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C: Physical Processes in Nanomaterials and Nanostructures

Stabilization of DOPA Zwitterions on the Laser-Generated Gold Nanoparticles; ONIOM Computational Study of the Charge-Dependent Structural and Electronic Changes of DOPA Adsorbed on the Gold Nanosurface Farid Hajareh Haghighi, Hassan Hadadzadeh, Hossein Farrokhpour, Zahra Amirghofran, and Seyede Zohreh Mirahmadi-Zare J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02366 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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The Journal of Physical Chemistry C Stabilization of DOPA Zwitterions on the Laser-Generated Gold Nanoparticles; ONIOM Computational Study of the Charge-Dependent Structural and Electronic Changes of DOPA Adsorbed on the Gold Nanosurface Farid Hajareh Haghighi,†,‡ Hassan Hadadzadeh,*,† Hossein Farrokhpour,*,† Zahra Amirghofran,§ and Seyede Zohreh Mirahmadi-Zare‡ †

‡

Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran

Department of Molecular Biotechnology, Cell Science Research Center, Royan Institute for Biotechnology, ACECR, Isfahan, Iran §

Department of Immunology, School of Medicine, Shiraz University of Medical Sciences, Shiraz, Iran

*Corresponding

authors,

Hassan Hadadzadeh Professor of Inorganic and Bioinorganic Chemistry Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran E-mail address: [email protected] Phone number: +983133913240 Hossein Farrokhpour Associate Professor of Physical Chemistry Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran E-mail address: [email protected] Phone number: +983133913243

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Abstract

A stable colloidal solution of gold nanoparticle–DOPA zwitterion conjugates (Au NP–zDOPA) was prepared using nanosecond-laser ablation of a gold target in an aqueous solution of zwitterionic DOPA (z-DOPA). The spectroscopic data revealed that the Au NPs strongly interact with z-DOPA, which lead to a significant change in the electronic structure of z-DOPA. The adsorbed z-DOPA is highly stable against the oxidation in the aqueous solution, indicating a significant stabilizing effect of the Au NPs surface on this zwitterion. The electronic structures and geometries of z-DOPA and the other forms of DOPA (including uncharged (u-DOPA), cationic (c-DOPA), and anionic (a-DOPA) forms) adsorbed on the Au(111) nanosurface were determined using the ONIOM calculations. The geometry and electronic structure of each DOPA form are significantly affected by the surface upon the adsorption process. The analysis of the frontier orbitals confirmed the significant stabilizing effect of the Au NPs on z-DOPA. The calculations are consistent with the variations observed in the recorded absorption spectra of the z-DOPA due to its interaction with Au NPs. The Au nanosurface does not necessarily have a stabilizing effect on all DOPA forms. The cytotoxicity of z-DOPA and Au NP–z-DOPA against the Jurkat T-cells was evaluated.



1. INTRODUCTION The unique catecholic amino acid 3,4-dihydroxyphenylalanine (DOPA) is an immediate precursor for biosynthesis of the neurotransmitter dopamine (DA) in the brain.1,2 DOPA is also considered as the most effective and widely used medication to control the symptoms of Parkinson's disease (PD).3,4 PD is the second most common neurodegenerative disorder after Alzheimer disease which is primarily associated with the loss of dopamine-producing nerve cells in the brain.5 In order to compensate the dopamine deficiency, DOPA is commonly prescribed because it can easily cross from the bloodstream into the brain, whereas dopamine itself cannot.6 When DOPA gets inside the brain, it is then converted into dopamine by DOPA decarboxylase, which results in enhancing the dopamine concentration in the brain.2 Besides the biological and clinical importance of DOPA, it belongs to a special group of antitumor compounds possessing the ortho-quinol moiety.7,8 Previous studies in the field of cancer therapy have demonstrated that DOPA has antitumor activity against a variety of melanoma tumors.9 It is suggested that the cytotoxicity of DOPA is related to its inhibitory effect on the DNA synthesis.10 The chemical stability is one of the most important characteristics of a drug substance since it has a direct effect on the quality, safety, and the bioavailability of the drug in biological environments.11 However, many drugs are susceptible to some form of degradation, leading to loss of their therapeutic efficacies and toxicological consequences.12 Therefore, it is essential to determine ways that can enhance the stability of the drug substances and protect them from degradation in biological systems.13 Clinical studies have shown that about 1% of the orally administrated dose of DOPA can reach to the central nervous system (CNS) as the target site.14,15 The sensitivity of DOPA to chemical and enzymatic degradations results in its low availability in the brain.16 Furthermore, the degradation of DOPA can lead to the generation of toxic and reactive metabolites (such as quinones and free radicals), which cause adverse side effects on the 3 ACS Paragon Plus Environment

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neurons, especially in the long-term treatment.17 Figure S1 (see Supporting Information) shows the degradation pathways of DOPA through enzymatic decarboxylation and chemical oxidation. The decarboxylation of DOPA leads to the formation of dopamine, whereas the oxidation forms melanin as the final product.12 As can be seen in eq 1, the oxidation reaction is pH-dependent and the stability of DOPA significantly enhances under the strong acidic conditions.18 DOPA + 2H2O

Dopaquinone + 4H+ + 4ē

(eq 1)

Nanotechnology has provided new tools to develop drug delivery vehicles such as liposomes,19–21 polymeric nanoparticles,22,23 carbon nanotubes,24,25 silica nanoparticles,26 quantum dots (QDs),27 gold nanoparticles (Au NPs),28 and gold nanoclusters (Au NCs).29 The nanoscale manipulation of drug carriers leads to a more effective delivery of therapeutics, which are either unstable in biological systems, have poor penetration into their target sites, are poorly soluble in water, or have low biocompatibility.30 In the past decades, Au NPs have attracted considerable interest due to their unique features such as ease of synthesis, chemical inertness, tunable physical and optical properties, high biocompatibility, and versatility in surface modification.31 More importantly, recent studies have demonstrated that Au NPs have great potential to transport the therapeutic drugs across the blood-brain barrier (BBB) into the brain, which is a challenging organ for drug delivery.32–35 The most commonly used method for the preparation of Au NPs in solution is based on the chemical reduction of the Au ions (Au3+) in the presence of stabilizing agents (such as polymers, surfactants, or small molecules).36 However, the chemical techniques are not always the best choice for the synthesis of Au NPs. For instance, the reducing agents, chemical precursors, and other related substances used in these methods can give rise to a surface contamination of the NPs, which restricts their biological applications.37 Thus, it is worthy to 4 ACS Paragon Plus Environment


