Amino acid catalysis of 2-alkylfuran formation from lipid oxidation-derived α,β-unsaturated aldehydes

Article abstract of DOI:10.1021/jf202448v

The formation of 2-alkylfurans from the corresponding lipid-derived α,β-unsaturated aldehydes under dry-roasting conditions was investigated in detail. The addition of an amino acid to an α,β- unsaturated aldehyde drastically increased 2-alkylfuran formation. Peptides and proteins as well were able to catalyze 2-alkylfuran formation from the corresponding α,β-unsaturated aldehydes. Further investigation of 2-alkylfuran formation showed the need of oxidizing conditions and the involvement of radicals in the reaction. This way, the formation of 2-methylfuran from 2-pentenal, 2-ethylfuran from 2-hexenal, 2-propylfuran from 2-heptenal, 2-butylfuran from 2-octenal, 2-pentylfuran from 2-nonenal, and 2-hexylfuran from 2-decenal was shown. The impact of amino acids on 2-alkylfuran formation from lipid-derived α,β-unsaturated aldehydes represents an interesting example of the complex role of amino acids in the multitude of chemical reactions occurring during thermal processing of lipid-rich foods.

Full text of DOI:10.1021/jf202448v

ARTICLE
pubs.acs.org/JAFC
Amino Acid Catalysis of 2-Alkylfuran Formation from Lipid
Oxidation-Derived α,β-Unsaturated Aldehydes
An Adams,† Capucine Bouckaert,† Fien Van Lancker,† Bruno De Meulenaer,‡ and Norbert De Kimpe*,†
†Department of Sustainable Organic Chemistry and Technology, Faculty of Bioscience Engineering, and ‡Department of Food Safety
and Food Quality, Faculty of Bioscience Engineering, Ghent University, Coupure links 653, B-9000 Ghent, Belgium
ABSTRACT: The formation of 2-alkylfurans from the corresponding lipid-derived α,β-unsaturated aldehydes under dry-roasting
conditions was investigated in detail. The addition of an amino acid to an α,β-unsaturated aldehyde drastically increased 2-alkylfuran
formation. Peptides and proteins as well were able to catalyze 2-alkylfuran formation from the corresponding α,β-unsaturated
aldehydes. Further investigation of 2-alkylfuran formation showed the need of oxidizing conditions and the involvement of radicals
in the reaction. This way, the formation of 2-methylfuran from 2-pentenal, 2-ethylfuran from 2-hexenal, 2-propylfuran from
2-heptenal, 2-butylfuran from 2-octenal, 2-pentylfuran from 2-nonenal, and 2-hexylfuran from 2-decenal was shown. The impact of
amino acids on 2-alkylfuran formation from lipid-derived α,β-unsaturated aldehydes represents an interesting example of the
complex role of amino acids in the multitude of chemical reactions occurring during thermal processing of lipid-rich foods.
KEYWORDS: 2-Alkylfurans, lipid oxidation, amino acids, α,β-unsaturated aldehydes
’ INTRODUCTION
2-octenal from (E)-2-octenal as minor compounds, but the cor-
responding 2-alkylfurans were not detected. From (E,E)-2,4-
decadienal, (E)-4,5-epoxy-2-decenal was formed, but in addition,
4-hydroxy-2-nonenal and 2-pentylfuran were identified. The
authors postulated an allylic hydroperoxidation for the forma-
tion of 4-oxygenated compounds, possibly cyclizing toward the
corresponding 2-alkylfurans.
Two of the most important reactions responsible for the deve-
lopment of flavor in processed foods are the Maillard reaction
between carbohydrates and amino acids, on the one hand, and
lipid degradation, on the other hand. In food, however, both
reactions are closely related: Lipid oxidation products or inter-
mediates can influence the course of the Maillard reaction and
vice versa. Specific carbonyl compounds and heterocyclic aroma
compounds can result from both reaction pathways.1,2
In our previous research, we studied the formation of volatile10
and nonvolatile11 reaction products of amino acids with various
lipid oxidation products, in comparison with Maillard reaction
systems. As we observed that amino acids were efficient catalysts
of the formation of 2-alkylfurans from the corresponding α,
β-unsaturated aldehydes,10 we studied this reaction in detail. The
impact of amino acids on 2-alkylfuran formation from lipid-
derived α,β-unsaturated aldehydes may represent an interesting
example of Maillard type carbonylÀamine interactions occurring
in lipid-rich foods. The reaction mechanism of 2-alkylfuran
formation from α,β-unsaturated aldehydes and amino acids
requires, therefore, further attention.
