Polyaniline Based Materials as a Method to Eliminate Haloanisoles in

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Polyaniline based materials as response to eliminate haloanisoles in spirits beverages Oscar Valdés, Adolfo Marican, Fabian Avila-Salas, Ricardo I. Castro, John Amalraj, V. Felipe Laurie, and Leonardo S. Santos Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01139 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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Industrial & Engineering Chemistry Research

Polyaniline based materials as response to eliminate haloanisoles in spirits beverages

1 2 3 4

Oscar Valdés*,a, Adolfo Marican b, Fabian Avila-Salas c, Ricardo Ignacio Castro d, John Amalraj b,

5

Victor Felipe Laurie e, and Leonardo Silva Santos *,f

6 7

a

Vicerrectoría de Investigación y Postgrado, Universidad Católica del Maule, Talca, Chile.

8

b

Instituto de Química de Recursos Naturales, Universidad de Talca, Talca, Chile.

9

c

Centro de Nanotecnología Aplicada, Facultad de Ciencias, Universidad Mayor, Huechuraba 8580000, Región Metropolitana, Chile.

10 11

d

Instituto de Ciencias Biomédicas, Universidad Autónoma de Chile, Talca, Chile.

12

e

Facultad de Ciencias Agrarias, Universidad de Talca, Talca, Chile.

13

f

Laboratorio de Síntesis Asimétrica, Instituto de Química de Recursos Naturales, Universidad de

14

Talca, Talca, Chile.

15 16 17 18 19 20 21 22 23

* Corresponding Authors:

24

*L.S.S.: Tel, +56(71)2201575; fax, +56(71)2200448; Email, [email protected]

25

*O.V.: Tel, +56712203100; Email, [email protected]

26

Notes: The authors declare no competing financial interest

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract

29

In this research, the ability of two polyaniline-based materials (PANI-EB and PANI-ES) was

30

evaluated as a potential fining agent to eliminate 2,4,6-trichloroanisole (TCA) and 2,4,6-

31

tribromoanisole (TBA). The results showed that the retention percentage of TCA and TBA were

32

higher than 60% for all the materials tested in methanol, and they vary according to the interaction

33

time and the quantity of polymer used. The polymers were also tested in whisky following the same

34

procedures and considering the results obtained in the methanol tests. The analyses indicated that

35

polyaniline-based materials are effective in removing TBA and TCA, with retention percentages

36

around 80% and 12%, respectively. Electronic structure calculations and molecular dynamics

37

simulations helped to gain insight on the behavior of the PANI polymers in methanol, and simulated

38

whisky solution (ethanol/water), and their interactions with each haloanisole. Finally, the main

39

compounds present in the whisky were characterized in order to demonstrate that the purification

40

process did not modify significantly the aromatic profile of the product and the total phenolic

41

content.

42 43 44

Keywords: Trichloroanisole; tribromoanisole; total phenolics; polyaniline; whisky.

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1. INTRODUCTION

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Haloanisoles are chemical substances that are responsible for musty taints in a variety of

50

foods and beverages. The most representative compounds of this family are 2,4,6-trichloroanisole

51

(TCA) and 2,4,6-tribromoanisole (TBA), which have been identified as trace contaminants that

52

make a musty off-aroma in broiler chickens, essential oils, wine, water, grains, potatoes, raisins,

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sake and others.1,2 In alcoholic beverages, these haloanisoles are usually related to cork taint and are

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considered a major organoleptic defect that produces a moldy aroma. In wines, this defect is of

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crucial economic importance as it could affect 1-5% of the production, reaching values as high as

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30% of bottles being affected.3 Similarly, it is well-known that TCA and TBA taints can be

57

absorbed into packaging materials, ingredients, and other products by diffusion. Beverage industries

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have reported that chemically porous polymeric materials are vulnerable to contamination with

59

these taints. Correspondingly, the beverage can easily absorb them from contaminated materials.4

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Frequent consumers are more likely to notice contamination of food, beer, whisky and wine due to

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the unpleasant flavors and smells that these taints can also produce. Specifically, for wine and

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whisky the perception threshold value ranges from 5 to 10 ng L-1.5

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In respect to the formation of TCA and TBA, it commonly known that these haloanisoles

64

result from the microbiological methylation of chlorophenols, which may be derived from

65

contamination with chlorine during the production process.6 Haloanisoles can also be formed by the

66

degradation of chlorine compounds such as anthropogenic tri-, tetra-, and pentachlorophenols (TCP,

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TeCP, PCP).7 A third source of chloro-compounds is the microbiological formation of TCA by

68

“Basidiomycetes and Penicillium isolates. These microorganisms can degrade anthropogenic

69

phenols, and among them, some Basidiomycetes are able to convert chlorophenols to the

70

corresponding anisoles and result in O-methylation.7 This is a crucial step in the formation of TCA

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and TBA in cork.8 The efficiency of bioconversion of 2,4,6-trichlorophenol to 2,4,6-trichloroanisole

