Geometrical and Electronic Properties of Hydrated Sodium

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Geometrical and Electronic Properties of Hydrated Sodiumand Tetracycline Montmorillonite from DFT Calculations Silvina Pirillo, Carla Romina Luna, Ignacio Lopez-Corral, Alfredo Juan, and Marcelo J. Avena J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b04061 • Publication Date (Web): 26 Jun 2015 Downloaded from http://pubs.acs.org on July 2, 2015

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The Journal of Physical Chemistry

Geometrical and Electronic Properties of Hydrated Sodium- and Tetracycline Montmorillonite from DFT Calculations Silvina Pirilloa, Carla R. Lunab, Ignacio López-Corrala, Alfredo Juanb, Marcelo J. Avenaa* a

Instituto de Química del Sur (INQUISUR, UNS−CONICET) and Departamento de

Química, Universidad Nacional del Sur, Av. Alem 1253, B8000CPB, Bahía Blanca, Argentina. b

Instituto de Física del Sur (IFISUR, UNS−CONICET) and Departamento de Física,

Universidad Nacional del Sur, Av. Alem 1253, B8000CPB, Bahía Blanca, Argentina. *[email protected] Telephone number: + 54-291-4595101 (3579)

1

ACS Paragon Plus Environment


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Abstract The adsorption of fully protonated tetracycline (TC) using Na-montmorillonite (NaMMT) as an adsorbent is studied by DFT calculations. Geometric, electronic and magnetic properties are analyzed. To the best of our knowledge, this is the first theoretical DFT research about Na-MMT as an adsorbent of TC cationic specie. Two Na-MMT models are considered: dry and hydrated Na-MMT. The incorporation of four water molecules in the interlayer space results in the expansion of the d001 parameter from 10.13 Å (dry) to 12.85 Å (hydrated). Finally, when TC molecule is adsorbed in the basal space of MMT, the increment of the d001 parameter is about 10 Å respect to hydrated model. Dry and hydrated Na-MMT present a non-magnetic (µ = 0 µB) and semiconductor behavior. Nevertheless, the TC-MMT system becomes conductor, and presents a very important magnetic moment (µ = 10.60 µB). After TC adsorption in MMT a potential valley with a slight depth in the MMT interlayer is generated. For this reason, it is proposed that TC adsorption would provide active sites for the adsorption of new species (like Na ions and water molecules). Keywords: Tetracycline; Montmorillonite; Adsorption; DFT; Bonding.

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1. Introduction Widely used antibiotics are considered harmful to ecosystem health due to their potential adverse effects on important soil microbial processes and aquatic photosynthetic organisms, and their implications in the rise of antibiotic-resistant genes.1 Tetracyclines, for example, which enter the environment mainly because of its wide use as animal growth promoters, have shown to disrupt microbial soil respiration, Fe (III) reduction, nitrification and phosphatase activities.2 Tetracyclines are an antibiotic family that presents bacteriostatic activity toward gram-negative and gram-positive microorganisms.3,4 This family comprises a group of natural and semi-synthetic products that inhibit the synthesis of bacterial proteins. The socalled tetracycline (TC) is one of the members of this family and its chemical properties have been extensively studied.5-8 TC has in its chemical structure three different acid groups that can undergo protonation-deprotonation reactions, leading to the formation of different ionic species and the adoption of different structural conformations depending on the pH of the aqueous solution. The best known conformations of TC ions are the “extended” conformation, adopted in basic media, and the “twisted” conformation, which predominates in neutral or acid solutions.8-11 Clay minerals are ubiquitous in natural media and are good adsorbents of different ions and molecules. Through adsorption-desorption reactions, clay minerals can participate in the control of the environmental mobility of many sustances. In addition, and due to their adsorptive properties, clay minerals are widely used for the removal of dyes and coloring substances from wool, oils, wines, fruit juices, fats and waxes; as Ca2+ remover in water softening, and as adsorbent of many pollutants.12,13 Also, clay minerals are extensively used 3