have green approaches to foster even more the use of metal NPs in the medical technologies. During the past decade, pulsed-laser ablation (PLA) synthesis has been exhibited to be a reliable alternative to the conventional chemical techniques for the preparation of noble metal NPs in solution.36,38 In this physical method, an intense laser pulse is focused on a bulk metal target immersed in a liquid, resulting in the generation of highly pure and contaminant-free colloidal solution of NPs.39,40 The purity of laser-generated NPs is of great significance in their biological applications. Moreover, PLA synthesis provides opportunity to obtain bare metal NPs, which can be further functionalized with desired molecules such as biomolecules and drugs.37,41,42 The study of the interaction between drugs and Au nanosurfaces has paramount importance for the development and optimization of the Au nanodelivery systems. In the past decade, a few experimental and theoretical efforts have been devoted to the study of the adsorption configuration of DOPA on the Au surfaces.43,44 However, the electronic structure of DOPA when adsorbed on the metal surfaces has not been studied so far. In the presence of a metal, the electronic structure of a drug substance often differs from that observed in its isolated state.45,29 The influence of Au surface on the electronic levels and molecular orbitals of DOPA needs to be well understood to properly interpret the resulting changes in the chemical characteristics of DOPA after the adsorption onto a metal surface. Thus, investigation of both electronic structure and geometry of the adsorbed DOPA is essential for development of highly effective metal-based nanodelivery systems which can protect DOPA from degradation in biological solutions. Similarly to other amino acids, DOPA exists in different forms depending on its environment. Scheme 1 shows four possible forms of DOPA.44,46


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Scheme 1. Four possible forms of DOPA. In the liquid phase, DOPA can exist in cationic (c-DOPA), zwitterionic (z-DOPA), and anionic (a-DOPA) forms depending on the experimental conditions. In addition, it is zwitterionic in the bulk crystalline structure and uncharged (u-DOPA) in its isolated gaseous state.47 Till now, there is little knowledge about the adsorption of DOPA on the Au surface,43,44 however, its electronic structure still remains unclear. It is important to study the stabilizing effect of Au on DOPA. Moreover, it is of great significance to investigate which DOPA form has the most stable electronic configuration on the Au surface. In this study, we prepared a highly stable colloidal solution of gold nanoparticle–z-DOPA conjugates (Au NP–z-DOPA) using nanosecond pulsed-laser ablation of an Au foil in an aqueous solution of z-DOPA. In this one-step approach, the z-DOPA molecules are adsorbed directly on the bare surface of the Au NPs immediately after the generation of nanoparticles in the solution. It is worth mentioning that the stability of both free z-DOPA and Au NPs is pHdependent. Literature reports have been shown that the laser ablation in basic solutions provides 6 ACS Paragon Plus Environment


more stable Au NPs with a narrower size distribution in comparison with those in acidic solutions.42 In contrast, free DOPA is easily oxidized in basic solutions.12 To guarantee the stability of both free DOPA and Au NPs during the experiment, the laser ablation was carried out at near-natural pH (6.5). The interaction of z-DOPA with the laser-generated Au NPs was investigated by means of several spectroscopic techniques. We were also interested in modeling the interaction of z-DOPA and other forms of DOPA, viz., u-DOPA, c-DOPA, and a-DOPA, with the Au nanosurface to investigate the effect of gold surface on their electronic structures and geometries. To the best of our knowledge, there is no report on the electronic structure and bonding of different forms of DOPA on Au(111) nanosurface. The purpose is to determine how each individual DOPA form is oriented toward the Au surface and how its electronic properties are affected by the gold surface. In addition, the cytotoxic effect of z-DOPA and Au NP–zDOPA conjugates against the Jurkat T-cells was evaluated.

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS See Supporting Information.

3. RESULTS AND DISCUSSION

3.1. TEM Study. The morphology and size distribution of the Au NPs with a protective layer of z-DOPA were characterized by TEM. The results demonstrate that the Au NPs are spherical in shape with an average diameter of about 11 nm (Figure 1A). As shown in the histogram (Figure 1B), the synthesized Au NPs have relatively narrow size distribution range, with a predominance of the very small NPs (4–13 nm).


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Figure 1. (A) TEM image of the Au NPs prepared in the aqueous solution of z-DOPA (1.4 × 10–4 M), and (B) the size distribution histogram. Synthesis of size-selected Au NPs with a narrow size distribution is highly desirable for their application in the biological and biomedical fields.36,37 In general, the size distribution of Au NPs prepared by PLA in pure water tends to be broadened due to their postablation coalescence.48 However, the in situ conjugation of the laser-generated Au NPs with suitable ligands can effectively control their average size. The size distribution is also significantly decreased in comparison to PLA in pure water and is in good agreement with the dynamic formation mechanism of NPs by PLA.40,49 The z-DOPA molecules have suitable binding sites to interact with the Au NPs surface and form a protecting layer, which can effectively prevent the growth of the laser-generated Au NPs.

3.2. Absorption Spectra. The absorption spectroscopy was used to study the effect of the in situ conjugation on the electronic and spectral properties of z-DOPA and Au NPs. As can be seen in Figure 2, the freshly prepared z-DOPA solution exhibits a strong absorption band at 286 nm, which is attributed to the π → π* transition of the phenylene ring.3,50,51



Figure 2. Absorbance spectra of the free z-DOPA and Au NP–z-DOPA samples in different intervals of Au NPs generation. Figure 2 also shows the UV–vis spectra of the Au NP–z-DOPA system recorded in constant time intervals of Au NPs generation. The in situ conjugation of the laser-generated Au NPs with z-DOPA indicates a blue-shift of about 10 nm in the absorption band of z-DOPA, which suggests that the phenylene ring of z-DOPA is most likely involved in the conjugation process. In addition, the intensity of the z-DOPA absorption band increases during the generation of Au NPs, indicating that the electronic states of z-DOPA are changed by the formation of the Au NP–z-DOPA conjugates.37 The characteristic plasmon band of Au NPs appears at 521 nm, which does not show any significant shift in its wavelength. This result shows that the agglomeration of NPs can be prevented due to the formation of a protective z-DOPA layer on the bare surface of Au NPs immediately after their generation.48 In addition, the long-term stability of Au NPs with a protective layer of z-DOPA was confirmed by recording the absorption spectrum of Au NP–z-DOPA after spending 20 days at room temperature. As shown in Figure S2, no remarkable change is observed in the SPR band of 9 ACS Paragon Plus Environment

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the aged Au NP–z-DOPA colloidal solution compared with the spectrum of a freshly prepared solution. The absorption spectroscopy also reveals that about 5.5% of the free z-DOPA is adsorbed on the Au NPs at concentration of 7.1 × 10−9 M. The concentration ratio of [z-DOPA]Initial / [Au NP]Maximum is 1.4 × 10–4 M / 7.1 × 10−9 M = 19718, indicating the surface binding sites saturation.