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varies, but the most effective strains of Fusarium oxysporum, Penicillium citreonigrum and

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Trichoderma longibrachianum can reach 29%, 13% and 38%, respectively”.9


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Since these haloanisoles are considered the most important contributor to the organoleptic

75

error already mentioned, a lot of research has been done to identify and evaluate different products

76

that can be used to remove them. Currently, different methods have been followed to eliminate

77

TCA and TBA from contaminated products, such as the use of activated charcoal10 or polyethylene,

78

which were added as adsorbents.11 In addition, yeast cell wall preparations were also tested for the

79

removal of chlorinated anisoles.12 Moreover, other methodologies exist to resolve taste and odor

80

problems produced by haloanisoles in drinking water, such as the use of heterogeneous catalytic

81

ozonation developed by Fei Qi et al.13 However, new techniques and tests of other polymeric

82

compounds are interesting for the industry, especially for whisky production, which is the main

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objective of this article.

84

In this article, we evaluated the effects of two polyaniline-based materials (PANI-EB and

85

PANI-ES) as potential remediation compounds for the problems caused by the presence of TCA

86

and TBA in whisky. In addition, quantum mechanics calculations and molecular dynamics

87

simulations were performed with the objective of analyzing the behavior of PANI polymers in

88

simulated whisky and methanol solutions. These calculations made possible the characterization of

89

the main intermolecular interactions that stabilize the affinity of the PANI polymers for both

90

haloanisole.

91

2. EXPERIMENTAL SECTION

92

2.1. Materials

93

Folin–Ciocalteu reagent, sodium carbonate (Na2CO3), 2,4,6-trichloroanisole (TCA) and

94

2,4,6-tribromoanisole (TBA) were purchased from Sigma–Aldrich, while methanol (≥99.5%) and

95

acetonitrile (HPLC grade) were acquired from Merck. Scotch whisky (Johnnie Walker Red Label)

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was purchased in commercial stores. Other chemicals were used without further purification.

97

2.2. Synthesis and characterization of PANI polymers

98 99 100

The synthesis and characterization of polyaniline emeraldine salt (PANI-ES) and emeraldine base (PANI-EB) was prepared according to a reported procedure by Marican et al.14 2.3. Absorption of TCA and TBA by polyaniline-based materials in methanol solution ACS Paragon Plus Environment

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101

A screening study was conducted using methanol to calculate the retention capacity of 2,4,6-

102

trichloroanisole and 2,4,6-tribromoanisole by the PANI-ES and PANI-EB. A methanol solution was

103

used with a concentration of 20 ng L-1 of both haloanisoles. The amount of polymers used in

104

treatment was 100 mg L-1, 300 mg L-1, and 500 mg L-1. The samples were agitated in 10 mL amber

105

vials, at 20 rpm, for 1, 8 and 24 h, at room temperature. The samples were filtered through

106

polyvinylidene fluoride (PVDF) filters (0.45 µm) and the free concentration of TCA and TBA was

107

determined by GC-MS analyses, giving the retained amount of TCA and TBA by the polymers.

108

2.4. The affinity of polyaniline-based materials towards TCA and TBA in whisky

109

Following the screening study performed in methanol solution, the ability of polyaniline-

110

based materials to capture TCA and TBA was evaluated in whisky. A sample of the whisky chosen

111

was spiked with a desired aliquot of TCA and TBA methanol solutions to reach a final

112

concentration of 20 ng L-1. The chosen concentration of TCA and TBA is higher that the recognized

113

organoleptically but was chose in order to test the capturing abilities of the polymers tested. The

114

amount of polymers used, interaction time and other conditions were the same as in the screening

115

study (Section 2.3). The samples were filtered through PVDF filters (0.45 µm), and the remaining

116

concentrations of TCA and TBA were determined by GC-MS.

117

2.5. Total phenolic content (TPC)

118

The TPC was determined by the Folin–Ciocalteu procedure.15 For each 160 µL of samples of

119

whisky, 1.44 mL of water was added, and 100 µL of Folin–Ciocalteu reagent. The mixture was

120

incubated for 10 min and then 300 µL of Na2CO3 at 20% were added. After 30 min, the absorbance

121

was measured at 750 nm using a UV spectrophotometer. The results were expressed as gallic acid

122

equivalents (GAE) in milligrams per dL of whisky (mg GAE/dL of whisky). Additional dilution

123

was conducted if the absorbance value measured was over the linear range of the standard curve.

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All determinations were carried out in triplicates.

125

2.6. UV analysis


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126

UV–Vis

spectrometric

investigations

were

performed

using

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a

spectrophotometer

127

Spectroquant Pharo 300 MERCK. The conditions for determining the TPC content were described

128

in the previous section.

