Chemosphere 91 (2013) 1026–1034

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Molecular products from the pyrolysis and oxidative pyrolysisof tyrosine

0045-6535/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.chemosphere.2013.01.071

⇑ Corresponding author. Tel.: +1 225 578 6759.E-mail address: barryd@lsu.edu (B. Dellinger).

Joshua K. Kibet, Lavrent Khachatryan, Barry Dellinger ⇑Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA

h i g h l i g h t s

" Thermal degradation of tyrosine in N2 and 4% O2 in N2 using the System for Thermal Diagnostic Studies (STDS)." Product distribution of major reaction products with degradation temperature." A detailed mechanistic description of formation of p-tyramine and its degradation to phenol and p-cresol." Biological compounds of interest including dibenzo-p-dioxin, dibenzofuran, hydroquinone and p-benzoquinone." The environmental and biological impacts of toxic compounds from the thermal degradation of tyrosine.

a r t i c l e i n f o

Article history:Received 13 November 2012Received in revised form 7 January 2013Accepted 16 January 2013Available online 11 March 2013

Keywords:DeaminationDecarboxylationChain reactionsMass spectrometry

a b s t r a c t

The thermal degradation of tyrosine at a residence time of 0.2 s was conducted in a tubular flow reactor inflowing N2 and 4% O2 in N2 for a total pyrolysis time of 3 min. The fractional pyrolysis technique, in whichthe same sample was heated continuously at each pyrolysis temperature, was applied. Thermal decom-position of tyrosine between 350 and 550 �C yielded predominantly phenolic compounds (phenol, p-cre-sol, and p-tyramine), while decomposition between 550 and 800 �C yielded hydrocarbons such asbenzene, toluene, and ethyl benzene as the major reaction products. For the first time, the identificationof p-tyramine, a precursor for the on of formation of p-tyramine and its degradation to phenol and p-cre-sol, and toxicological discussion of some of the harmful reaction products is also presented.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Amino acids are the fundamental building blocks of proteinsand have applications in the food, agricultural, cosmetics, andpharmaceutical industries. Pyrolytic processes are commonly usedin their manufacture (Chiavari and Galletti, 1992; Wu et al., 2010).Consequently, pyrolysis studies of amino acids is critical becauseformation of mutagenic and carcinogenic products in pyrolysatesof proteinaceous foods is a health concern in the fields of food pro-cessing, preservation, and safety (Djilas et al., 1994). The pyrolyticbehavior of common amino acids has been investigated in detailbut despite this effort, potentially diagnostic fragments bearing po-lar functional groups, e.g. COOH, NH, OH frequently escape detec-tion because of thermal instability, low volatility, and highadsorptivity (Chiavari et al., 2001).

Tyrosine is a large amino acid found in substantial quantities inmany animal and plant proteins (Mahoney et al., 2007) as well in

tobacco (Sharma et al., 2006) and is the metabolic precursor forthe synthesis of catecholamine neuron transmitters, dopamineand norepinephrine in the central and peripheral nervous system(Neri et al., 1995; Mahoney et al., 2007). Amino acids in the formof proteins are the main source of nitrogen in wood (Hanssonet al., 2004). Most biomass materials such as bagasse, straw, wood,and tobacco contain nitrogen which can be converted to environ-mentally harmful products such as hydrogen cyanide, phenols,nitrosamines, furans, and aromatic compounds (Chiavari et al.,2001; Li et al., 2008). The thermodynamic end products of aminoacids are simple inorganic compounds (CO2, H2O, NH3, and CO);however, more complex chemicals are formed as by-products(amines, nitriles, amides, and hydrocarbons) (Chiavari et al.,2001; Li et al., 2006; Ren et al., 2011; Ren and Zhao, 2012). Com-pared to other bio-polymeric materials (i.e., cellulose, lignin, andpectin), tyrosine undergoes decomposition at fairly high tempera-tures (> 300 �C).