3.3. Fluorescence Measurements. Fluorescence measurements were performed to determine the affinity of z-DOPA for Au NPs. The emission spectra of z-DOPA were measured in the presence of increasing concentrations of Au NPs (from 0.0 to 7.1 × 10–9 M). Figure 3 shows the emission spectra of z-DOPA in the absence and presence of Au NPs.

Figure 3. Fluorescence spectra of the z-DOPA solution (1.4 × 10–4 M, λex = 236 nm) in the presence of different concentrations of the Au NPs (0.0–7.1 × 10−9 M). The free z-DOPA solution exhibits a strong emission peak at 320 nm, when exited at 236 nm.50 By increasing the concentration of the Au NPs, the fluorescence intensity of the z-DOPA solution decreases without any change in λmax, which clearly demonstrates the quenching effect 10 ACS Paragon Plus Environment


of the Au NPs on the fluorescence of z-DOPA. These results confirm that the electronic structure of z-DOPA is significantly changed due to the conjugation process.37,52,53 The quenching efficiency of the Au NPs and the mechanism of their quenching were investigated by the wellknown Stern-Volmer, Lineweaver-Burk, and modified Stern-Volmer equations37 (See Supporting Information, Figures S3–S6). The results demonstrate that the Au NPs strongly interact with z-DOPA molecules and quench their fluorescence with a static quenching mechanism.

3.4. FT-IR Spectra. The FT-IR spectrum of the free z-DOPA is shown in Figure S7. Some of the characteristic vibrational bands of the free z-DOPA are assigned in Figure S7. In the high frequency region of the z-DOPA spectrum, the bands located at 3385 and 3213 cm–1 are attributed to the stretching vibrations of the phenolic hydroxyl groups. In addition, the NH3+ group shows a band at 3070 cm–1 (N–H stretching mode).50 The rocking and umbrella vibrations of the NH3+ group are located at 1121 and 1499 cm–1, respectively, and its other bending vibrations are observed at 1570, 1592, and 1606 cm–1.44,54 The asymmetric stretching of the carboxylate group (–COO–) appears at 1656 cm–1, whereas its symmetric vibration is observed at 1406 cm–1. Another prominent characteristic bands in the spectrum of the free z-DOPA are those at 1528, 1458, and 1440 cm–1, which can be identified with C=C stretching vibrations of the phenylene ring.44,50 The FT-IR spectra of the free z-DOPA and Au NP–z-DOPA conjugates are shown in Figure 4. This figure shows the assignment of the characteristic bands of z-DOPA with one-toone correspondence between the free and adsorbed z-DOPA bands. As can be seen in Figure 4, the IR spectrum of the Au NP–z-DOPA conjugates clearly exhibits the characteristic bands of zDOPA, which confirms the presence of the z-DOPA on the surface of Au NPs. As expected, the 11 ACS Paragon Plus Environment

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frequency and the relative intensity of some vibrational bands change upon the interaction of zDOPA with the nanosurface, so that two main important changes are recognized in the IR spectrum of the adsorbed z-DOPA. Upon the conjugation process, the peaks related to the vibrations of the phenylene ring and carboxylate group are significantly shifted to the lower wavenumbers, suggesting that these two functional groups are involved in the conjugation process. As shown in Figure 4, the bending vibrations of the NH3+ group at 1499, 1592, and 1606 cm–1 are shifted to higher frequencies due to the adsorption process. This pattern has been also observed in the interaction of cysteine with Au20 NC.54

Figure 4. FT–IR spectra of the free z-DOPA (red line (A)) and the adsorbed z-DOPA (Au NP–zDOPA) (green line (B)). The assignment are based on references 44, 50, and 54. Abbreviations: ν = stretching vibration; δ = bending vibration; vib = vibration; s = symmetric; and as = asymmetric.



In the aqueous solution, the oxidation reaction is the major pathway for degradation of zDOPA. IR spectroscopy is a sensitive and effective method to study the stability of the adsorbed z-DOPA against the oxidation reaction.3 The stability of z-DOPA was evaluated by recording the FT-IR spectrum of the colloidal solution of Au NP–z-DOPA after spending 14 days at room temperature (see Figure S8), which did not show any significant changes in the characteristics peaks of the adsorbed z-DOPA. This demonstrates that the adsorbed z-DOPA molecules are highly stable even in the presence of dissolved oxygen for a long period of time (at least two weeks) at room temperature. It is worth mentioning that the IR spectra do not provide detailed information about the orientation and bonding of z-DOPA on the Au nanosurface. The computational methods can help us obtain molecular-level insights into the configuration, bonding, and electronic structure of the z-DOPA molecules adsorbed on the surface of Au NPs.

3.5. Geometries and Orientations of Different Forms of DOPA on the Au(111) Nanosurface. The second aim of this work is to study the geometries and electronic structures of different forms of DOPA (including uncharged (u-DOPA), cationic (c-DOPA), zwitterionic (z-DOPA), and anionic (a-DOPA)) adsorbed on the Au(111) nanosurface in the gas and water phases. At first, the structure of each isolated DOPA form was optimized in the gas phase. The structural analysis of the isolated DOPA forms is provided in the Supporting Information (see Figure S9 and Tables S1,S2). The results show that each DOPA form has a lowsymmetric structure, in which the atoms of the CO2 group and the phenylene ring are not in the same plane. The optimized structures of four DOPA forms on the Au(111) nanosurface, obtained from the ONIOM calculations in the gas phase, are shown in Figure 5. As shown in Figure 5, the


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geometry of each DOPA form significantly changes upon its adsorption on the gold surface compared to its isolated optimized structure. It is worth mentioning that the presence of aliphatic C–C bonds in DOPA provides high flexibility for the molecule to change its structure on the Au surface to obtain higher stability.