129

2.7. Gas chromatography–mass spectrometry (GC–MS)

130

The quantitative study of TCA and TBA in all studies was performed by GC–MS, using a GC

131

Trace 1300 (Thermo Scientific, Italy) coupled to a triple quadrupole mass spectrometer (TSQ

132

8000), with an AS 3000 autosampler. The column was a Restek Rtx-5MS w/integra-guard (30 m,

133

0.25 mm ID, 0.25 µm ft). The working conditions were as follows: the carrier gas was helium with

134

a flow rate of 1.2 mL min-1. The split/splitless injector port and mass spectrometer interface line

135

were heated to 270 °C and 250 °C, respectively. The oven temperature was programmed from 40

136

°C, held for 3 min, and then to 230 °C at 10 °C min-1, held for 2 min. Splitless injections were made

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with a splitless time of 1 min. The mass spectrometer was operated in electron impact ionization

138

mode with a source temperature of 250 °C and an emission current of 70 eV. TCA was detected by

139

SIM (m/z 174, 176, 195, and 210) and m/z 210 was selected for quantification. TBA was detected

140

by SIM (m/z 141, 303, 329, and 344) and m/z 329 was selected for quantification.

141

2.8. Analysis of volatile compounds by GC–MS

142

The samples were injected directly to the GC, to avoid the extractions and derivatization of

143

this type of compounds in these matrices. The calibration curves were performed in the same way.

144

The initial oven temperature was set to 60 ºC (for 2 min), then increased in two steps: 60 to 100 ºC,

145

at 20 ºC/min; and 100 to 240 ºC at 5 ºC/min (held 10 min). The injector temperature was 250 ºC and

146

the transfer line was held at 240 ºC. The carrier gas was Helium with constant pressure of 1.2

147

mL/min. The detection was performed by a TSQ 8000 mass spectrometer in the electronic impact

148

(EI) mode (ionization energy, 70 eV; temperature source, 230 ºC). The electron multiplier was set

149

to the auto tune procedure. The acquisition was made in full scan mode (the mass-to-charge ratio

150

range used was m/z 30–400). The compounds were identified by comparison of mass spectra data

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obtained from the sample with that taken from the technical guide for analyzing alcoholic beverage

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by GC and the mass spectra obtained from the NIST library. ACS Paragon Plus Environment

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2.9. Computational details

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2.9.1. Building Molecular Structures

155

Structures of TBA, TCA, PANI-EB monomer and PANI-ES monomer (at neutral and low

156

pH) were design through GaussView software.16 The geometric optimization of the 3D molecular

157

structures was carried out using Gaussian0317 software. The calculations were performed at Density

158

Functional Theory18 level with B3LYP19 method and with 6-311G+(d,p) as basis set.

159

Chains of 4, 6 and 8 monomers were built using LEAP program of AmberTools software

160

package.20 Then, using PACKMOL21 program, the chains were used to generate four PANI particles

161

of approximately 12,000 atoms each one of them (Figure S1).

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2.9.2. Interaction energies (∆ ∆E) study at semiempirical quantum mechanical level

163

The strategy that mixes a Monte Carlo sampling and semiempirical quantum mechanical

164

calculations was used to estimate the ∆E between pairs of molecules (molecule1-molecule2).22-25 For

165

this study, the molecule1 represents the monomers of PANI and molecule2 represents TCA and

166

TBA. The ∆E for each pair was obtained directly through a supermolecular approach as the

167

difference between the complex energy (molecule1-molecule2) and the sum of the energies of their

168

isolated parts. Finally, the ∆E1,2 is calculated as indicated by equation 1:

169

∆, = ∆ , − ∆ + ∆

(1)

170

Where the heat of formation (∆Hf) was extracted from single point energy calculations at

171

semiempirical quantum mechanical level using the parameterization Method 7 (PM7),26 which is

172

implemented in MOPAC2016TM packaged program, version 16.111L.27

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2.9.3. Molecular dynamics simulation (MDS) study

174

Four all-atom MDS were carried out with the aim of analyzing and describing the behavior of

175

the PANI particles and haloanisole in a methanol solution and in whisky. Two MDS considered the

176

protonation state of PANI polymers at neutral pH (in methanol solution) and two MDS at low pH

177

considering a 40:60 mixture of ethanol:water, in order to simulating the main components of

178

whisky.28


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179

In the first two simulations, PANI-EB and PANI-ES particles (at neutral pH) were

180

incorporated in the center of a methanol box of 130 Å x 130 Å x 130 Å. In the other two

181

simulations, PANI-EB and PANI-ES particles (at low pH) were added in the center of an

182

ethanol/water mixture box of 130 Å x 130 Å x 130 Å. The numbers of methanol, ethanol and TIP3

183

water molecules were calculated on the basis of their experimental molecular density (0.792 g cm-3

184

for methanol, 0.789 g cm-3 for ethanol and 1 g cm-3 for water). In each box, 20 molecules of TCA

185

and TBA were randomly added (8 Å away from the polymer). The systems were built using the

186

PACKMOL program.21 Orthorhombic boxes with a boundary distance of 0.1 nm were generated.