Furans are among the most ubiquitous heterocyclic com-
pounds in thermally processed foods.3,4 Carbohydrates (in the
presence or absence of amino acids) are primary sources of furans
in food, for instance, furfural, 2-acetylfuran, and 2-methylfuran.
2-Alkylfurans, especially those with longer alkyl side chains,
mainly result from the degradation of lipids, and are common
among the flavor volatiles of, for example, meat products and
coffee.3 In this respect, 2-alkylfurans are sometimes mentioned
as markers of lipid oxidation. 2-Pentylfuran, for instance, is
commonly reported as an oxidation product of linoleic acid
and possesses a buttery flavor, resembling green beans.5 2-Ethyl-
furan, evoking a powerful burnt, sweet, coffeelike flavor,3 has also
been suggested to represent the extent of lipid oxidation in food
systems, for example, in cod liver oil and cooked salmon.6,7
Secondary lipid oxidation products, in particular 4-hydroxy-2-
alkenals, are probable precursors of 2-alkylfurans in foods, but the
mechanism involved is poorly described in the literature. Cycli-
zation of a 4-hydroxy-2-alkenal followed by dehydration can yield
the corresponding 2-alkylfuran. α,β-Unsaturated aldehydes, how-
ever, are potential precursors of 2-alkylfurans as well. Quantita-
tively, these 2-alkenals are more important than the correspond-
ing 4-hydroxy-2-alkenals in food products.8 Grein et al.9
described the water-mediated oxidative decomposition of α,
β-unsaturated aldehydes (0.02 M concentration of aldehydes,
pH 6.5, room temperature). They reported the formation of 4-
hydroxy-(E)-2-hexenal from (E)-2-hexenal and of 4-hydroxy-(E)-
’ MATERIALS AND METHODS
Chemicals. Alanine (99%), glutamic acid (99%), arginine mono-
hydrate (98%), lysine monohydrate (99%), methionine (98%), (E)-2-
pentenal (97%), (E)-2-hexenal (99%), (E)-2-heptenal (97%), (E)-2-
decenal (95%), 2-ethylfuran (99%), L-(+)-ascorbic acid (99%), sodium
bisulfite, copper(II)chloride, iron(III)chloride, butylated hydroxyani-
sole (BHA, 96%), and azobisisobutyronitril (AIBN, 98%) were from
Acros Organics (Geel, Belgium). Glycine (99%), aspartic acid (98%),
glutamine (98%), threonine (98%), histidine (98%), leucine (99%),
diglycine (>99%), triglycine, tetraglycine, casein, (E)-2-octenal (94%),
Received: June 20, 2011
Accepted: September 19, 2011
Revised:
September 14, 2011
Published: September 19, 2011
r
2011 American Chemical Society
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Figure 1. Influence of the addition of various amino acids, peptides, and casein (2 equiv) on the formation of 2-ethylfuran from (E)-2-hexenal under dry-
roasting conditions (180 °C, 20 min), as measured by SPME-GC-MS peak area. Values are means of three repetitions with standard deviations as error
bars. Blank refers to only (E)-2-hexenal. 2-Ethylfuran was not detected in case of addition of proline, benzylamine, and dibenzylamine.
(E)-2-nonenal (97%), and 18O-labeled water (Isotec, 97% 18O atoms)
were from Sigma-Aldrich (Bornem, Belgium). Serine (99%), phenyla-
lanine (98.5%), and cysteine (97%) were from Janssen Chimica (Geel,
Belgium). Tyrosine was from Difco Laboratories (BD, Erembodegem,
Belgium). 2-Propylfuran(99%), 2-butylfuran (98%), 2-pentylfuran (98%),
and 2-hexylfuran (97%) were from Alfa Aesar (Karlsruhe, Germany).