Figure 5. The optimized structures of four DOPA forms on the Au(111) nanosurface obtained from the ONIOM calculations (all hydrogen atoms are omitted for more clarity) in the gas phase. In the following, the structural differences between deformed and isolated structures of DOPA are investigated. In addition, the orientation and proximity of the deformed structures of DOPA with respect to the Au(111) nanosurface will be discussed in details in this section. Figure 6 shows the juxtaposition of the isolated structures of DOPA on their deformed structures after the adsorption on the gold surface. To compare each deformed structure of DOPA with its isolated one, the dihedral angles of the deformed structures are listed in Table S3. Table S4 shows the difference between the dihedral angles of the isolated and deformed structures. In all DOPA forms, the value of the



dihedral angle C4–C3–C7–C8 changes significantly due to the deformation process. In the deformed structures, the values of this dihedral angle are in the range of 168.1 to 179.2°, which means that the aliphatic carbons tend to locate in the same plane with the phenylene ring atoms upon the deformation of DOPA. Thus, in the deformed structures, three individual moieties including the phenolic oxygen atoms, the phenylene ring, and the CO2 group, can simultaneously interact with the Au surface.

Figure 6. Juxtaposition of the optimized structures of the isolated DOPA forms on their deformed structures after the adsorption on the gold surface (the atom numbering is shown only for the isolated u-DOPA). As shown in Table S4, the degree of the structural deformation depends on the DOPA charging state. In u-DOPA and c-DOPA, the dihedral angles C4–C3–C7–C8, C7–C8–C9–O3, and N1–C8–C9–O3 significantly change upon the deformation process. The main change in the structure of z-DOPA is related to the values of dihedral angle C4–C3–C7–C8, which is 106.7° in the isolated form and increases to 175.9° in the deformed structure. In the case of a-DOPA, the 15 ACS Paragon Plus Environment

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most important structural changes are related to the dihedral angles C4–C3–C7–C8 and C3–C7– C8–N1. The angle between the planes made by the CO2 group and phenylene ring in each deformed structure of DOPA is shown in Table S1. Comparing these angles with those of the isolated DOPA structures shows that the structural deformation changes the orientation of the CO2 group with respect to the phenylene ring in all DOPA forms. Especially in the deformed cDOPA structure, a significant change (about 37.5°) is observed in the mentioned angle. In the final part of this section, the geometry and proximity of each deformed structure with respect to the Au surface are investigated. Table S5 summarizes the distance of the DOPA atoms from the surface in each Au/DOPA system. In addition, the tilt angles of the phenylene ring and the plane of the CO2 group with respect to the Au surface are listed in Table 1 for each DOPA form. In all Au/DOPA systems (see Figure 5), the nitrogen atom (N1) does not have significant interaction with the Au surface and this atom is tilted out of the surface plane and placed far away from the surface (above 4 Å, see Table S5). Thus, we will mainly focus on other binding groups. In the Au/u-DOPA system (see Figure S10 for more clarity), the phenylene ring is placed nearly flat on the surface with a tilt angle of about 3.4°. Table 1. The tilt angles (°) of the planes of the phenylene ring and CO2 group with respect to the Au surface in the gas phase and water (in parentheses) u-DOPA

c-DOPA

z-DOPA

a-DOPA

phenylene ring

3.4 (6.0)

7.0 (8.1)

3.9 (6.3)

7.8 (7.3)

CO2 group

10.7 (9.1)

13.6 (8.7)

64.6 (35.1)

46.2 (36.2)

3.15 Å

2.99 Å 16 ACS Paragon Plus Environment


The O1 and O2 atoms of the phenolic hydroxyl groups are placed at the distances of 3.09 and 2.99 Å from the Au surface, respectively. In addition, the plane of the CO2 group is oriented toward the Au surface by 10.7°, which allows O3 to come closer to the Au surface. The nearly planar orientation of the phenylene ring of u-DOPA provides the best overlap between the delocalized π orbitals of the ring and 5d orbitals of Au. As can be seen in Figure S11 and Table S5, the geometry of the adsorbed c-DOPA exhibits a slight difference as compared to that of u-DOPA. The phenylene ring and the CO2 planes of c-DOPA show more deviations from the parallel orientation in comparison to those of u-DOPA. In c-DOPA, the planes of the phenylene ring and CO2 group are tilted toward the Au surface by 7.0 and 13.6°, respectively, which allow O2 and O3 to come closer to the Au surface as compared with O1 and O4, respectively. The distance of the c-DOPA atoms from the Au surface (Table S5) confirm the slightly tilted orientation of the phenylene ring and CO2 group with respect to the surface. As shown in Figures S12 and S13, z-DOPA and a-DOPA show different geometries compared with u-DOPA and c-DOPA. The presence of the deprotonated O4 atoms in z-DOPA and a-DOPA results in a significant deviation of the CO2 plane from the parallel orientation relative to the Au surface. In z-DOPA and a-DOPA, the O3 atom of the CO2 group closely approaches to the Au atoms, resulting in a relatively perpendicular orientation of the CO2 plane toward the surface. In this configuration, the deprotonated O4 atoms are tilted out of the surface plane and placed far away (above 4 Å) from the Au surface. Consequently, they do not significantly participate in the interaction process. It should be mentioned that the tilt angle of the CO2 plane in z-DOPA is larger than that in a-DOPA. These results indicate a significant difference in the orientation of the CO2 group in z-DOPA and a-DOPA versus u-DOPA and c17 ACS Paragon Plus Environment

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DOPA. However, the phenylene rings of z-DOPA and a-DOPA remain almost parallel relative to the Au surface with the tilt angles of about 3.9 and 7.8°, respectively. In a-DOPA, one of the phenolic hydroxyl groups (HO2) is deprotonated, thus there is an interplay between two competing processes. The π structure of the phenylene ring favors its flat orientation toward the Au surface (to increase the overlap with 5d orbitals of Au). In addition, the deprotonated O2 atom tends to approach the Au surface as closely as possible. As a result, the phenylene plane of the adsorbed a-DOPA exhibits a larger tilt angle (7.8°) relative to the Au surface when compared to other DOPA forms. To provide information about the interaction of the DOPA forms with the Au surface in the aqueous medium, the geometry of each adsorbed DOPA form was calculated by considering the solvent effect. The optimized structures of four DOPA forms on the Au(111) nanosurface, obtained from the ONIOM-PCM calculations, are presented in Figures S14–S17. For the aqueous phase, the distance of the DOPA atoms from the surface in each Au/DOPA system is listed in Table S5. In addition, the tilt angles of the phenylene ring and the plane of the CO2 group with respect to the Au surface are presented in Table 1. The adsorption patterns and binding modes of the DOPA forms in the aqueous phase are similar with those in the gas phase. However, a slight difference is observed in the orientation of each DOPA form relative to the surface in water compared with that in the gas phase. The difference is related to the interaction of water with the DOPA forms in the solution. The results obtained for the geometries of the adsorbed DOPA forms in the gas and aqueous phases can be summarized as follows: (i)