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in pharmaceuticals as carriers or suspending agents. All these facts, justify the interest in the possible interactions between clay particles and diferent types of substances, especially antibiotics or other drugs if the environmental mobility of drugs wants to be understood or the perfomance of clays as a pharmaceutical excipient needs to be investigated.14-18 Among clays, montmorillonite (MMT) is extensively used in pharmacological applications due to its laminar structure with high surface area and its high cation exchange capacity. The properties of MMT make this clay able to interact with several drugs, affecting their bioavailability and activity.8,19 MMT belongs to the smectite family of 2:1 clay minerals and has a layered structure consisting of two tetrahedral sheets linked to an octahedral sheet through the sharing of apical oxygens; it possesses a negative layer charge due to isomorphic substitutions mainly in the octahedral sheet (Mg2+ for Al3+). The excess negative charge is normally compensated by exchangeable, typically interlayer hydrated cations (Na+, K+, Ca2+, or Mg2+ in natural samples).20 These clay minerals easily undergo hydration, and have the tendency to swell by the incorporation of large amounts of water molecules.13 They can also exchange interlayer cations (usually in hydrated form) and incorporate various species, from small cations to polyelectrolytes into the interlayer space.21 There are several reports that analyze the interaction between TC and different clays, including MMT.22 Adsorption isotherms, X-ray diffraction (XRD) meaurements and spectroscopic techniques reveal high adsorption and intercalation of TC in the MMT interlayer space at acidic pH whereby there is an increase in the basal spacing (d001), whereas, at circumneutral to basic pH there is low adsorption which only becomes important in the presence of divalent cations.1,23 The high adsorption at low pH is usually 4


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attributed to the presence of the fully protonated TC molecule, which is positively charged and adsorbs by cation exchange. Only a few ab initio studies have considered the smectites hydrated phases.24-32 Berghout et al.13 showed by means of density functional theory (DFT) calculations how different types of interlayer inorganic cations and their different hydration affect the d001 and mechanical-elastic (bulk modulus, B0) properties of MMT. These authors concluded that the d001 and B0 are governed by the size and the degree of hydration of the countercations. The aim of the present work is to study by means of theoretical calculations the adsorption process of the fully protonated TC using Na-MMT as an adsorbent. DFT calculations are performed and geometric, electronic and magnetic properties are analyzed. To the best of our knowledge, this is the first theoretical DFT research about hydrated NaMMT as an absorbent of TC cationic species.

2. Computational Method and Model All calculations were performed using the Vienna Ab−initio Simulation Package (VASP) code within the framework of the generalized gradient approximation of the density functional theory (GGA−DFT), with the exchange−correlation functional of Perdew−Wang 91.33,34 DFT calculations were performed taking into account dispersion corrections. The semi−empirical van der Waals correction was included as described by Grimme35, where this kind of interactions is described via a simple pair-wise force field, which is optimized for several popular DFT functionals. It is well known that DFT computations that utilize conventional (semi) local exchange correlation functionals are 5



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unable to account adequately for vdW interactions and hence lead to poor predictions of equilibrium structures, densities, cohesive energies, and bulk moduli of some systems. Nevertheless, Liu et al.36 note that the PBE-D2 scheme leads to significant improvements in the predictions of structural properties of polymers with respect to the purely (semi)local functionals. The cutoff energy for the plane-wave basis was 650 eV. It was considered a 5×5×5 Monkhorst−Pack k−point grid for sampling the Brillouin zone in the case of the MMT bulk and 1×1×1 for isolated fully protonated TC and the system (fully protonated TC with MMT) because of the size of the computational cells.37 The convergence criteria was set to be 10-3 eV/Å on each atom and 10-4 eV for the full relaxation total energy changes without constraints using conjugate-gradient algorithm. During the ionic relaxation process, the forces acting on the ions were calculated using the Hellmann−Feynman theorem, including the Harris−Foulkes corrections.38 Compositional variety and structural complexity of clay minerals usually require certain simplifications of the models used in theoretical calculations. The used structural models were derived from the monoclinic structure of Wyoming MMT with structural formula Na0.33(Al1.67Mg0.33)Si4O10(OH)2.20,39 Only Al3+/Mg2+ substitution in the octahedral sheets were included. Similar models have also been used in a theoretical study of hydration steps of several cations in the interlayer space.13,20 Following the ideas of Berghout et al.13 we considered two partially hydration models of Na-MMT: a dry model (no water molecules); and a one water layer model with four water molecules coordinated to the Na cation in the interlayer space. According to Berghout et al.13, four water molecules represent an ideal coordination in planar cation complex and are also consistent with experimental observations.40,41 From the comparison of dehydrated and hydrated Na6


