported by The Minerva Center for Biohybrid Complex Systems. REFERENCES (1)

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