187

The final systems were relaxed with the default multistage protocol implemented Desmond

188

program, followed by a series of short NVT and NPT simulations (constant Number, Volume and

189

Temperature and constant Number, Pressure and Temperature, respectively).29 The temperature was

190

established to 300 K with the Nose-Hoover chain thermostat30,31 and considering 1.0 ps as

191

relaxation time. Pressure was established to 1 bar with the Martyna-Tobias-Klein barostat and

192

considering 2.0 ps as isotropic coupling and a relaxation time.32 To integrate equations of motions

193

the RESPA integrator33 was used. 9 Å was applied as a cutoff for non-bonded interactions. The

194

smooth particle-mesh Ewald method (SPME)34 was used to treat long-range electrostatics

195

considering a tolerance of 10-9. Finally, four MDS were carried out for about 15 ns using the

196

Desmond-Maestro program version 4.4.35 NPT ensemble was selected and the OPLS force field36-38

197

was applied to the systems.

198

For each MDS a total of 3000 conformations were obtained, which were analyzed using

199

VMD 1.9.2 software39 and Gnuplot 4.4.40 In order to characterize polymer structure in solutions, the

200

Solvent Accessible Surface Area (SASA).41,42 The capture of TCA and TBA within a distance of 4

201

Å with respect of any atom of the PANI particles were calculated through TCL scripts of VMD.

202

3. RESULTS AND DISCUSSION

203

3.1. Affinity of polyaniline-based materials towards TCA and TBA in methanol solution

204

The results were expressed as retention percentages of TCA and TBA by PANI-EB and

205

PANI-ES, respectively. The experimental variables were coded between -1 and 1 to have the same ACS Paragon Plus Environment

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206

statistical weight. All experiments were done under same conditions described in section 2.4. The

207

retention percentages of TCA and TBA by PANI-EB treatment were 43.8% to 67.9%, and 54.0% to

208

80.7%, respectively. In the case of PANI-ES treatment, the retention percentages obtained for TCA

209

and TBA were 52.7% to 66.6%, and 47.7% to 80.9%, respectively.

210

The results of the interaction between PANI-EB, TCA and TBA at different concentrations

211

and contact times, in methanol solution, is shown in Figure 1. In the case of TCA (Figure 1a and

212

1c), the Pareto chart shows that the applied dose of PANI-EB is statistically significant because it

213

exerts a negative influence on the concentration of haloanisoles. The time of interaction exerted a

214

positive influence in the case of TCA and TBA. The time quadratic effect exerted a negative

215

influence in the case of both compounds. The estimated response surfaces (Figure 1b and 1d) show

216

that the percentage of retention increased when the time of contact was increased, reaching a

217

maximum value near the half of the interval.

218 219

Figure 1. (a, c) Standardized Pareto chart for Percentages of TCA and TBA Retention due to PANI-

220

EB treatment, respectively. (Where: A, time of reaction; B, concentration of PANI polymer; and

221

AB, AA and BB interactions. The blue line represents the critical t-value, 95% confidence); and (b,


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222

d) Estimated Response Surface. (For interpretation of the references to color in this figure legend,

223

the reader is referred to the web version of this article.)

224 225

Taking into account the statistical significance of the variables, the regression equations of

226

the models for PANI-EB treatment are:

227

! = 70.3 + 6.95 ∗ − 2.33 ∗ + − 16.1 ∗ ^2 (R2=94.79)

(2)

228

+ ! = 79.6 + 4.13 ∗ − 21.6 ∗ ^2 + 5.56 ∗ ∗ + (R2=88.44)

(3)

229

These models predicted the following optimum conditions: for TCA, a time of reaction of

230

15.5 h was predicted, and a PANI-EB dose of 178 mg L-1. In the case of TBA, a time of reaction of

231

15.1 h was predicted, and a dose of 500 mg L-1. It is important to note that in order to use the same

232

interaction time and unique amount of PANI-EB for the affinity experiment, we used the

233

Derringer’s desirability function (D) because of its additional benefits – user flexibility in selecting

234

optimum conditions for analyzing a variety of sample matrices.43 In our case, this was used to yield

235

substantial time savings and allowed the effective use of polyaniline-based materials. In Figure 2,

236

the Pareto chart and the Response Surface for D are presented.

237 238

Figure 2. (a) Standardized Pareto chart for D due to PANI-EB treatment (where: A, time of

239

reaction; B, concentration of PANI polymer; and AB, AA and BB interactions. The blue line

240

represents the critical t-value, 95% confidence); and (b) Estimated Response Surface.

241 242 243

The regression coefficients for D are: / = 1.02 + 0.158 ∗ − 0.796 ∗ ^2 + 0.164 ∗ ∗ + (R2=92.93) ACS Paragon Plus Environment

(4)

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244 245


The optimum values predicted by the model were: Time of reaction, 14.8 h, and PANI-EB dose, 492 mg L-1.

246

The interaction between PANI-ES, TCA and TBA in methanol solution is presented in

247

Figure 3. The Pareto chart shows that time quadratic interaction is statistically significant, exerting

248

a negative influence in both TCA and TBA. The dose of PANI-ES was not statistically significant

249

for both compounds. The estimated response surfaces (Figure 3b and 3d) show that the percentage

250

of retention increased when the time of contact was increased, reaching a maximum value near the

251

half of the interval.