The health consequences resulting from consumption of tobac-co products has been blamed on the production of toxic molecularproducts as well as free radicals during tobacco burning. Radicals

J.K. Kibet et al. / Chemosphere 91 (2013) 1026–1034 1027

produced during combustion of tobacco are very reactive and capa-ble of causing oxidative stress, cancers, and reproductive healthdiseases. For example, tyrosyl radical has been reported from thefractional pyrolysis of bright tobacco (Maskos et al., 2008). Tyrosylradical may originate either from the decomposition of protein-containing tyrosine residues or from free tyrosine molecules (Mas-kos et al., 2008).

Our investigations of the thermal degradation of tyrosine avoidmany experimental pitfalls by using a continuous flow reactor sys-tem, with collection of the reactor effluent with an in-line GC–MSat the head of the GC column at �60 �C. Our efforts were directedtoward identification of some intermediate products, e.g. the exis-tence of p-tyramine as a product has been expected, but its actualformation is controversial (Li et al., 2008). A p-tyramine is reportedto displace amines such as dopamine from its vesicles and is a falseneuron transmitter as well as a metabolic product of catechol-amine biosynthesis (Kitahama et al., 2005). It is one of the aminespresent in the human brain thought to influence behavior (Hiroiet al., 1998).

This study will demonstrate the formation of p-tyramine and itssubsequent degradation to toxicologically important pollutants,such as phenol, and p-cresol. A mechanism of p-tyramine forma-tion and degradation from the thermal decomposition of tyrosineis presented for the first time.

2. Experimental section

2.1. Materials

Tyrosine (> 99%, purity) used in this study was purchased fromsigma Aldrich (USA) and used without further treatment.

2.2. Fractional pyrolysis and the System for Thermal Diagnostic Studies(STDS)

The fractional pyrolysis technique was applied in this study andis an experimental procedure in which the same sample is contin-uously pyrolyzed at each pyrolysis temperature (Agblevor et al.,2010). The fractional pyrolysis technique offers some advantagesin comparison with conventional pyrolysis. First, only one loadingof sample is used and can be heated multiple times. Secondly, itprovides partial accumulation of any fraction and analysis forproducts in the gas phase as well as in the charred material.Thirdly, the intermediate neutral, but unstable products may becollected before they disappear in secondary processes.

The System for Thermal Diagnostic Studies (STDS) is a well-established technique for analysis of a wide range of organic mate-rials including biomass materials (Rubey and Grant, 1988). TheSTDS contains various units: the reactor compartment whichhouses a high temperature furnace and the reactor, the tempera-ture control console which controls the pyrolysis temperaturewithin a temperature gradient of ±5 �C, the injection port, the cryo-genic trap, and the in-line detection system. The experimental de-tails for STDS are given elsewhere (Rubey and Carnes, 1985; Rubeyand Grant, 1988).

2.3. Pyrolysis and oxidative pyrolysis

The thermal degradation of tyrosine was investigated in a tubu-lar flow reactor over the temperature range of 200–800 �C at atmo-spheric pressure, typically in 50 �C increments under two reactionregimes (pyrolysis in N2 and oxidative pyrolysis in 4% O2 in N2)using the System for Thermal Diagnostic Studies (STDSs) (Rubeyand Grant, 1988). The concentration of oxygen in the oxidativepyrolysis experiments was kept low to optimize the formation of

oxidative products (Sharma et al., 2004a,b). The gas flow ratewas designed to maintain a constant residence time of 0.2 s.30 ± 0.2 mg of tyrosine were loaded into the tubular quartz reactor(0.3 cm i.d. � 17.7 cm, volume 1.25 mL) and held in place by quartzwool to avoid being swept by carrier gas flowing through the reac-tor. The reactor containing the sample was then placed inside anelectrically heated furnace at a heating rate of 10 �C s�1 for a totalpyrolysis time of 3 min. The furnace was then turned off and thesample cooled with flowing N2 while exposing the reactor to acooling fan. This method of thermolysis of tyrosine closely resem-bles the TGA technique wherein a sample boat is used to hold thesample in the reactor. The benefits of this technique are two-fold:(1) the sample is held intact in the reactor, and (2) the carrier gasflows uniformly through the sample during the entire analysis,resulting in highly reproducible analyses. Due to high flow rates,the contact time with charred material is short (0.2 s) to minimizesecondary reactions.