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MMT the effect of the water in the interlayer space on the geometric, electronic and magnetic properties were analyzed. The initial structure for simulations was the MMT bulk with exchangeable Na+ cation hydrated by four water molecules (Na-MMT) (Figure 1 a). The Na-MMT model has initial cell parameters a = 5.20 Å, b = 9.20 Å, c = 12.63 Å and β = 98.99°. The model of fully protonated TC intercalated in MMT (TC-MMT) was build from the optimized NaMMT model replacing the Na+ hydrated by the TC cation (Figure 1 b). In order to simplify calculations, no water molecules were considered to be present in the interlayer space. Since the size of the TC cation is considerably larger than that of a water molecule, introducing water molecules would not change appreciably this spacing. The initial cell parameters were a = 10.52 Å, b = 17.53 Å, c = 22.85 Å and β = 95.34°. Calculations on the isolated fully protonated TC (twisted conformation) were performed in a supercell in order to minimize mutual interactions between neighboring cells.

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Figure 1. Initial structures of Na-MMT and TC-MMT models. O,

H,

C,

N,

Al,

Si,

Mg,

Na

3. Results and Discussion 3.1. Adsorption Geometry Table 1 lists the values of the cell parameters (a, b, c and β) and basal spacing (d001) corresponding to the equilibrium volume for dry and hydrated Na-MMT and TC-MMT after optimization. For comparison some theoretical and experimental values reported in the literature are also included.

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Table 1. Optimized cell parameters (a, b, c and β) and basal spacing (d001) for dry and hydrated Na-MMT and TC-MMT. Some values are compared with results collected from the literature. dehydrated Na-MMT this work

theo. a

hydrated Na-MMT exp.

this work

theo.

exp.

this work

exp.

5.26

5.23

a

-

10.52

-

a (Å)

5.20

5.24

b (Å)

9.20

9.08a

8.97-9.01b

8.77

9.08a

-

17.54

-

c (Å)

10.13

10.36a

10.05-10.20b

12.85

12.55a

-

22.86

-

β (°)

98.99

98.70a

99.50-101.40b

95.34

97.80a

-

95.34

-

d001 (Å)

10.13

10.24a, 10.17c

9.55d,e, 9.60e,f, 9.83g

12.81

12.43a, 11.51c

12.5e, 13.80h,i

22.79

22.00h

a

5.18

b

TC-MMT

Ref. 13. bRef. 39. cRef. 24. dRef. 42. eRef. 43. fRef. 44. gRef. 45. hRef. 8. iRef. 46.

Table 1 shows that the calculated cell parameters present only a slight difference with respect to previously reported values. For the d001 distance the difference with the experimental reported values is approximately 1 Å. This difference is due to the fact that natural clay samples present two hydration states, with one and two intercalated water layers in the interlayer space.8 The incorporation of four water molecules (hydrated NaMMT) in the interlayer space with a one-layer arrangement results in the expansion of the d001 parameter from 10.13 Å (dry) to 12.81 Å (Figure 2 a and b, respectively). Ferrage et al.40 studied by X-ray diffraction (XRD) hydration of SWy-1 source clay (low-charge montmorillonite) under controlled relative humidity (RH) on Li-, Na-, K-, Mg-, Ca-, and Sr-saturated specimens. Particularly, for Na-SWy-1 with a RH of 35 % and 60 % the d001 value was 12.44 Å and 12.47 Å, respectively. Furthermore, Bérend et al.43 by different experimental methods studied the mechanism of adsorption and desorption of water vapor of five homoionic montmorillonites saturated by alkaline cations on a single Na-MMT. 9