252 253

Figure 3. (a, c) Standardized Pareto chart for Percentage of TCA and TBA Retention due to PANI-

254

ES treatment, respectively. (Where: A, time of reaction; B, concentration of PANI polymer; and

255

AB, AA and BB interactions. The blue line represents the critical t-value, 95% confidence); and (b,

256

d) Estimated Response Surface.

257 258

Considering the statistical significance of the variables, the regression equations of the

259

models for TCA and TBA are:

260

! = 66.3 − 8.22 ∗ ^2 (R2=37.68) ACS Paragon Plus Environment

(5)

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+ ! = 84.2 + 3.78 ∗ − 23.8 ∗ ^2 − 4.22 ∗ ∗ + (R2=96.40)

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(6)

262

The optimum experimental conditions for the capturing of these compounds were: for TCA,

263

an interaction time of 13.2 h, and a PANI-ES dose of 100 mg L-1. In the case of TBA, it was

264

predicted an interaction time of 13.4 h, and a dose of 300 mg L-1. It was observed that the optimum

265

predicted time of interaction was similar for both compounds. On the other hand, the optimum

266

predicted PANI-ES dose is different for TCA and TBA. Considering this aspect, we used the D, as

267

we did above. In Figure 4, the Pareto chart and the Response Surface for D are presented.

268 269

Figure 4. (a) Standardized Pareto chart for D due to PANI-ES treatment (where: A, time of

270

reaction; B, concentration of PANI polymer; and AB, AA and BB interactions. The blue line

271

represents the critical t-value, 95% confidence); and (b) Estimated Response Surface.

272 273

The regression coefficients for D are:

274

/ = 1.09 + 0.142 ∗ − 0.782 ∗ ^2 (R2=84.96)

275 276 277 278

(7)

The optimum values predicted by the model were: Time of interaction, 14.2 h, and PANI-ES dose, 188 mg L-1. The results of the retention percentages of TCA and TBA-spiked methanol solution for PANI-EB and PANI-ES are shown and summarized in table 1.

279 280

Table 1. TCA and TBA retention percentage for all materials, in optimum conditions obtained by

281

screening tests in methanol model solution.


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Polymer

282

Time *

Dose *

Retention TCA

Retention TBA

-1

hours

mg L

%

%

PANI-EB

14.8

492.60

71.89

85.44

PANI-ES

14.2

188.35

68.15

84.30

* These results were obtained using the Derringer’s desirability function

283 284

3.2. Affinity of polyaniline-based materials towards TCA and TBA in whisky

285

Following the same protocol of subsection 2.2 (with the optimum conditions for the spiked

286

methanol solution) the haloanisoles retention percentage was determined in whisky, as shown in

287

Figure 5. It was observed that polyaniline-based materials capture more than 70% of TBA

288

compared to only the 12% of TCA. The best results were found for the PANI-EB at 75.8% and

289

15.7% for TBA and TCA, respectively.

290 291

Figure 5. Retention percentages of TCA and TBA using PANI-ES and PANI-EB in whisky.

292

MDS were performed to characterize the PANI polymers and their interactions with TCA and

293

TBA at the intermolecular level. A comparative visual approach between the trajectories of PANI

294

systems at 0 and 15 ns are shown in Figure 6. The polymeric structure of both PANI polymers

295

generated internal and superficial microcavities that increase or decrease in size depending on the

296

change in its structure when subjected to a different environment.


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297 298

Figure 6. Snapshots of trajectories for PANI EB and ES systems (blue and green colors,

299

respectively) in methanol and whisky (ethanol-water mixture), at 0 and 15 ns of simulated time.

300

In the simulated whisky medium (ethanol/water mixture) both the PANI-EB and PANI-ES

301

structures undergo considerable compaction, minimizing the internal microcavities, and on the other

302

hand, reducing the size of the surface microcavities. In Figure 7, the decrease of the solvent

303

accessible area (SASA) can be observed by changing the polymers from a solution with methanol to

304

a simulated whisky medium.

305 306

Figure 7. Comparative plots of SASA values for PANI ES and EB systems at methanol and whisky

307

(ethanol-water mixture).


14

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308

Considering an interaction distance of 4.0 Å, the contacts between the PANI polymers and

309

the haloanisoles were measured, in order to estimate the average capture of each polymer during the

310

15 ns of simulation (Figure 8). Both PANI polymers captured between 15 and 17 molecules of

311

TCA and TBA in methanol. In the simulated whisky solution the polymers maintained a high TBA

312

capture rate, however, they barely managed to maintain a stable interaction with only 3 molecules

313

of TCA on average, similar to what happened experimentally (Table 1 and Figure 5). The above-

314

mentioned in addition to the structural characterization shown in Figure S6, allow for the

315

conclusion that the generated size of the surface microcavities are adjusted to the TBA size,

316

blocking the ability to interact with TCA drastically.