Model Reactions. For this purpose, 20 mL solid phase microex-
traction (SPME) vials (Gerstel, M€ulheim a/d Ruhr, Germany) were sub-
sequently filled with 0.25 mmol of carbonyl compound, 2 g of sand
(purified sea sand, 50À70 mesh, Sigma-Aldrich) and, when required,
0.5 mmol of amino acid (or 1 g of casein) and/or various additives in
amounts specified in the tables. The vials were hermetically closed with a
magnetic crimp cap with a septum (silicone/PTFE; 55° Shore A; 1.5 mm,
Gerstel), mixed well by means of vortex, and placed in an oven (equipped
with a fan, Memmert, Schwabach, Germany), which had been preheated
to and stabilized at 180 °C. The reaction mixtures were heated for exactly
20 min. After they were heated, the vials were removed from the oven
and rapidly cooled in an ice bath. All model reactions were performed in
triplicate. To determine the influence of water on the SPME extraction
efficiency, authentic standards were analyzed. For this purpose, standard
solutions of 2-alkylfurans in methanol (5% v/v) were prepared, and 10 μL
of this solution was sprayed on 2 g of sand. To this mixture, varying
amounts of water (5À25 μL) or ethanol (10À100 μL) were added, fol-
lowed by vortex mixing. All standards were prepared in triplicate.
SPME Parameters. SPME extraction and desorption were per-
formed automatically by means of an MPS-2 autosampler (Gerstel). For
the analysis of 2-alkylfurans, SPME extraction was performed for 30 min
at 35 °C, and desorption took place for 2 min in the hot GC inlet
(250 °C). A divinylbenzene/Carboxen/polydimethylsiloxane (DVB/
Carboxen/PDMS) fiber (Supelco, Bornem, Belgium) was used.
Desorption of blank 2-alkenal mixtures at three different tempera-
tures (230, 250, and 270 °C) showed the formation of traces of the cor-
responding 2-alkylfurans, with no significant difference among the three
desorption temperatures. As a result, artifactual formation of 2-alkyl-
furans on the SPME fiber during fiber desorption (250 °C, 2 min) is
negligible.
(0À10 min), 40À300 (10À20 min), and 40À400 (>20 min). Volatile
substances were identified by comparison of their mass spectra and
retention times with those of reference substances and by comparison
with the Wiley (6th ed.) and the NIST Mass Spectral Library (Version
1.6d, 1998).
’ RESULTS AND DISCUSSION
To study in detail the reaction mechanism of the formation of
2-alkylfurans from the corresponding α,β-unsaturated alde-
hydes, several model reactions were performed. Model mixtures
of an α,β-unsaturated aldehyde, in the presence or absence of
various additives, were heated under dry-roasting conditions
(180 °C, 20 min). After they were cooled, the headspace of the
reaction mixtures was sampled by means of SPME (DVB/Car/
PDMS), followed by GC-MS analysis. Results are expressed as
GC-MS peak areas and provide as such semiquantitative data.
Because of the lack of labeled standards for 2-alkylfurans, reliable
quantification by SPME (by means of stable isotope dilution
analysis) was unfortunately not possible. Matrix effects were
minimized by applying low amounts of precursors in a matrix of
sand. By means of automated SPME extraction and desorption,
careful control of all of the parameters involved was ensured, and
repeatable analyses were accomplished. The relative standard
deviation (RSD) of SPME-GC-MS measurements of replicate
samples ranged from 2.4% until, occasionally, 48.7% (for com-
pounds present in very low amounts). Analysis of 2-ethylfuran
standard mixtures showed a logarithmic response of the SPME-
GC-MS peak areas on varying amounts of 2-ethylfuran (data not
shown). These standard tests indicated that the analyzed peak
areas represent amounts of 2-alkylfurans in the low μmol range.
In the first instance, the formation of 2-ethylfuran from (E)-2-
hexenal in model systems was investigated. Whereas upon heat-
ing of neat (E)-2-hexenal no 2-ethylfuran was detected, the
addition of an amino acid remarkably increased the formation
of 2-ethylfuran (Figure 1). Previous studies showed that aldol
type reactions prevailed in model mixtures of unsaturated alde-
hydes with amino acids.10 To prevent the prevalence of such (E)-
2-hexenal self-condensation reactions, 2 equiv of amino acid was
applied in the first instance. Later experiments showed, however,
that catalytic amounts (0.1 equiv) of the amino acid were suffi-
cient to increase 2-alkylfuran formation (data not shown). In
Figure 1, considerable differences in reactivity between the variety
of amino acids tested can be noted. The highest amounts of
Mass Spectrometry. For the analysis of volatile reaction products,
a Hewlett-Packard 6890 GC Plus coupled with a HP 5973 MSD (Mass
Selective Detector-Quadrupole type), equipped with a CIS-4 PTV (Pro-
grammed Temperature Vaporization) Injector (Gerstel) and a AT5-MS
capillary column (30 mm  0.25 mm i.d.; coating thickness 0.25 μm)
was used. Working conditions were as follows: injector, 250 °C; transfer
line to MSD, 260 °C; oven temperature, start 40 °C, hold 3 min;
programmed from 40 to 200 at 4 °C minÀ1 and from 200 to 240 at
30 °C minÀ1, hold 2 min; split ratio, 10:1; carrier gas (He), 1.2 mL minÀ1
;
ionization EI, 70 eV; and acquisition parameters, scanned m/z 40À200
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2-ethylfuran were detected in the presence of glutamic acid and
tyrosine, followed by serine and threonine.