2.4. GC–MS characterization of molecular products

The gas chromatography–mass spectroscopy (GC/MS) analysisof the pyrolysate was conducted with an Agilent 6890N gas chro-matograph equipped with a 5973N mass selective detector(MSD) (Kibet et al., 2012) with an ion source in the electron impact(EI) mode at 24 eV to minimize extensive fragmentation (Sharmaet al., 2004a,b). A detailed description of product characterizationhas been described elsewhere (Kibet et al., 2012). The compoundswere identified using a NIST library (standard mass spectral database developed by NIST) and confirmed by enhanced data library(mass spectral data base developed by Agilent technologies). Thiswas to ensure the mass fragmentation pattern was well matchedand enhance the confidence of compound identification. Further-more, each of the compounds identified was thoroughly checkedagainst literature data to ensure the correct reaction product wasreported. In addition, pure compounds were used to compare chro-matographic retention time as well as mass spectral patterns withthose of the analyte. Accordingly, critical emphasis has been givento those products which can easily be correlated with the structureof the starting material (tyrosine). Representative GC–MS spectrafrom pyrolysis and oxidative pyrolysis of tyrosine are provided assupporting information. The experimental results were averagedfrom two replicates.

3. Results

3.1. Pyrolysis

This investigation revealed the principal products of tyrosinepyrolysis in an N2 atmosphere were phenolic compounds (phenol,p-cresol, and o-cresol), acetonitrile, benzaldoxime, ethylbenzene,and toluene. The maximum release of phenolic compounds andnitrogen containing compounds of low molecular weight occurredbetween 350 and 450 �C, while the maximum concentration of aro-matic hydrocarbons and nitrogen containing compounds of highmolecular weight occurred between 550 and 650 �C. Phenol andp-cresol reached maximum concentrations at about �450 �C,Fig. 1A. Acetonitrile and propionitrile achieved a maximum con-centration at about �400 �C, (cf. Fig. 1B). Hydrogen cyanide wasformed in significant amounts throughout the entire pyrolysistemperature range, (cf. Fig. 1B). The major hydrocarbon products:ethyl benzene, toluene, and benzene, peaked between �550 and650 �C (cf. Fig. 1C).

The hydrocarbon products are believed to be the result of ther-mal cracking and concerted rupture of the C–C chain followed bymolecular growth to form aromatic species (Li et al., 2008). Gener-

50x109

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Are

a C

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800700600500400300

Temperature (ºC)

phenol p-cresol 2,3-dimethylphenol o-cresol p-tyramine 4-ethylphenol

A6x109

4

2

0

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a C

ount

s

700600500400300Temperature (ºC)

acetonitrile hydrogencyanide propionitrile pyrrole urea

B

3.0x109

2.5

2.0

1.5

1.0

0.5

Are

a C

ount

s

800700600500400

Temperature (ºC)

ethylbenzene toluenebenzene

styrene

C400x106

300

200

100

0

Are

a C

ount

s

700650600550500450400350Temperature (ºC)

propene 1-butene propane 2-butene

D

Fig. 1. Yields (based on GC area counts) of the major phenol products (A) and other major products (B–D) from the pyrolysis of tyrosine in N2.

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ally, product profile concentrations first increased with increase inpyrolysis temperature, before falling off at high temperatures dueto decomposition. Low molecular weight hydrocarbons (propeneand 1-butene) yields were lowest (cf. Fig. 1D).