317 318

Figure 8. Comparative graphs of simulated capture of TCA and TBA by PANI ES and PANI-EB

319

at: a) methanol and b) whisky (ethanol-water mixture).

320

During systematic analysis of non-bonded contacts for PANI-haloanisole systems, π-π

321

interactions have been found (Figure 9), mainly between the reduced quinoide ring of PANI and


15


Page 16 of 40

322

the aromatic ring of both haloanisoles. However, this does not show a significant difference to

323

explain the affinity obtained experimentally for TCA and TBA.

324

In recent years, several experimental studies

43-46

have shown that chlorine and bromine

325

atoms are involved in non-covalent dipolar interactions in molecule-molecule complexes and

326

contribute significantly to binding affinity with aromatic macromolecules.47 Figure 9 shows that the

327

chlorine and bromine atoms of TCA and TBA, respectively, are in close contact with the centroid of

328

aromatic rings of PANI polymers. From the geometric analysis of Cl–π and Br–π interactions, two

329

distinct geometries were identified: the edge-on approach of chlorine and bromine atoms to a ring

330

atom and the face-on approach toward the ring centroid of PANI with an average interatomic

331

distance of 3.8 Å for TCA and 3.6 Å for TBA. The above-mentioned together with the π-π

332

interactions would allow the stabilization and permanence of the haloanisole in the pockets and

333

microcavities of PANI surfaces. The difference of 0.2 Å in the interaction distance for bromine is

334

because of its larger van der Waals radius. These results show a more favorable interaction for

335

bromine than chlorine with the aromatic ring of PANI, as explained by Matter, and et al.48 where it

336

is also indicated that the Cl/Br π interactions were described as important contributions to the

337

binding affinity between proteins and ligands.

338 339

Figure 9. Snapshots of the distance between Br···centroid and Cl···centroid for: a) TBA and b)

340

TCA, respectively.

341


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342

Theoretical calculations were carried out to study the geometrical, energetic and electronic

343

parameters involved in the interaction between haloanisoles and polyaniline derivatives. This type

344

of study helps to explain the affinity differences found in the whisky experiment mentioned above.

345

Several studies indicate there is an effect of pH on the properties and structure of PANI. 49-51 PANI-

346

EB and PANI-ES are very sensitive to environments with different pH; this produces changes in the

347

backbone of both PANI forms due to protonation and deprotonation of the imine nitrogen of the

348

quinoid segment.52 The pH of whisky is 4.43, and when PANI-EB (Figure 10a) was mixed with

349

whisky, its pH was increased to 4.94, which suggests that the imine nitrogens were partially

350

protonated (Figure 10b). On the other hand, when PANI-ES (Figure 10a) was mixed with whisky,

351

the pH of the solution was decreased to 3.75, which suggests that the imine nitrogens of PANI-ES

352

were partially deprotonated (Figure 10b) when it was subjected to a low pH environment.53-55 The

353

structural similarity of both forms of PANI in the whisky matrix would explain the small difference

354

in the experimental retention results of haloanisoles.

355

Quantum mechanics calculations for PANI monomers and the haloanisoles were performed

356

in order to characterize the electronic properties through the spatial distribution of their HOMO and

357

LUMO orbitals. Figure 10a and 10b show that the distributions of the HOMO and LUMO orbitals

358

in the doped and un-doped monomers show small changes. The HOMO mainly localizes around the

359

benzenoid ring, while the LUMO is mainly distributed at the reduced quinoid ring. These results are

360

consistent with those presented by Chen et al.,56 who indicated that the protonation and

361

deprotonation of the PANI backbone caused changes in the distribution of HOMO and LUMO

362

orbitals.


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363 364

Figure 10. Localization of the HOMO and LUMO orbitals by B3LYP/6- 311+G(d,p) for: (a)

365

monomers of PANI-EB and PANI-ES, (b) protonated PANI-EB and deprotonated PANI-ES in

366

whisky and (c) TBA and TCA.

367

A conformational sampling of 100,000 conformations for each pair molecule1-molecule2

368

(PANI monomer-haloanisole) was generated to calculate the interaction energies at the

369

semiempirical quantum mechanics level. The average of interaction energies between PANI-EB and

370

TCA and TBA were -0.54 kcal mol-1 and -1.48 kcal mol-1, respectively. The average interaction

371

energies between PANI-ES and TCA and TBA were -0.51 kcal mol-1 and -1.47 kcal mol-1,

372

respectively. The results showed a good correlation with the experimental values of the retention

373

percentage (Figure 5), this would indicate that the use of the strategy proved to be fast and accurate

374

to identify the affinity that has a particular form of PANI polymer (protonated and deprotonated) for

375

each haloanisole.


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376 377

Figure 11. (a) Distribution of LUMO orbitals for PANI-ES and PANI-EB in whisky. (b) Spatial

378

distributions for the lowest energy conformations of monomers-haloanisole pairs. The colored

379

spheres represent the mass centers of TCA (green) and TBA (orange).