Table 1. Formation of 2-Ethylfuran from (E)-2-Hexenal and
of 2-Propylfuran from (E)-2-Heptenal in the Presence of 2 equiv
of Phenylalanine under Dry-Roasting Conditions (180 °C,
20 min), as Measured by SPME-GC-MS Peak Area (Â106)
The formation of 2-ethylfuran from (E)-2-hexenal in the
presence of glycine peptides (diglycine, triglycine, and tetra-
glycine) was significantly higher than from glycine itself. Thus,
the addition of peptides and a protein (casein) also enhanced
the formation of 2-ethylfuran from (E)-2-hexenal. Although the
addition of an amino acid has an important influence on 2-alkyl-
furan formation, it is difficult to distinguish the specific nature of
this effect.
additives
2-ethylfuran
2-propylfuran
2-alkenal not heated
2-alkenal
31.43 ( 3.88 aa
43.89 ( 3.85 c
68.48 ( 6.69 d
7.44 ( 3.70 a
89.41 ( 9.41 b
NAb
+ 5 μL of water
+ 15 μL of water
+ 25 μL of water
+ 15 μL of 2 N NaOH
+ 15 μL of 2 N HCl
+ N2 atm
87.40 ( 10.53 e 153.43 ( 13.93 c
When comparing the influence of acidic and basic amino acids
on 2-ethylfuran formation, no clear trend could be observed:
Whereas the addition of glutamic acid induced the highest 2-
ethylfuran formation, the formation of 2-ethylfuran in the pre-
sence of aspartic acid was much lower (Figure 1). In the presence
of lysine, the 2-ethylfuran peak was very low, while tyrosine
increased 2-ethylfuran formation, although both side chains have
a similar pKa (around 10). Thus, acid/base catalysis by the amino
acid side chain is most probably not responsible for the increase
in 2-alkylfuran formation upon addition of an amino acid.
If increased cyclization toward the 2-alkylfuran would be due
to the formation of an imine in a first step, a secondary amine
would be more advantageous than a primary amine. However, in
the presence of proline, no 2-ethylfuran was detected upon
heating of (E)-2-hexenal (data not shown). In the presence of
benzylamine, only traces of 2-ethylfuran were found, while no
2-ethylfuran was detected in the presence of dibenzylamine (data
not shown). Moreover, low amounts of 2-ethylfuran were noted
in case of lysine, having a very reactive ε-amino group. These
results imply that the formation of an imine prior to cyclization
cannot explain the positive effect of the amino acids on 2-alkyl-
furan formation. Conversely, those amino compounds expected
to be more prone to imine formation showed very low yields of
2-ethylfuran. As the Strecker aldehyde phenylacetaldehyde was
not detected in the phenylalanine model systems, Strecker degra-
dation does not occur to a considerable extent. This indicates
again that imine formation is probably not a key step in the reac-
tion mechanism.
The addition of an amino acid, peptide, or protein to the α,
β-unsaturated aldehyde induced browning and enhanced the
general reaction rate in the model systems. The 2-alkylfurans
were actually minor compounds in a complex reaction mixture.
An overview of the different reaction products of these model
systems, among which are 2-alkylfurans, has been discussed else-
where.10 Measurement of the degree of browning of the model
systems, however, showed no correlation between browning and
2-ethylfuran formation: In some cases, a high extent of browning
was accompanied with low amounts of 2-ethylfuran and vice versa.