In both gas and aqueous phases, the amine group does not participate in the interaction of DOPA with the Au surface. In fact, DOPA interacts with the surface through its three binding groups including the phenolic oxygen atoms, the phenylene ring, and the CO2 group. 18 ACS Paragon Plus Environment


(ii)

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The tilt angles of the phenylene rings are less than 10°, which show that the rings are oriented almost parallel to the Au surface. The slight tilting of the phenylene rings toward the Au surface can be explained by the interaction between the phenolic oxygens (O1 and O2 atoms) and Au atoms. In the parallel orientation of the phenylene ring, the π orbitals are extended perpendicular to the Au surface, providing a π-metal interaction. These results are consistent with the adsorption pattern of the aromatic amino acids on the Au surface.55

(iii)

The geometry optimization of Au/DOPA systems indicates a significant difference in the orientation of the CO2 group of u-DOPA and c-DOPA versus zDOPA and a-DOPA in both gas and aqueous phases. The CO2 planes of u-DOPA and c-DOPA are oriented almost parallel toward the Au surface, whereas those of the other forms are significantly tilted. However, in the aqueous phase, the tilt angles of the CO2 planes of z-DOPA and a-DOPA are lower than those observed in the gas phase.

(iv)

In the aqueous phase, the distance of the DOPA binding sites from the surface in each Au/DOPA system is larger than that observed in the gas phase. The effect is assigned to the interaction of the adsorbed DOPA forms with water.

3.6. Interaction, Adsorption, and Deformation Energies of Each DOPA Form on the Au(111) Nanosurface in Both Gas and Liquid Phases. The 𝑢𝑛−𝑐𝑜𝑟𝑟 interaction energy (𝐸𝑖𝑛𝑡 ) of each DOPA form was calculated in the gas and aqueous phases

using eq 2. 𝑢𝑛−𝑐𝑜𝑟𝑟 𝐸𝑖𝑛𝑡 = 𝐸𝑆𝑢𝑟/𝐷𝑂𝑃𝐴  (𝐸𝐷𝑂𝑃𝐴 + 𝐸𝑠𝑢𝑟 )

(2) 19

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𝑢𝑛−𝑐𝑜𝑟𝑟 where 𝐸𝑖𝑛𝑡 is the interaction energy without considering the basis set superposition error

(BSSE) (𝛿 𝐵𝑆𝑆𝐸 ). 𝐸𝑆𝑢𝑟/𝐷𝑂𝑃𝐴 , 𝐸𝑆𝑢𝑟 , and 𝐸𝐷𝑂𝑃𝐴 are the electronic energies of the Au/DOPA system, the Au surface, and the deformed DOPA structure, respectively. The calculated interaction energies were also corrected for the BSSE according to eq 3. 𝑐𝑜𝑟𝑟 𝑢𝑛−𝑐𝑜𝑟𝑟 𝐸𝑖𝑛𝑡 = 𝐸𝑖𝑛𝑡 + 𝛿 𝐵𝑆𝑆𝐸

(3)

𝑐𝑜𝑟𝑟 In the gas phase, the corrected interaction energies (𝐸𝑖𝑛𝑡 ) of four DOPA forms are

found to be in the range of –67.72 to –25.10 kcal/mol (Table 2). The observed negative values of 𝑐𝑜𝑟𝑟 𝐸𝑖𝑛𝑡 imply a stable adsorption of all DOPA forms onto the Au surface. 𝑢𝑛−𝑐𝑜𝑟𝑟 𝑐𝑜𝑟𝑟 Table 2. The uncorrected interaction (𝐸𝑖𝑛𝑡 ), corrected interaction (𝐸𝑖𝑛𝑡 ), adsorption

(𝐸𝑎𝑑𝑠 ), and deformation (𝐸𝑑𝑒𝑓 ) energies (kcal/mol) of each DOPA form on the Au(111) nanosurface in the gas phase and water (in parentheses) u-DOPA

c-DOPA

z-DOPA

a-DOPA

𝑢𝑛−𝑐𝑜𝑟𝑟 𝐸𝑖𝑛𝑡

–38.60 (–33.86)

–46.25 (–35.10)

–40.85 (–32.41)

–81.21 (–38.34)

𝑐𝑜𝑟𝑟 𝐸𝑖𝑛𝑡

–25.10 (–21.10)

–33.08 (–22.46)

–27.43 (–19.96)

–67.72 (–26.30)

𝐸𝑎𝑑𝑠

–36.02 (–30.94)

–41.46 (–30.94)

–38.49 (–29.20)

–71.80 (–30.99)

𝐸𝑑𝑒𝑓

2.58 (2.92)

4.79 (4.16)

2.36 (3.21)

9.41 (7.35)

The interaction energies increase in the order: u-DOPA < z-DOPA < c-DOPA < a-DOPA. As can be seen in Table 2, the interaction energy of a-DOPA exhibits a significant difference with those of the other forms. The results show that the magnitude of the corrected interaction energy of a-DOPA is about 2.7 times higher than that of u-DOPA. In the aqueous phase, the interaction energy of each DOPA form decreases as compared to the gas phase, which is related to the solvent effect. The corrected interaction energies are 20 ACS Paragon Plus Environment


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within the range of –26.30 to –19.96 kcal/mol. The z-DOPA and a-DOPA forms have the lowest and highest interaction energies, respectively. To better understand the individual features of the adsorption of DOPA onto the Au(111) nanosurface, the adsorption energy (𝐸𝑎𝑑𝑠 ) for each form of DOPA was calculated in both gas and aqueous phases using eq 4. The 𝐸𝑎𝑑𝑠 is defined as the difference between the total energy of the Au/DOPA system and the sum of the total energies of the isolated DOPA (𝐸′𝐷𝑂𝑃𝐴 ) and the Au surface: 𝐸𝑎𝑑𝑠 = 𝐸𝑆𝑢𝑟/𝐷𝑂𝑃𝐴  (𝐸′𝐷𝑂𝑃𝐴 + 𝐸𝑠𝑢𝑟 )

(4)