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Typical XRD patterns showed for hydrated Na-MMT a d001 value of 12.5 Å. These values are similar to those obtained in this work. Finally, when the TC molecule is within the basal space of MMT, the increment of the d001 parameter is about 10 Å (from 12.81 Å to 22.79 Å) (see Figure 2 c), a value that is also in agreement with experiments. Unfortunately there is not available any experimental data about cell parameters values for hydrated Na-MMT and TC-MMT.

Figure 2. Optimized structures of dehydrated and hydrated Na-MMT and TC-MMT models. O,

H,

C,

N,

Al,

Si,

Mg,

Na

Table 2 presents the bond distances for selected atoms after optimization. It can be seen that the Si─Si, Al─Al and Al─Mg distances in the hydrated Na-MMT and TC-MMT systems are longer (approximately 5.0 %, 8.0-9.5 % and 2.4-5.0 %, respectively) than in the dehydrated Na-MMT system. This increase in bonding distances indicates that the bonds are debilitated after the expansion of the interlayer space. In addition, the Na─Si bond is 10


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also debilitated after the hydration of Na-MMT (from 3.37-3.81 Å to 3.61-4.64 Å). For basal oxygen atoms located in the upper and lower MMT sheets, the O─O distance is practically the same for dehydrated and hydrated Na-MMT, being this distance 3 % smaller than that of TC-MMT. The O─O distance for internal oxygen atoms practically does not vary for all the systems studied (the difference is lower than 2 %). Finally, based on the O─O distances corresponding to surfaces oxygen atoms belonging to the same MMT layer, it can be concluded that the whole MMT structure is not impacted by the enlargement, only the interlayer space is enlarged. The distances between Na and oxygen atoms from water are about 2.31 Å. Similar values were computed by Berghout et al. (2.40 Å).13 In the case of dry MMT the Na ion is located approximately in the midplane of the clay interlayer space, at 2.38 Å and 2.44 Å from basal oxygens belonging to the layers that are above and below the cation. Nevertheless in hydrated Na-MMT, Na ion is clearly not located symmetrically in the midplane. The distances are 2.23 Å and 3.76 Å to its first neighbors basal oxygens (see Figure 2 b). Finally, calculations show that the TC molecule is located at ∼ 5.11 Å from the layer above and at ∼ 2.02 Å from the layer below TC, as it can be seen in Figure 2 c.

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Table 2. Bond distances for different atoms after geometries optimization, where the first and second value correspond to the shorter and larger ones, respectively. distance (Å) dehydrated Na-MMT

hydrated Na-MMT

TC-MMT

Si─Si

3.02-3.18

3.17-3.35

3.17-3.35

Na─Si

3.37-3.81

3.61-4.64

-

Al─Al

3.07-3.11

3.36

3.36

Al─Mg

2.92-2.99

3.36

3.36

O─O (1)a

2.66-2.72

2.67-2.71

2.57-2.67

O─O (2)a

2.81-2.89

2.83

2.83

O─O (3)a

6.69-6.70

6.77-6.90

6.77-6.91

a

The O─O (1) and the O─O (2) distances correspond to basal oxygen atoms located in the upper and lower MMT layers, and to oxygen atoms of the inner MMT layers. The O─O (3) distances correspond to surfaces oxygen atoms belonging to the same layer.