3.2. Oxidative pyrolysis

The principal products in this experiment were p-tyramine andphenol with a combined yield of over 80%. The formation of p-tyra-mine, with a maximum yield between 350 �C and 400 �C, (cf.Fig. 2A) was a very important observation. This compound hasbeen known to have a low volatility and is not easily transportedfor detection. Li et al. (2008) concedes that the low volatilitybehavior of p-tyramine/4-(2-aminoethyl)phenol was responsiblefor eluding detection in their experiments. A p-tyramine shouldbe an important signature of tyrosine pyrolysis formed from decar-boxylation reactions. In our study, p-tyramine was observed inhigh concentration under oxidative pyrolysis conditions and lowconcentrations from pyrolysis (cf. Figs. 1A and 2A, respectively).Oxidative pyrolysis also formed compounds of biological interest:hydoquinone, benzofuran, dibenzofuran, and dibenzo-p-dioxin, aswell as phenolic compounds (phenol, p-cresol, and o-cresol). Themaximum release of hydroquinone, benzofuran, dibenzo-p-dioxin,phenol, p-cresol, benzonitrile, and benzaldoxime was generally be-tween 400 and 450 �C (cf. Fig. 2A–C). Hydrogen cyanide wasformed in low amounts throughout the entire temperature range,(cf. Fig. 2B). Benzene was the dominant product among the aro-matic compounds, with a maximum concentration being observedat �550 �C. Ethyl benzene, which was one of the main products inpyrolysis experiments, was formed in nearly trace amounts underoxidative pyrolysis conditions and exhibited a maximum yield atabout 450 �C (cf. Fig. 2D). Oxidative pyrolysis of tyrosine did not re-sult in significant formation of hydrocarbons.

The majority of products from the thermal degradation of tyro-sine pass through a maximum. This observation is characteristic ofthe nature of fractional pyrolysis or oxidative pyrolysis. Three rea-sons are explored to explain this phenomenon: (1) exhaustion ofreaction products from the sample, (2) degradation of some inter-mediates, for instance p-tyramine to other major reaction productssuch as p-cresol and phenol or (3) oxidation of products to CO, CO2,and H2O, etc. during oxidative pyrolysis.

4. Discussion

4.1. Decomposition/oxidation pathways for tyrosine

4.1.1. Initial decompositionThe mechanisms of thermal degradation of amino acids have

been extensively studied (Simmonds et al., 1972; Ratcliff et al.,1974; Chiavari and Galletti, 1992; Basiuk, 1998; Basiuk and Douda,1999, 2000; Li et al., 2008). Consequently, this investigation will fo-cus primarily on the mechanistic pathways of new, major productsfrom oxidative pyrolysis of tyrosine (p-tyramine, phenol, and p-cresol, Fig. 2A).

The main decomposition pathway for p-tyramine is likely a con-certed, 4-center decarboxylation pathway. The decarboxylation ofamino acids has been predicted using density functional theory,which found the decarboxylation channel for high molecularweight amino acids, including tyrosine, proceeds from the high-er-energy anti carboxylic hydrogen conformer and involves the di-rect heterolytic loss of CO2 accompanied by direct proton transfer(Li and Brill, 2003) (Fig. 3A). The calculated activation energy fordirect decarboxylation in tyrosine was found to be 72 kcal mol�1

in the absence of water (Li and Brill, 2003). While in the presenceof water, the direct decarboxylation is catalyzed, and the calculatedenergy barrier drops to an average of 45 kcal mol�1 (Li and Brill,

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p-tyramine phenol p-cresol 2,3-dimethylphenol o-cresol

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141210

86420

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benzonitrile benzaldoxime acetonitrile hydrogencyanide

B

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700600500400300Temperature (ºC)

hydroquinone benzofuran benzoquinone dibenzo-p-dioxin dibenzofuran

C 800x106

600

400

200Are

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ount

s

700650600550500450400350Temperature (ºC)

D benzene toluene ethyl benzene styrene

Fig. 2. Yields (based on GC area counts) of major products from (A–D) from the oxidative pyrolysis of tyrosine in 4% O2 in N2.