To identify important parameters in the formation of 2-alkyl-
furans from the corresponding α,β-unsaturated aldehydes, the
cyclizations of (E)-2-hexenal to 2-ethylfuran and of (E)-2-hep-
tenal to 2-propylfuran were investigated under different reaction
conditions, in the presence of phenylalanine. Phenylalanine was
selected as model amino acid, as it had an intermediate effect on
2-alkylfuran formation among the amino acids tested. In addi-
tion, phenylalanine degradation products, such as the Strecker
aldehyde phenylacetaldehyde, are easily detectable by means of
gas chromatography and may be an indication of specific reaction
mechanisms that occur. The resulting GC peak areas of the
2-alkylfurans are displayed in Table 1.
115.03 ( 13.75 f
34.04 ( 5.26 a
NA
129.53 ( 23.94 c
78.76 ( 22.10 e 252.90 ( 38.70 d
11.39 ( 1.87 g
91.40 ( 8.55 e
11.49 ( 0.89 a
+ 0.01 equiv of vit C in 15 μL
of water
177.75 ( 10.81 c
+ 1 equiv of NaHSO3
+ 1 equiv of NaHSO3 in 15 μL
of water
24.92 ( 0.66 b
110.11 ( 6.60 e
29.00 ( 6.82 ab 111.34 ( 15.27 e
+ 15 μL of 2 M CuCl2
+ 10 μL of EtOH
295.62 ( 38.68 h 359.36 ( 43.90 f
29.02 ( 9.53 abi 62.05 ( 8.84 g
+ 10 μL of 0.1 M BHA in EtOH 27.16 ( 2.21 ab
96.06 ( 14.23 b
+ 100 μL of EtOH
+ 100 μL of 0.01 M AIBN
in EtOH
15.01 ( 1.99 i
NA
c
321.18 ( 119.01 h 294.10 ( 143.13 df
a Different letters within one column indicate significant differences
(α = 0.05). b NA, not analyzed. c In 10 μL of ethanol.
(except in the presence of 100 μL of ethanol and with NaOH). On
the contrary, 2-propylfuran was detected upon heating of (E)-2-
heptenal in most cases (except in the nonheated sample and in the
presence of sodium bisulfite); the resulting peak areas remained
a factor 3À10 lower than in the presence of phenylalanine.
At first, the effects of heating and of the addition of small
amounts of water to the dry model system were evaluated. The
addition of phenylalanine seemed sufficient to catalyze the for-
mation of 2-ethylfuran and 2-propylfuran from the correspond-
ing α,β-unsaturated aldehydes, even without heating. Heating
the model systems (180 °C, 20 min) significantly increased 2-
alkylfuran formation. The addition of small amounts of water
(0À25 μL) additionally increased 2-alkylfuran formation upon
heating. The 2-ethylfuran peak area increased linearly with the
amounts of water added (R2 = 0.976). Standard tests analyzing
2-ethylfuran (5 μmol) in the presence of increasing amounts of
water (0À25 μL) showed that this effect was partly due to an
enhanced SPME extraction of 2-ethylfuran in the presence of
small amounts of water (data not shown). This can be attributed
to an increased expulsion of 2-ethylfuran to the headspace. On
the basis of the slopes of the fitted straight lines, however, the
increase in 2-ethylfuran formation upon heating was almost three
times higher than the corresponding increase in SPME extraction
efficiency caused by the addition of water. Therefore, it can be
stated that the presence of water effectively increased 2-ethylfur-
an formation. As the addition of 18O-labeled water showed no
incorporation of the 18O-label in 2-ethylfuran, water does not
take part in the reaction as a hydroxylating reagent itself. Pro-
bably, the effect of water is merely due to an increased mobility of
the reagents.