3.2. Electronic and Magnetic Properties DOS curves are shown in Figure 3 and some computed physical and chemical properties are summarized in Table 3. As mentioned by other authors, the properties of the modified clays depend on the concentration and spatial arrangement of the cations in the interlayer space.47,48

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Table 3. Band Gap (Eg), Magnetic Moment (µ), distance between Fermi Level (EF) and Valence and Conduction Band (∆VB and ∆CB) for dehydrated Na-MMT, hydrated NaMMT and TC-MMT. system

Eg (eV) µ (µB) ∆VB (eV) ∆CB (eV)

dehydrated Na-MMT

2.56

0.00

0.28

2.28

hydrated Na-MMT

0.74

0.00

0.29

0.45

TC-MMT

0.00

10.60

0.00

0.00

isolated TC

2.04

1.00

0.33

1.71

Dehydrated and hydrated Na-MMT behave as semiconductors, with a band gap of 2.56 eV and 0.74 eV respectively. This value is typically underestimated, due to well-know limitations in DFT calculations.49,50 It is well known that the band gap energy is a critical factor governing clay photocatalytic activity.51,52 The incorporation of water molecules in the MMT interlayer leads to a considerable band gap reduction, around 70 %, respect to dry MMT. Moreover, we noted the presence of occupied localized energy states below Fermi level (see Figure 3 a to 3 c) as a result of mixing states between dry MMT and Na ions. Similar behavior appears in hydrated Na-MMT as shown in Figure 3 d to 3 f. In Figure 3 a and 3 d it can be seen that the Fermi level is closer to VB, then the dehydrated and hydrated Na-MMT behaves as a p-type semiconductor. Consequently, the promotion of photoelectrons to the conduction band is followed by rapid thermalization to the band edge, resulting in an excess free-carrier density like.53

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The total DOS and PDOS curves for TC-MMT are plotted in Figure 3 g to 3 i. From TC PDOS curve (Figure 3 i) it can be seen that TC remains being semiconductor after adsorption in MMT. Nevertheless, the TC-MMT system has a conductor behavior then it has not photocatalytic activity, contrary dehydrated and hydrated Na-MMT systems. A similar behavior was reported by Koutselas et al.54 In this work those authors employed several experimental techniques, between them photoluminescence experiments. Their results showed that the detectable luminescence was due only to the organic molecules within the clay interlayer. Regarding the magnetic behavior of dry and hydrated Na-MMT, the computed magnetic moment was zero in both cases. For this reason the total DOS and PDOS curves for spin up-down contribution are symmetrical. Nevertheless, for TC-MMT a very important magnetic moment was noted, with a value of 10.60 µB. Mao et al.55 found a similar behavior studying the functionalization of graphene (a single graphite layer) by the addition of transition metal atoms of Mn, Fe and Co to its surface. The total DOS curve is not symmetric around the Fermi level, between -3 eV and 3 eV.

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Figure 3. DOS and projected DOS curves for dehydrated Na-MMT, hydrated Na-MMT and TCMMT. The dotted vertical line indicates the Fermi level.

3.3. Electrostatic Potential and Bader′′s Charge Analysis Figure 4 shows the computed electrostatic potentials along to the z-direction for dry Na-MMT and hydrated Na-MMT. It can be seen that the Na species, located in the interlayer of dry MMT, act like electron donor due to the fact that there is a potential peak around them (their charge states are positive). In addition to this, the alternate charge states of the MMT are well noticed. The O atoms act like acceptors (a depth potential valley) and the Si, Mg and Al atoms have positive charge states (peak of potential). This behavior was 15



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obtained in several studies.56 In Figure 4 a it can be observed that valleys around z = 3 Å and 6 Å, maybe can provide adsorption active sites for new species, i.e. water molecules. In the case of hydrated Na-MMT, the water molecules lead to a new charge distribution around Na ions (see Figure 4 b). For this reason a potential valley around z = 5 Å is noticed, then it is possible to say that more water molecules would be adsorbed in the MMT interlayer. However the alternate charge states of the MMT are not modified significantly if dry and hydrated Na-MMT are compared. Similar results were obtained by Chang et al.57 Finally, the TC adsorption in MMT, besides increasing the d001, generates a potential valley with a slight depth (z ∼ 3.5 Å) in MMT interlayer (see Figure 5). This suggests that after TC adsorption this valley would still remain as an active site for interaction with for example a water molecule. This will be the subject of a future investigation.