O

O

H

H

N H

R

H

C C C C

H

N H

R

H

H O

Odecarboxylation

TransitionState

A

B

Fig. 3. A: Transition state during decarboxylation of high molecular weight amino acids in the gas phase (Alexandrova and Jorgensen, 2011) and B: Bond dissociation energiesfor important bonds in tyrosine (Davico et al., 1995; Wu et al., 1996; Lucarini et al., 2002; Blanksby and Ellison, 2003; Tumanov and Denisov, 2003; Gomes et al., 2004; Mulderet al., 2005).

J.K. Kibet et al. / Chemosphere 91 (2013) 1026–1034 1029

2003). More recently, a statistical mechanical investigation (QM/MM) (Alexandrova and Jorgensen, 2011) indicated the most likelypathway for decomposition of amino acids in the presence of wateroccurs via direct decarboxylation, where CO2 elimination is thefirst as well as the rate determining step (Alexandrova and Jorgen-sen, 2011). For instance, the computed free energy of activation fordecarboxylation of glycine in the presence of water was found tobe 45 kcal mol�1, and the resultant rate constant was 10�21 s�1 at25 �C (Alexandrova and Jorgensen, 2011), which was in agreementwith experimental data (Li and Brill, 2003). The high activation en-

ergy and low pre-exponential factor for decarboxylation of aminoacids results in a very slow process at room temperature whichaccelerates rapidly with increasing temperature.

p-cresol formation is probably initiated by the cleavage of bond# 5 with an estimated bond strength of 72 kcal mol�1, (Fig. 3B)(Davico et al., 1995; Wu et al., 1996; Lucarini et al., 2002; Blanksbyand Ellison, 2003; Tumanov and Denisov, 2003; Gomes et al., 2004;Mulder et al., 2005). Cleavage of bond # 5 results in the formationof p-methylene phenolic radical, which forms p-cresol by donationof a hydrogen by a suitable organic donor, RH. (Rxns 1 and 2,

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Scheme 1). Typically, the activation energy for simple bond cleav-age reactions, such as Rxn 1 is closely related to the enthalpy ofreaction, 72 kcal mol�1. This is close to the activation energy(72.6 kcal mol�1) for decarboxylation of tyrosine which producesp-tyramine (Li and Brill, 2003).

The pre-exponential factors for decarboxylation reactions of dif-ferent amino acids span a wide range, from 1010 s�1 for Met-aminoacid (methionine amino acid) to 1016 s�1 for a-Aib (a-amino isobu-tyric) amino acid (Li and Brill, 2003). The pre-exponential valuesfor a complex bond scission reactions are 1015–1017 s�1 (Hiattand Benson, 1972). The generally higher pre-exponential factorfor bond scission makes the cleavage of bond # 5 (Fig. 3B) favorableover decarboxylation reactions for simple amino acids. As a result,p-cresol is one of the major products in tyrosine pyrolysis, Fig. 1A.

Phenol has been proposed to form from further decompositionof p-cresol (Li et al., 2008). Because the concentration of phenol is alittle higher than that of p-cresol for both pyrolysis and oxidationexperiments (cf. Figs. 1A and 2A), it would appear there is an addi-tional mechanistic channel for the formation of phenol. For in-stance it may be the result of cleavage of bond # 4 (cf. Fig. 3A),with an estimated bond energy of 100 kcal mol�1, leading to theformation of p-hydroxylated phenyl radical (and latter to phenolby abstraction of hydrogen from RH) or displacement of the entireside-chain by H�. This pathway may be feasible, if we compare itwith one of the important channels, deamination of amino acids(Alexandrova and Jorgensen, 2011) which occurs by participationof the bond # 6 with exactly the same bond energy as bond # 4,100 kcal mol�1 (cf. Fig. 3A).