The calculated adsorption energies are listed in Table 2. In the gas phase, the adsorption energies of the DOPA forms follow the same order as those observed in the case of the interaction energies. The high negative values of the adsorption energies for all DOPA forms (– 71.80 to –36.02 kcal/mol) provide the evidence of their strong adsorption on the Au(111) nanosurface that occurs through the O–Au coordination bonds. The results demonstrate that the adsorption energy of a-DOPA is about two-fold higher than that of u-DOPA. The values of interaction and adsorption energies indicate that the electronic structure of a-DOPA is much more affected by the adsorption process as compared with those of the other forms. The results are consistent with the adsorption behaviors observed for all DOPA forms on the Au surface and confirm the simultaneous interactions of the phenolic oxygens, CO2 group, and phenylene ring with the gold surface in all Au/DOPA systems. As expected, in the aqueous phase, the adsorption energies of all DOPA forms decrease with respect to those in the gas phase. The values of adsorption energies in the aqueous phase are


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very close to each other and in agreement with the adsorption energy of DOPA on the metal surfaces (~30 kcal/mol).43 To obtain the additional insight into the adsorption of the DOPA forms onto the Au surface, the deformation energies (𝐸𝑑𝑒𝑓 ) were calculated (Table 2). For each DOPA form, the 𝐸𝑑𝑒𝑓 value is easily obtained by subtraction of the uncorrected interaction energy from the adsorption energy (eq 5). 𝑢𝑛−𝑐𝑜𝑟𝑟 𝐸𝑑𝑒𝑓 = 𝐸𝑎𝑑𝑠  𝐸𝑖𝑛𝑡

(5)

In the gas phase, the calculations demonstrate that a-DOPA has a large deformation energy of about 9.41 kcal/mol. In contrast, the other three DOPA forms have small 𝐸𝑑𝑒𝑓 values (2.36 to 4.79 kcal/mol). In the aqueous phase, the deformation energies of the DOPA forms follow the same pattern as that observed in the gas phase. The deformation energy of a-DOPA is found to be 7.35 kcal/mol, while the deformation energies of the other DOPA forms are in the range of 2.92 to 4.16 kcal/mol.

3.7. Study of the Charge Transfer Between the Adsorbed DOPA Forms and the Au(111) Nanosurface. The natural bonding charges (NBOs) were used as a probe to evaluate the extent of the charge transfer between DOPA and Au surface in each Au/DOPA system in the gas phase. For this purpose, the changes in the NBO charges of the DOPA atoms were calculated when each DOPA form was adsorbed onto the Au(111) nanosurface. The NBOs atomic charges of the isolated and adsorbed DOPA forms are listed in Tables S6 and S7, respectively.



Table S8 shows the changes in the charges of the selected atoms in each DOPA form upon the adsorption process. In each Au/DOPA system, two competing charge transfer processes can be recognized including the charge transfer from DOPA to the Au surface (forward charge transfer) and vice versa (back charge transfer). The forward charge transfer mainly occurs through the electron-rich oxygen atoms. In addition, the phenylene ring mostly participates in the back charge transfer from the Au surface to DOPA. The degree of these two charge transfer processes determines the direction and magnitude of the net charge transfer in each system. The results show that the degree of the charge transfers between DOPA and Au surface depends on the charging state of DOPA. In u-DOPA, c-DOPA, and z-DOPA, the degree of the back charge transfer is higher than the forward charge transfer. Thus these three DOPA forms are negatively charged after the adsorption onto the Au surface. The theoretical results verify that the negative charges of –0.56 and –0.40 are added to u-DOPA and z-DOPA, respectively, upon their adsorption on the surface. The results also show that c-DOPA accepts a higher negative charge (–0.85) compared to u-DOPA and z-DOPA. In the case of a-DOPA, the net charge transfer is from the adsorbate to the surface. The findings demonstrate that a negative charge of –0.31 is transferred from a-DOPA to the Au surface upon the adsorption process. Thus in the Au/aDOPA, the forward charge transfer is higher than the back charge transfer. It is worth mentioning that the highest and the lowest net charge transfer occur in the Au/c-DOPA and Au/a-DOPA systems, respectively. The analysis of NBO charges was used to obtain detailed information about the interaction of the CO2 and phenolic hydroxyl groups with the Au surface in each Au/DOPA system. Comparing the atomic charges of the isolated forms with those of the adsorbed ones demonstrates that in all systems, the phenolic oxygens and especially the O2 atom have more 23 ACS Paragon Plus Environment

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contribution in the forward charge transfer when compared with the oxygen atoms of the CO2 group. As expected, the highest charge transfer of the phenolic oxygen atoms is observed in the Au/a-DOPA system. In addition, the changes observed in the NBO charges show that all four oxygen atoms of the u-DOPA and c-DOPA simultaneously participate in the forward charge transfer. However, in z-DOPA and a-DOPA, the analysis of the atomic charges confirms that the O4 atom of the CO2 group has no significant interaction with the Au surface. The degree of the forward and back charge transfers in each Au/DOPA system is directly related to the electron-donating/accepting character of the DOPA form. The electron donor/acceptor strength of each DOPA form strongly depends on its frontier molecular orbitals (FMOs) energies. In the following section, the trend observed in the magnitudes and directions of the net charge transfers will be discussed in detail based on the FMOs energies of the DOPA forms.

3.8. The Effect of the Au(111) Nanosurface on the Electronic Structures of the DOPA Forms. To investigate the effect of the surface on the electronic structures of the DOPA forms, the FMOs of each DOPA form were calculated in the isolated, deformed, and adsorbed states. Figure S18 illustrates the calculated highest occupied and the lowest unoccupied molecular orbitals (HOMOs and LUMOs) of the isolated structures with their energy levels in the gas phase. The results show that the energy levels of the FMOs of DOPA depend on its charging state. As shown in Figure S18, the FMOs of a-DOPA are considerably higher in energy than those of the other forms of DOPA. In contrast, the c-DOPA has lower energy levels as compared to the other forms. In addition, there is no significant difference between the FMOs energies of u-DOPA and z-DOPA. The HOMO and LUMO energy difference (Eg) is a typical



