Molecular Mechanisms in Metal Oxide Nanoparticle-Tryptophan Interactions

Abstract

One of the crucial metabolic processes for both plant and animal kingdoms is the oxidation of the amino acid tryptophan (TRP) that regulates plant growth and controls hunger and sleeping patterns in animals. Here, we report revolutionary insights into how this process can be crucially affected by interactions with metal oxide nanoparticles (NPs), creating a toolbox for a plethora of important biomedical and agricultural applications. Molecular mechanisms in TRP-NP interactions were revealed by NMR and optical spectroscopy for ceria and titania and by X-ray single-crystal study and a computational study of model TRP-polyoxometalate complexes, which permitted the visualization of the oxidation mechanism at an atomic level. Nanozyme activity, involving concerted proton and electron transfer to the NP surface for oxides with a high oxidative potential, like CeO₂ or WO₃, converted TRP in the first step into a tricyclic organic acid belonging to the family of natural plant hormones, auxins. TiO₂, a much poorer oxidant, was strongly binding TRP without concurrent oxidation in the dark but oxidized it nonspecifically via the release of reactive oxygen species (ROS) in daylight.

Figure 1

Molecular structures of the polyoxometalate ion (W₁₂PO₄₀^3–, left) and the protonated TRP amino acid (HTRP⁺, right) studied here.

Figure 2

Titration of 0.5 mM TRP with TiO₂–I (A), TiO₂–II (B), CeO₂(−)(C), and CeO₂(+)(D) showing the H6 proton resonance of TRP at 7.728 ppm. The following colors of curves obtained at different ratios of NP/TRP were used. (A, B) TRP:TiO₂–I or TiO₂–II:0.5 mM:0 mM (black); 0.5 mM:0.031 mM (blue); 0.5 mM:0.0625 mM (red); 0.5 mM:0.125 mM (green); 0.5 mM:0.250 mM (light blue); 0.5 mM:0.5 mM (light green); and 0.5 mM:1.0 mM (rose). (C, D) TRP:CeO₂(−)(C)/or CeO₂(+)(D):0.5 mM:0 mM (black); 0.5 mM:0.0005 mM (blue); 0.5 mM:0.001 mM (red); 0.5 mM:0.002 mM (green); 0.5 mM:0.005 mM (light blue); and 0.5 mM:0.007 mM (light green).

Figure 3

1D ¹H spectrum with assignment of the proton resonances of the oxidized product of TRP obtained in a mixture with CeO₂(−)(C) NP. Expanded spectra in the aromatic 8.0–6.5 ppm region are presented. The assignment, performed using the superposition of the HMBC and HSQC spectra (Figure S4), and numbering are shown according to the structure on the panel. The lower normalized spectrum corresponds to overnight treatment of 1 mg/mL TRP in the presence of 8 mg/mL CeO₂(−) immediately after centrifugation and gives 2.9 ± 0.5% of the primary product by integration. The upper spectrum is for the sample after removal of ca. 90% CeO₂(−) by centrifugation and 1 month storage at 5 °C. The primary oxidized product content is 6.4 ± 0.4% by integration.

Scheme 1. Proposed Molecular Mechanism of TRP Oxidation Based on Observed Structural Features of the POM–TRP Complex, Theoretical Calculation, and the Identified Nature of the Reaction Product

The latter is apparently generated in a neutral form, but will then, of course, transform into a zwitterion.

Figure 4

1D ¹H spectra of free TRP (A) and its complex (HTRP)₃PW₁₂O₄₀·5H₂O (B). Assignments of the TRP in the complex performed based on HSQC and HMBC experiments (Figure S7) are shown on top of the proton resonances.

Figure 5

Molecular structure of (HTRP)₃PW₁₂O₄₀·5H₂O with hydrogen bonds indicated (A) and its packing motif (B).

Figure 6

FTIR spectra of the solids isolated by drying buffered TRP solutions and that of the pure POM–TRP complex.