4.1.2. The main channels of oxidative pyrolysisWhereas p-tyramine is the major product during oxidative

pyrolysis of tyrosine, it is formed in low concentrations in pyrolysis(cf. Figs. 22A and 4A). This phenomenon can be understood if amore favorable, free radical mechanism is considered in presenceof oxygen. For instance the initiation pathway presented in Rxn3, Scheme 1 (assuming the activation energy is equal to the bonddissociation energy �86.5 kcal mol�1), k3 = 3.2 � 1015 exp(�86500 cal mol�1/RT) S�1 can be accelerated significantly in pres-ence of oxygen, Rxn 4 (activation energy is around 40–42 kcal mol�1), k4 = 10 � 1012 (10�1014) exp (�41500 cal mol�1/RT) cm3 molecule�1 S�1 (Khachatryan et al., 2003). The concentra-tion of oxygen in the system was 4%, the equivalent of 4.50 � 1017 -mol cm3 at 673 K. Therefore the ratio of the rates R4/R3 can becomputed and found to be in favor of Rxn 4 (R4/R3 � 1.0 � 105) at673 K.

Reactions 3 and 4 (cf. Scheme 2) form tyrosyl radical (Maskoset al., 2008). It is remarkable that the observable amounts of tyrosil

Scheme 1. Chain reaction leading to the formation of p-methy

radical (Tyr�) were produced at <380 �C from tobacco pyrolysis(Maskos et al., 2008), which matches well with the maximumyields of tyramine (370 �C) from tyrosine pyrolysis in this study,Fig. 1A. Further decarboxylation of Tyr� favors formation of p-tyraminyl radical, Rxn 5, and subsequent formation of p-tyraminevia Rxn 6, (cf. Scheme 3).

Statistical mechanical investigations (QM/MM) indicated that inpresence of water the decarboxylation of amino acids is more fac-ile, and the activation energy drops from 72 kcal mol�1 (withoutwater) to 45 kcal mol�1 (Li and Brill, 2003; Alexandrova and Jor-gensen, 2011). Hydroxyl radicals (�OH), which are the major reac-tive species in oxidation processes (Benson, 1976), may have asimilar effect towards decarboxylation as water. Furthermore theprocesses of formation Tyr� will be accelerated in Rxn 4 (cf.Scheme 2) when �OH are the main chain carrier radical (abstractionof hydrogen from phenolic hydroxyl group).

These reactions (4–6) are the main pathways that promote theformation of p-tyramine, which is the major product during oxida-tive pyrolysis. Therefore the rate of decomposition of tyrosine isenhanced under oxidative pyrolysis because the process occurs ina reactive regime, in the presence of O2 and �OH. For this reason,decarboxylation reaction will also proceed via a free radical mech-anism in addition to a molecular process under pyrolysis. This ex-plains why the concentration of p-tyramine for oxidative pyrolysisexperiments is much higher than that from pyrolysis.

Mechanisms presented here as well some additional reactionsfor formation of major products from tyrosine pyrolysis/oxidativepyrolysis, based also on literature (Li et al., 2008), are summarizedin Scheme 3.

There are two additional pathways for the formation of p-tyra-mine from the oxidative pyrolysis of tyrosine, Scheme 3 (parallel tothe initiation reactions by molecular oxygen): (1) abstraction of Hfrom the hydroxyl group of tyrosine by hydroxyl radical to formtyrosyl radical followed by CO2 elimination to form tyraminyl rad-ical and ultimately the formation of p-tyramine (2) abstraction of Hfrom acidic group of tyrosine by hydroxyl radical to form interme-diate I followed by CO2 elimination leading to intermediate II andeventually p-tyramine.