380

Figure 11b shows the behavior of the haloanisoles versus PANI monomers. The spatial

381

distributions of these structures showed a clear interaction between the haloanisole and the region

382

where the LUMO orbitals of the monomers are located (Figure 11a), and at the same time, shows

383

that PANI-EB has a slightly better affinity for the haloanisoles than PANI-ES.

384

3.3. Study of the whisky’s volatile composition after the treatments with polyaniline-based

385

materials

386

In order to know the volatile composition of whisky after the treatments with polyaniline-

387

based materials, the volatile compounds present were analyzed using the GC-MS technique. A total

388

ion chromatogram generated from studied whisky samples through the experimental conditions

389

discussed above are shown in Figure S2.

390

Fifteen compounds were identified, including ethyl esters, higher alcohols, acetates, fatty

391

acids, carbonyl compounds, phenols, etc. (Table 2). It is important to highlight that this work was

392

focused only on the signal that had more than 0.4 relative peak area percentages.

393

Table 2 shows the results obtained in the study of the percentage area of the main

394

chromatographic peaks found in whisky before and after treatment with polyaniline-based ACS Paragon Plus Environment

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395

materials. Only statistically significant differences were observed in the following compounds:

396

Isobutyric acid, 2-phenyl ethane, Ethyl octanoate, Ethylhexanedecanoate, Ethyl dodecanoate, and

397

Dodecanol. For that reason, we concluded that the elimination of TCA and TBA did not greatly

398

modify the volatile matrix of whisky, in terms of which the compounds where differences are

399

observed change their proportion, but do not disappear completely from the matrix.

400 401

Table 2. Percentage area of the main chromatographic peaks found in whisky before and after

402

treatment with polyaniline-based materials. The same letter beside the percentage area in the same

403

row indicates no statistical differences between the means, at 95%, by LSD ANOVA.


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ID Compounds *

Chromatographic

Whisky

Whisky

Whisky

peak

(Percentage

w/PANI-EB

w/PANI-ES

(Retention Time)

Area)

treatment

treatment

[min]

[% ± RSD]

(Percentage

(Percentage

Area)

Area)

[% ± RSD]

[% ± RSD]

1

Isobutyric acid

5.10

5.32b ± 0.31

4.05a ± 0.61

4.74ab ± 0.74

2

Acetic acid

6.67

1.97a ± 0.44

1.25a ± 1.07

2.14a ± 0.51

3

2-phenyl ethane

8.89

15.40a ±

20.95b ± 3.41

17.44ab ± 0.94

13.64a ± 0.40

13.42a ± 0.40

1.75 4

Octanoic acid

9.72

12.81a ± 0.61

5

Ethyl octanoate

10.23

5.28b ± 0.15

4.43a ± 0.30

5.01b ± 0.06

6

Hydroxymethylfurfural

11.05

1.04a ± 0.50

1.35a ± 0.52

1.21a ± 1.01

7

2-phenylethyl acetate

11.69

2.27a ± 0.19

2.47a ± 0.36

2.57a ± 0.20

8

Decanoic acid

13.98

12.41a ±

14.44a ± 0.68

14.17a ± 1.36

17.92a ± 0.63

19.88b ± 1.17

0.81 9

Ethylhexanodecanoate

14.65

23.49c ± 0.31

10

Dodecanoic acid

18.47

1.96a ± 0.58

2.90a ± 0.51

2.72a ± 1.07

11

Ethyl dodecanoate

19.21

15.25b ±

12.80a ± 0.73

13.43a ± 0.95

0.74

404

12

Dodecanol

21.05

0.76a ± 0.12

1.02b ± 0.12

0.93ab ± 0.08

13

Ethyl stearate

23.53

0.72a ± 0.15

0.86a ± 0.06

0.84a ± 0.06

14

Hexadecanol

25.3

0.58a ± 0.17

0.82a ± 0.04

0.74a ± 0.24

15

Ethyl tetradecanoate

27.49

0.74a ± 0.33

0.83a ± 0.04

0.75a ± 0.24

* Number in the chromatogram of Figure S2

405 406

One of the properties associated with the consumption of alcoholic beverages that contain

407

phenolic compounds such as wine and whisky is the antioxidant power in the plasma, compatible

408

with suggestions of moderate consumption.57 Specifically, the presence of polyphenols has a

409

protective effect on gastric mucosal reducing ethanol-induced damage,58 and represent an additional


21


Page 22 of 40

410

benefit as a cardiovascular protection.59 For that reason, the quality of whisky could also be linked

411

with the amount of the phenolic that contains.