Figure 4. Electrostatic potential in the normal direction of increase MMT (z direction) for (a) dehydrated Na-MMT and (b) hydrated Na-MMT. O,

H,

C,

N,

Al,

Si,

Mg,

Na

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Figure 5. Electrostatic potential in the normal direction of increase MMT (z direction) for TC-MMT. O,

H,

C,

N,

Al,

Si,

Mg,

Na

Also, it is possible to observe that the positive charge of the protonated TC molecule is aligned with the negative charge presented in the MMT structure due to isomorphic substitutions mainly in the octahedral sheet (ions Mg2+ for Al3+).

4. Conclusions By employing periodic DFT calculations it was found that cell parameters (a, b, c and β) and basal spacing (d001) have shown a good agreement with previously reported experimental and theoretical data for Na-MMT and hydrated Na-MMT. The incorporation of four water molecules in the MMT interlayer space results in the expansion of the d001 parameter around 27 % respect to dry MMT. Finally, when the hydrated Na cation is exchanged by the fully protonated TC in the basal space of MMT, the increment of the d001 17



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parameter is about 78 %. For the first time theoretical data about geometric parameters, electronic and magnetic properties for hydrated Na-MMT and TC-MMT are reported.

Acknowledgements The authors acknowledge SGCyT (UNS), PICT 2186 y PIP CONICET 2014-2016 N° 11220130100436, Departamento de Química (UNS) and Departamento de Física (UNS), for financial support. All authors are members of CONICET.

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References (1) Aristilde, L.; Marichal, C.; Miéhé-Brendlé, J.; Lanson, B.; Charlet L. Interactions of Oxytetracycline with a Smectite Clay: A Spectroscopic Study with Molecular Simulations. Environ. Sci. Technol. 2010, 44, 7839–7845. (2) Zielezny, Y.; Groeneweg, J.; Vereecken, H.; Tappe, W. Impact of Sulfadiazine and Chlorotetracycline on Soil Bacterial Community Structure and Respiratory Activity. Soil Biol. Biochem. 2006, 38, 2372–2380. (3) Krafft, C.; Hinrichs, W.; Orth, P.; Saenger, W.; Welfle, H. Interaction of Tet Repressor with Operator DNA and with Tetracycline Studied by Infrared and Raman Spectroscopy. Biophys. J. 1998, 74, 63–71. (4) Parolo, M. E.; Savini, M. C.; Vallés, J. M.; Baschini, M. T.; Avena, M. J. Tetracycline Adsorption on Montmorillonite: pH and Ionic Strength Effects. Appl. Clay Sci. 2008, 40, 179–186. (5) Stezowski, J. J.; Prewo, R. Chemical-Structural Properties of Tetracycline Derivatives. 3. The Integrity of the Conformation of the Nonionized Free Base. J. Am. Chem. Soc. 1977, 99, 1117–1121. (6) Lambs, L.; Brion, M.; Berthon, G. Metal Ion-Tetracycline Interactions in Biological Fluids. Part 4. Potential Influence of Ca2+ and Mg2+ Ions on the Bioavailability of Chlortetracycline and Demethylchlortetracycline, as Expected from their ComputerSimulated Distributions in Blood Plasma. Inorg. Chim. Acta 1983, 106, 151–158. (7) Jin, L.; Amaya-Mazo, X.; Apel, M. E.; Sankisa, S. S.; Johnson, E.; Zbyszynska, M. A.; Han, A. Mg2+ Bind Tetracycline with Distinct Stoichiometries and Linked Deprotonation. Biophys. Chem. 2007, 128, 185–196. 19



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Geometrical and Electronic Properties of Hydrated Sodium

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