As discussed above, p-tyramine is formed in relatively highyields in oxidative pyrolysis compared to a pyrolysis (cf. Figs. 1Aand 2A). Consequently, p-tyramine is an important precursor forformation of many other reaction products identified in this study.Two pathways for the formation of phenol and p-cresol through p-tyramine and tyrosine are included in Scheme 3. The first pathwayinvolves the scission of bond #5 (cf. Fig. 3A) to form p-hydroxymethylene radical before forming p-cresol and eventually phenol(ref. Scheme 1, Rxn 2). The second channel proceeds from the scis-

lene phenolic radical and ultimate formation of p-cresol.

Scheme 2. Chain reactions showing the formation for tyrosyl radical and formation of p-tyramine.

J.K. Kibet et al. / Chemosphere 91 (2013) 1026–1034 1031

sion of an amino group (cf. Scheme 3) to form p-hydroxy methy-lene radical and finally p-cresol and phenol as shown in Scheme 3.

The formation of other major products (by decreasing yieldsafter p-tyramine, phenols and cresols) such as benzonitrile andacetonitrile are probably the result of dipeptide or polypeptidedecomposition reactions. Dipeptide forming reactions occur read-ily because they are simple dehydration reactions which are usu-ally enhanced by increase in temperature (Simmonds et al.,1972; Ratcliff et al., 1974; Sharma et al., 2004a,b; Li et al., 2008).Although the concentration of dipeptide is considered low, it is be-lieved to play a critical role in the formation of many observedproducts of amino acid pyrolysis (Chiavari and Galletti, 1992; Shar-ma et al., 2004a,b). For instance the formation of acetonitrile frompyrolysis of tyrosine may proceed via the decomposition of cyclicdipeptides (Sharma et al., 2004a,b). This channel involves a molec-ular process, and a free radical mechanism in which acetonitrile iseventually formed from dehydration of acetamide.

4.1.3. New classes of compounds not reported in literatureOxidative pyrolysis of tyrosine yielded other important com-

pounds of biological interest: hydroquinone, p-benzoquinone, ben-zofuran, dibenzofuran, and dibenzo-p-dioxin. The main precursorfor formation of hydroquinone and ultimately p-benzoquinone isp-cresol (cf. Scheme 4). An OH radical displaces the methyl in p-

cresol, yielding hydroquinone. Subsequently, p-benzoquinone for-mation is initiated via endothermic dissociation of a phenoxyl-hydrogen (DHrxn = 81.3 kcal mol�1) or H� abstraction by �OH toform p-semiquinone radical (Lucarini et al., 2002; McFerrin et al.,2009). Subsequent loss of phenoxyl-hydrogen by unimoleculardecomposition (DHrxn = 87 kcal mol�1) (Mulder et al., 2005) orabstraction (DHrxn = 40 kcal) (Wiater et al., 2000) by OH resultsin the formation of p-benzoquinone.

The formation of dibenzo-p-dioxin and dibenzofuran from oxi-dative pyrolysis of tyrosine has captured our attention because ofthe health impacts of the chlorinated analogues of these com-pounds (Dellinger et al., 2008; Zheng et al., 2008). Although thesecompounds are reported extensively in literature, never beforehave they been documented during the combustion of amino acids.Hydroxyl radical is believed to play a critical role during oxidativepyrolysis of tyrosine and influences the reaction products ob-served. The precursor for these compounds is phenol.

When subjected to heat, phenol forms both dibenzofuran (Wiateret al., 2000) and dibenzo-p-dioxin (Evans and Dellinger, 2003a,b;Khachatryan et al., 2003; Evans and Dellinger, 2005a,b). The forma-tion pathway for dibenzo-p-dioxin/dibenzofuran proceeds via freeradical mechanisms either through radical–molecule or radical–radical pathways (Louw and Ahonkhai, 2002; Evans and Dellinger,2003a,b; Khachatryan et al., 2003; Asatryan et al., 2005; Evans and

Scheme 3. Mechanistic pathways for formation of major phenolic compounds from decomposition of tyrosine.