It must be noted that, in the absence of an amino acid, no 2-
ethylfuran was detected for almost all reaction conditions evaluated
To elucidate the effect of acid/base catalysis further, the
addition of an acidic (2 N HCl) and an alkaline solution
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Scheme 1. Hypothesized Reaction Mechanism for the Formation of 2-Alkylfuran from the Corresponding (E)-2-Alkenal
Table 2. Formation of the Corresponding Substituted Furans from α,β-Unsaturated Aldehydes in the Absence and Presence of
2 equiv of Phenylalanine under Dry-Roasting Conditions (180 °C, 20 min), as Measured by SPME-GC-MS peak Area (Â106)
without Phe + watera
+ 2 M CuCl2
carbonyl
with Phe + watera
+ 2 M CuCl2
a
a
carbonyl
2-methylfuran from 2-pentenal
2.90 ( 0.71 ab
NDc
ND
2.21 ( 0.59 a
6.05 ( 0.96 b
43.89 ( 3.85 a
6.84 ( 2.92 b
87.40 ( 10.53 b
153.43 ( 13.93 d
67.24 ( 10.08 a
260.10 ( 6.32 d
151.47 ( 14.10 d
20.86 ( 2.82 c
295.62 ( 38.68 c
359.36 ( 43.90 e
513.63 ( 19.50 b
465.70 ( 128.57 e
481.08 ( 122.26 e
2-ethylfuran from (E)-2-hexenal
ND
ND
2-propylfuran from (E)-2-heptenal 11.51 ( 1.78 a 15.42 ( 3.06 a
52.37 ( 10.41 b 89.41 ( 9.41 c
2-butylfuran from (E)-2-octenal
2-pentylfuran from (E)-2-nonenal
2-hexylfuran from (E)-2-decenal
a 15 μL. b Different letters within one row indicate significant differences (α = 0.05). c ND, not detected.
ND
ND
77.10 ( 3.23 a
73.36 ( 4.96 b
55.07 ( 7.08 c
ND
7.61 ( 0.50 a
13.89 ( 5.48 a
137.93 ( 5.50 c
46.23 ( 2.63 c
11.70 ( 1.49 a 19.01 ( 0.84 b
(2 N NaOH) wasinvestigated. Asalreadysuggestedfromthevarious
amino acids applied, these results were also inconclusive: In the case
of (E)-2-hexenal, the addition of an alkaline solution decreased
2-ethylfuran formation, while the addition of an acid solution did
not have a significant effect (Table 1). From (E)-2-heptenal on the
other hand, more 2-propylfuran was formed in case of acid catalysis,
whereas alkaline catalysis did not show a significant influence.
To be able to cyclize into the corresponding 2-alkylfuran,
hydroxylation in the γ-position of the carbonyl group of the α,
β-unsaturated aldehyde is required, followed by isomerization of
the double bond from the E to the Z configuration. The inter-
mediate 4-hydroxy-2-alkenals formed are known as very reactive
compounds. Especially 4-hydroxy-2-hexenal and 4-hydroxy-2-
nonenal are described as highly toxic lipid oxidation-derived
compounds, reacting readily with proteins and aminophospho-
lipids under physiological conditions.12 However, their forma-
tion from the corresponding α,β-unsaturated aldehydes is not
described in literature.
Considering the oxidative nature of the mechanism of 2-alkyl-
furan formation proposed in Scheme 1, the effect of various
oxidizing and reducing agents was investigated. Performing the
reaction under inert nitrogen atmosphere induced a very sig-
nificant reduction in 2-alkylfuran formation, thus suggesting the
importance of oxygen in the oxidation step (Table 1). The addi-
tion of the food-grade antioxidant vitamin C (in low amounts)
did not significantly change the 2-alkylfuran peak areas. The
reducing agent sodium bisulfite significantly reduced 2-alkylfuran
formation, in dry and aqueous conditions (as compared to the
corresponding model systems not containing sodium bisulfite),
although an unexpected small increase in the 2-propylfuran peak
area was observed upon the addition of sodium bisulfite to 2-
heptenal. Besides, as a reducing agent, bisulfite is known to
stabilize imines and may thus have a dual effect in the model
reactions.13,14
results clearly shows the importance of oxidative reaction con-
ditions in an efficient formation of 2-alkylfurans from the corres-
ponding α,β-unsaturated aldehydes. Because of the strong cata-
lytic effect of copper ions in 2-alkylfuran formation, it cannot be
excluded that varying levels of trace metals in the amino acid
reagents are partly responsible for the varying influence of the
amino acids noted.
The importance of radicals in the mechanism was evaluated by
means of the addition of a radical scavenger butylated hydro-
xyanisole (BHA) and a radical initiator azobisisobutyronitrile
(AIBN), as ethanolic solutions. The effect of BHA was small and
not clear: 2-Ethylfuran formation was not significantly influ-
enced, while 2-propylfuran formation showed a slight increase.
The addition of AIBN clearly enhanced the formation of the
respective 2-alkylfurans, although the variability on the results
obtained was quite high. Thus, the oxidation mechanism prob-
ably follows a radical pathway.