quantity to describe the reactivity of the system. The Eg values of the DOPA forms are in the range of 2.50 to 3.95 eV, which increase in the order: a-DOPA < c-DOPA < z-DOPA < uDOPA. These values show that the HOMO→LUMO electronic transition of a-DOPA occurs in the visible region (400–700 nm), while the electronic excitations of the other three forms are observed in the UV region (200–400 nm). The FMOs energies and Eg values show that the aDOPA and c-DOPA forms have the highest and the lowest reactivity against the oxidation reaction in the UV-vis region, respectively. These results are consistent with literature about the pH-dependent stability of DOPA in aqueous solutions. The FMOs of the deformed structures of DOPA are shown in Figure S19. The results show that the energies of the FMOs are changed due to the deformation process. The comparison of Figures S18 and S19 shows that the FMOs energies of the DOPA forms increase upon the deformation process. However, the a-DOPA and c-DOPA forms still show the highest and the lowest HOMOs and LUMOs energy levels, respectively. In addition, the deformed u-DOPA and z-DOPA structures have rather similar FMOs energies. The Eg values of the deformed structures follow the same order as that observed in the case of the isolated ones. The calculations clearly demonstrate that the FMOs energies of a-DOPA are more affected by the deformation process compared with those of the other three forms, which is consistent with the Edef value obtained for a-DOPA (see Section 3.6, Table 2). The energy levels of the HOMO and LUMO of the deformed structures also provide detailed information about their electron donating/accepting ability. As mentioned in the previous section (Section 3.7), the degree of the forward and the back charge transfers in each Au/DOPA system is directly related to the electron-donating/accepting character of the DOPA form. The electronic structures of the deformed DOPA forms are consistent with the magnitudes and directions of the net charge transfers observed for the 25 ACS Paragon Plus Environment

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Au/DOPA systems. The trend observed in the magnitudes and directions of the net charge transfers is discussed in the Supporting Information based on the FMOs energies of the DOPA forms (see Figure S20).

Figure 7. The calculated FMOs of the adsorbed DOPA forms and their energy levels in the gas phase. The FMOs of the different forms of DOPA adsorbed on the Au surface are shown in Figure 7. As can be seen, the adsorption of DOPA forms on the Au surface leads to the changes in their electronic structures. The FMOs energies of DOPA forms in the adsorbed state have the same order as those observed in the case of the deformed and isolated states. Comparing the energies of HOMOs and LUMOs of the adsorbed structures with those of the isolated ones shows that the FMOs energies of c-DOPA become unstable upon its adsorption on the Au surface, while in the other three forms of DOPA, the adsorption process results in the stabilization of the FMOs. It can be concluded that the reactivity of c-DOPA is increased on the Au surface in comparison to its isolated state. It is worth mentioning that the HOMO and LUMO



of a-DOPA are much more stabilized by the adsorption process compared with those of the other forms, which is consistent with the calculated value of Eads for a-DOPA (see Section 3.6, Table 2). The adsorption process results in increasing the Eg value of z-DOPA, while in the case of the other three forms, the Eg values decrease upon the adsorption. The change of Eg in u-DOPA is very small. As expected, the Eg values of the adsorbed structures do not follow the same trend as that observed in the case of the isolated structures. As can be seen in Figure 7, the Eg values increase in the order: a-DOPA < c-DOPA < u-DOPA < z-DOPA. These values are in the range of 2.36 to 4.23 eV, which are different with the Eg values observed for the isolated DOPA forms. The results indicate that the optical and chemical properties of the DOPA forms change due to the adsorption process. The Eg values of the adsorbed DOPA structures indicate that the HOMO→LUMO electronic transitions of c-DOPA and a-DOPA are observed in the visible region, while the excitations of u-DOPA and z-DOPA occur in the UV region. Thus the adsorbed c-DOPA and a-DOPA forms are in their exited electronic states in the visible region. Based on the FMOs energies and Eg values, it can be inferred that the adsorbed z-DOPA form shows the lowest reactivity against the oxidation reaction under the visible light irradiation. From the above considerations, it can be concluded that the Au surface does not necessarily have a stabilizing effect on all DOPA forms, so that the adsorption of c-DOPA on the Au surface leads to the increase of its reactivity compared to its isolated state. In addition, the electronic structure of the adsorbed a-DOPA form indicates its low photostability in the visible region. In contrast, the Au surface exhibits a stabilizing effect on u-DOPA and z-DOPA. Interestingly, z-DOPA is more stabilized by the Au surface compared to u-DOPA. The calculations indicate that the adsorbed z-DOPA form has the most stable electronic configuration on the Au surface.


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The FMOs of the adsorbed DOPA forms were also calculated in the aqueous phase using the ONIOM-PCM method (see Figure S21). The FMOs energies follow the same order as that observed in the gas phase. The obtained Eg values are in the range of 3.59 to 4.17 eV, which increase in the order: c-DOPA < a-DOPA < u-DOPA < z-DOPA. These values show that all adsorbed DOPA forms are photostable in the visible region. The results also confirm the stable electronic structure of z-DOPA on the Au NPs in water. Until now, the energy changes of the FMOs in the DOPA forms due to their deformation and adsorption on the Au surface were discussed. It is worth mentioning that the deformation and adsorption processes also result in the change of electron density of HOMO and/or LUMO in uDOPA, c-DOPA, and z-DOPA. However, the electron density of FMOs of a-DOPA is not significantly affected by the deformation and adsorption processes. For instance, the changes observed in the electron densities of the FMOs for z-DOPA are discussed in the following. The changes in the electron density of the other three DOPA forms (including u-DOPA, c-DOPA, and a-DOPA) are also discussed in the Supporting Information (see Figures S22–24). Figure 8 shows the HOMOs and the LUMOs of z-DOPA in its three states. In z-DOPA, the electron densities of the FMOs are not significantly affected by the structural deformation. In both of the isolated and deformed structures, the electron densities of HOMOs are concentrated on the CO2 group. In the isolated form, a low electron density is also distributed on the phenylene ring (except C1 and C4 atoms), and the oxygen atoms of phenolic groups. Upon the structural deformation, this small portion of the electron density is transferred to the other groups. As shown in Figure 8, both HOMOs of the isolated and deformed structures show a mixed n-π-character with a slight difference in the nonbonding and π contributions of their atoms. In the isolated form, the phenolic oxygen atoms and phenylene ring have small 28 ACS Paragon Plus Environment


contribution in the nonbonding- and π-character of the HOMO, respectively, while in the deformed structure, these two groups do not have any contribution in the HOMO character. In the LUMOs, the electron density is essentially concentrated on the phenylene ring and a small portion is distributed on the phenolic oxygen atoms and the NH3 group. Thus the LUMOs show the relatively similar mixed n-π*-character. Upon the structural deformation, a low electron density is transferred from the phenylene ring to the aliphatic C7 atom and no considerable change is observed in the HOMO and LUMO energies.