412

Table 3 shows that the results of the optimum dose of each polymer in whisky have

413

influences on Total Phenolic Content (TPC). It can observe that the PANI-EB has statistical

414

influence on the total phenol content of the whisky, due to the fact that it captured about 14% of

415

total phenols. In previous studies, it appears that the capture of polyphenols in wine was affected by

416

the higher dose of a used polymer.14,60

417 418

Table 3. Influence of optimum dose of each polymer on Total Phenolic Content in Red Label

419

whisky, expressed as Gallic Acid (GA). Same letter in the same row indicate no statistical

420

differences between the means (Multiple range tests for the mean, by Tukey HSD, at 95% level of

421

confidence (n=3). Polymers

Red Label whisky / GA / Mean ± SD [mg L-1]

Blank

129.61b ± 1.35

PANI-EB

111.10a ± 3.88

PANI-ES

126.70b ± 3.89

422 423

4. CONCLUSIONS

424

In this work, it has been demonstrated that polyaniline-based materials have a remarkable

425

ability to scavenge TBA from whisky. Since these materials have several advantages such as

426

insolubility, easy to handle, easy to prepare and easy to separate from the test solution, they are an

427

alternative to the existing fining agents for the purification of whisky. We demonstrated that

428

polyaniline-based materials are more selective for TBA. The results showed 70% and 12% of

429

capture for TBA and TCA, respectively. Finally, the elimination of TCA and TBA with these

430

polymers was not accompanied by a large reduction in volatiles and total phenolics, therefore, does

431

not affect its organoleptic properties.

432 ACS Paragon Plus Environment

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433


Acknowledgement

434

The authors acknowledge support from PIEI (Químico-Bio) Universidad de Talca. John

435

Amalraj and Oscar Valdes gratefully acknowledge the financial support of CONICYT through

436

projects FONDECYT INICIACION EN INVESTIGACION 11130087 and 11170008, respectively.

437

Fabian Avila thanks to Post-doctoral FONDECYT grant Nº 3170909. L.S.S. thanks FONDECYT

438

Regular 1180084. The authors thank the Academic Writing Center at Programa de Idiomas at

439

Universidad de Talca.

440 441

Supporting information available

442

The supporting information shows two figures. Figure S1 details the in-silico design process

443

of PANI particles at different pH, and Figure S2 shows a total ion chromatogram obtained from

444

studied whisky samples.

445 446

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447

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Figure 1. (a, c) Standardized Pareto chart for Percentages of TCA and TBA Retention due to PANI-EB treatment, respectively. (Where: A, time of reaction; B, concentration of PANI polymer; and AB, AA and BB interactions. The blue line represents the critical t-value, 95% confidence); and (b, d) Estimated Response Surface. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 160x104mm (300 x 300 DPI)



Figure 2. (a) Standardized Pareto chart for D due to PANI-EB treatment (where: A, time of reaction; B, concentration of PANI polymer; and AB, AA and BB interactions. The blue line represents the critical t-value, 95% confidence); and (b) Estimated Response Surface. 162x50mm (300 x 300 DPI)


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Figure 3. (a, c) Standardized Pareto chart for Percentage of TCA and TBA Retention due to PANI-ES treatment, respectively. (Where: A, time of reaction; B, concentration of PANI polymer; and AB, AA and BB interactions. The blue line represents the critical t-value, 95% confidence); and (b, d) Estimated Response Surface. 161x102mm (300 x 300 DPI)



Figure 4. (a) Standardized Pareto chart for D due to PANI-ES treatment (where: A, time of reaction; B, concentration of PANI polymer; and AB, AA and BB interactions. The blue line represents the critical t-value, 95% confidence); and (b) Estimated Response Surface. 162x50mm (300 x 300 DPI)


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Figure 5. Retention percentages of TCA and TBA using PANI-ES and PANI-EB in whisky. 82x48mm (300 x 300 DPI)



Figure 6. Snapshots of trajectories for PANI EB and ES systems (blue and green colors, respectively) in methanol and whisky (ethanol-water mixture), at 0 and 15 ns of simulated time. 162x78mm (300 x 300 DPI)


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Figure 7. Comparative plots of SASA values for PANI ES and EB systems at methanol and whisky (ethanolwater mixture). 84x67mm (300 x 300 DPI)



Figure 8. Comparative graphs of simulated capture of TCA and TBA by PANI ES and PANI-EB at: a) methanol and b) whisky (ethanol-water mixture). 168x132mm (300 x 300 DPI)


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Figure 9. Snapshots of the distance between Br···centroid and Cl···centroid for: a) TBA and b) TCA, respectively. 156x68mm (300 x 300 DPI)



Figure 10. Localization of the HOMO and LUMO orbitals by B3LYP/6- 311+G(d,p) for: (a) monomers of PANIEB and PANI-ES, (b) protonated PANI-EB and deprotonated PANI-ES in whisky and (c) TBA and TCA. 160x92mm (300 x 300 DPI)


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Figure 11. (a) Distribution of LUMO orbitals for PANI-ES and PANI-EB in whisky. (b) Spatial distributions for the lowest energy conformations of monomers-haloanisole pairs. The colored spheres represent the mass centers of TCA (green) and TBA (orange). 160x84mm (300 x 300 DPI)



81x43mm (300 x 300 DPI)


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Polyaniline Based Materials as a Method to Eliminate Haloanisoles in

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