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Dellinger, 2005a,b). In the radical–molecule pathway, the enol formof the phenoxy radical displaces a ring hydrogen of the phenol mol-ecule to form a hydroxyl biphenyl ether intermediate, Rxn 7(Scheme 4), followed by ring closure and ultimately the formationof dibenzo-p-dioxin (Wiater et al., 2000; Louw and Ahonkhai,2002; Lucarini et al., 2002; Evans and Dellinger, 2003a,b; Khachatr-yan et al., 2003; Asatryan et al., 2005; Evans and Dellinger, 2005a,b)(cf. Scheme 4). In the radical–radical pathway two keto mesomers(resonance structures) can react with each other (cf. Scheme 4, Rxn8) to form dibenzofuran, while keto- and enol- mesomers react toform dibenzo-p-dioxin (cf. Scheme 4, Rxn 9) (Khachatryan et al.,2003; Asatryan et al., 2005; Evans and Dellinger, 2005a,b).

4.1.4. Toxicological concerns for major reaction products from thethermal degradation of tyrosine

The toxicology of molecular products from pyrolysis of tyrosineis a subject of environmental concern (Kibet et al., 2012). Phenoland cresol can be oxidized by DNA to produce resonance stabilized

persistent free radicals (PFRs) with long lifetimes, which can causeextensive cellular damage, oxidative stress, tumors, and cancer(Dellinger et al., 2000, 2007; Smith and Hansch, 2000; Dellingeret al., 2001). The phenolic compounds found in biomass burningand cigarette smoke as well as in this work are also consideredtoxic and include phenol, substituted phenols (p-cresol, and o-cre-sol), and hydroquinone (Selassie et al., 1999; Smith and Hansch,2000). Phenol affects liver enzymes, lungs, kidneys, cardiovascularsystem, and may attack the nervous system (Talhout et al., 2011).Following H atom abstraction from the phenol hydroxyl group, theresultant phenoxy radical exhibits some electron-deficient charac-ter (Dellinger et al., 2007) which would be stabilized by electron-donating substituents such as amino, methoxy, and methyl groups,imparting longer life times (Smith and Hansch, 2000). Substitutedphenoxy radicals (e.g. p-cresol, o-cresol, p-tyramine, etc.) bearingelectron-donating substituents would therefore be expected to bemore toxic because they are more stable and have longer lifetimes(Dellinger et al., 2000, 2007; Smith and Hansch, 2000; Dellinger

Scheme 4. Formation of hydroquinone, p-benzoquinone, dibenzofuran, and dibenzo-p-dioxin.

J.K. Kibet et al. / Chemosphere 91 (2013) 1026–1034 1033

et al., 2001). Additionally, phenoxy radicals are precursors for for-mation of dibenzo-p-dioxin/dibenzofuran, which are easily chlori-nated in the presence of a redox-active transition metal such ascopper or iron to form polychlorinated dibenzo-p-dioxin/dibenzo-furan (PCDD/F) (Louw and Ahonkhai, 2002; Evans and Dellinger,2003a,b; Khachatryan et al., 2003; Kibet et al., 2012). Clearly, theformation of compounds of biological interest including, hydroqui-none, p-benzoquinone, benzofuran, dibenzofuran and dibenzo-p-dioxin suggests the thermal degradation of tyrosine is an impor-tant contributor to degradation in air quality when biomass isburned.

5. Conclusion

A detailed kinetic analysis for the formation of p-tyramine as aprincipal product in the thermal degradation of tyrosine has thor-oughly been discussed. For the first time, compounds of biologicalinterest including dibenzo-p-dioxin, dibenzofuran, hydroquinoneand p-benzoquinone are reported from oxidative pyrolysis of tyro-sine. We believe these toxic compounds are formed during heattreatment of proteinaceous foods, tobacco burning and biomassburning, and are precursors for oxidative stress and cancers.

Acknowledgements

The authors appreciate financial support from Reynolds’ Tobac-co Company.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.chemosphere.2013.01.071.

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