Figure 8. The comparison of the HOMOs and LUMOs of z-DOPA in the isolated, deformed, and adsorbed states in the gas phase. As shown in Figure 8, the interaction of z-DOPA with the Au surface leads to a significant change in the HOMO electron density. In the HOMO of the adsorbed form, a considerable charge transfer is observed from the CO2 and NH3 groups to the phenylene ring and the phenolic oxygen atoms. Thus the HOMO of z-DOPA obtains a new mixed n-π-character on the Au surface. Comparing the LUMOs of the adsorbed and deformed structures demonstrates 29 ACS Paragon Plus Environment

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that the electron density of the C7 atom is transferred to the phenylene ring due to the interaction process. The interaction of z-DOPA with the Au surface results in decreasing the energies of FMOs. The results show that the HOMO-LUMO gap of z-DOPA increases due to its interaction with the Au surface. Also, the comparison of the FMOs energies of the isolated and adsorbed structures shows that the energies of the FMOs decrease due to the adsorption process. As can be seen in Figure 8, the energy of the HOMO is more decreased than the LUMO energy, which leads to increasing the HOMO-LUMO gap. Consequently, the electronic excitation of the adsorbed z-DOPA form occurs at higher energies compared with its isolated state. The theoretical results confirm the experimental findings obtained in this work and give more insight into the interaction of z-DOPA with the laser-generated Au NPs. The experimental data can be interpreted using the results obtained in the computational section as follows: (i)

As mentioned in the experimental section, the in situ conjugation of z-DOPA with the laser-generated Au NPs indicates a blue-shift in the absorption band of zDOPA. The intensity of the absorption band also increases during the generation of Au NPs. The theoretical results show that the adsorption of z-DOPA on the Au surface leads to increasing the energy gap between the occupied and unoccupied molecular orbitals. Thus the electronic transition of the adsorbed z-DOPA form occurs at a higher energy compared with that of the isolated form, which is consistent with the observed blue-shift of the z-DOPA absorption band during the generation of Au NPs. The theoretical results also reveal that the interaction of zDOPA with the Au surface leads to the change in the electron density of the molecular orbitals, which effects on the probability of the electronic transition between the ground state and excited states. This phenomenon changes the 30 ACS Paragon Plus Environment


extinction coefficient of the transitions. Thus in the absorption spectra of the Au NP–z-DOPA system, the hypsochromic effect can be observed and the intensity of the z-DOPA band increases during the generation of Au NPs. (ii)

The IR spectra show that the adsorbed z-DOPA form exhibits a high stability in the aqueous solution, which is consistent with the changes of its electronic structure on the Au surface. The adsorption of z-DOPA on the Au surface results in decreasing the orbitals energies and increasing the HOMO-LUMO gap. Consequently, the adsorbed z-DOPA form becomes more stable against the oxidation compared to its isolated state.

3.9. MTT Cell Viability Assay. The in vitro cytotoxicity of z-DOPA and the Au NP–z-DOPA conjugates against the Jurkat T-cells was evaluated using the MTT cell proliferation assay. It is estimated that the free z-DOPA can show a significant cytotoxicity only at concentrations above 165 μg/mL. However, the obtained results show that at the total concentration of 8.3 ± 2 μg/mL, the Au NP–z-DOPA conjugates can inhibit 50% of the cell growth. The MTT assay clearly demonstrates that the Au NP–z-DOPA conjugates are dramatically more cytotoxic than the free z-DOPA against the Jurkat T-cells under the same experimental conditions. The detailed information is provided in the Supporting Information (Figure S25 and Table S9).

4. CONCLUSIONS Today DOPA is the most widely used prodrug for the treatment of Parkinson's disease. However, it is unstable and may degrade when exposed to the light or added to an aqueous solution (especially a basic solution), so that its therapeutic efficiency is greatly reduced. The use of nanoparticles as drug delivery systems is one of the most promising approaches to decrease 31 ACS Paragon Plus Environment

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the drug degradation. In the present work, we studied the in situ generation of the Au NP–zDOPA conjugates in a green method using the pulsed-laser ablation of gold foil in the aqueous solution of z-DOPA. The Au NPs with a protective layer of z-DOPA exhibit high colloidal stability for a long period of time at room temperature. The spectroscopic results showed that the Au NPs strongly interact with z-DOPA and have a significant stabilizing effect on the electronic structure of z-DOPA. In addition, the ONIOM calculations were used to investigate the effect of the Au(111) nanosurface on the electronic structures and geometries of all DOPA forms and the results showed that these forms strongly interact with the Au surface through the three binding groups (including the phenolic oxygens, the phenylene ring, and the CO2 group), as well as their adsorption on the Au surface results in the changes of electronic structures and geometries. Our findings showed that the structural and electronic changes of DOPA strongly depend on its charging state. Although the Au surface decreases the electronic energy of each DOPA form, it does not necessarily have a stabilizing effect on all DOPA forms. The analyses of the FMOs showed that the adsorption of c-DOPA on the Au surface leads to the increase of its reactivity against the oxidation reaction compared to its isolated form. The electronic structure of the adsorbed a-DOPA form also indicated its low photostability in the visible light. In contrast, the Au surface exhibits a stabilizing effect on u-DOPA and z-DOPA. Interestingly, z-DOPA is more stabilized by the Au surface compared to u-DOPA. The calculations revealed that the adsorbed z-DOPA form has the most stable electronic configuration on the Au surface. The computational findings are consistent with the experimental results and confirm the high stabilizing effect of the Au NPs on z-DOPA. Finally, the results demonstrated that the Au NPs significantly enhance the cytotoxic activity of z-DOPA against the Jurkat T-cells. In future works, we will investigate the interaction of other amino acids with Au NPs (or Au NCs). Furthermore, it is interesting to



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theoretically investigate the interaction of DOPA with Au NCs (such as Au20 cluster) and compare the results with those obtained in the present work.

ACKNOWLEDGEMENTS The authors would like to express their appreciation to the Isfahan University of Technology (IUT-IRAN) for financial support of this research. Supporting Information. Experimental and computational details; experimental setup; degradation pathways of DOPA; quenching efficiency and mechanism of Au NPs; emission of Au NP; IR of z-DOPA; UV–Vis and IR of the aged Au NP–z-DOPA; optimized structures of DOPA; FMOs of DOPA in the isolated and deformed states; FMOs of DOPA forms in the adsorbed states in water; electron-donating/accepting ability of each DOPA form; change of electron density of DOPA; MTT assay; the angle between CO2 and phenylene ring in the DOPA structures; dihedral angles in the DOPA structures; NBOs in the DOPA structures; changes of dihedral angles and NBOs charges; distance of the DOPA atoms from the surface

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