Following the same reaction mechanism, the formation of
2-methylfuran from (E)-2-pentenal, 2-butylfuran from (E)-2-
octenal, 2-pentylfuran from (E)-2-nonenal, and 2-hexylfuran
from (E)-2-decenal was shown (Table 2). It must be noted that
quantitative comparison of GC peak areas obtained for different
2-alkylfurans is not relevant, as each compound shows a different
response for SPME extraction. Analysis by means of SPME-GC-
MS of authentic standards of the different 2-alkylfurans in the
same amounts showed a clear increase in GC peak area with
increasing length of the alkyl side chain, from 2-ethylfuran to
2-hexylfuran, following a sigmoid curve (Figure 2). For all α,
β-unsaturated aldehydes tested, an increase in the 2-alkylfuran peak
area was found with the addition of water and a CuCl2 solution to
the carbonyl compound and with the addition of phenylalanine
to the model mixtures (Table 2). These results confirm the en-
hancing effect of the pro-oxidant copper ions on the 2-alkylfuran
formation and thus suggest the involvement of oxidation pro-
cesses in their formation from α,β-unsaturated aldehydes. The
subsequent intramolecular cyclization of 4-hydroxy-2-alkenals
The addition of an aqueous oxidizing CuCl2 solution strongly
increased 2-alkylfuran formation. The combination of these
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’ AUTHOR INFORMATION
Corresponding Author
*Tel: +32 9 264 59 51. Fax: +32 9 264 62 21. E-mail: norbert.
dekimpe@UGent.be.
Funding Sources
We are indebted to the Research Foundation—Flanders
(Belgium) (FWO-Vlaanderen) for a Postdoctoral Fellowship
of A.A. and an Aspirant Fellowship of F.V.L.
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alkyl chain of a 2-alkylfuran (from 2-ethylfuran to 2-hexylfuran; 0.45 mg
on 2 g of sand).
(2) Zamora, R.; Hidalgo, F. J. Coordinate contribution of lipid
oxidation and Maillard reaction to the nonenzymatic food browning.
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(3) Maga, J. Furans in food. CRC Crit. Rev. Food Sci. Nutr. 1979, 11,
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(4) Adams, A.; Borrelli, R. C.; Fogliano, V.; De Kimpe, N. Thermal
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Scheme 2. Hypothesized Amino Acid Catalysis of the Cycli-
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into the corresponding 2-alkylfurans is believed to require high
temperature,15 but our results indicate that it can also be facilitated
by amino acid catalysis. The resulting mechanism of 2-alkylfuran
formation, involving the radical oxidation by means of oxygen, is
proposed in Scheme 1. Because α-amino acids have received
attention before as (enantioselective) catalysts of various
transformations,16 we hypothesize that amino acids can catalyze
the cyclization of the (Z)-4-hydroxy-2-alkenals into the correspond-
ing 2-alkylfurans, in several possible ways (Scheme 2). First, an
amino acid can facilitate the cyclization process by acid/base
catalysis (Scheme 2A). The zwitterionic form of the amino acid
protonates the carbonyl group of the (Z)-4-hydroxy-2-alkenal,
thus increasing its electrophilicity, while at the same time the
hydroxyl function is deprotonated, increasing its nucleo-
philicity. An amino acid can regain its zwitterionic form when
the amino group takes up a proton while the carboxyl group is
deprotonated. This process as well may catalyze the cyclization of
a (Z)-4-hydroxy-2-alkenal (Scheme 2B). Finally, the hydrogen-
bonding ability of amino acids can be of high importance.16 Thus,
we present in Scheme 2C catalysis of the cyclization based on
hydrogen bonding by the carboxyl group of the amino acid.
The alcohol function of the side chain of serine, threonine, and
tyrosine may also be relevant in this respect, as these amino acids
showed the highest activity. Other relative differences between
the amino acids tested will probably depend on their specific
acid/base properties and on steric effects.
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(12) Esterbauer, H.; Schaur, R. J.; Zollner, H. Chemistry and
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(14) Adams, A.; Abbaspour Tehrani, K.; Kersiene, M.; De Kimpe, N.
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6-acetyl-1,2,3,4-tetrahydropyridine in proline/1,3-dihydroxyacetone
model systems. J. Agric. Food Chem. 2004, 52, 5685–5693.
(15) Garibyan, O. A.; Ovanesya, A. L.; Makaryan, G. M.; Petrosyan,
A. L.; Chobanyan, Zh.A. (2E)-4-hydroxy-2-alkenals and 2-substituted
furans as products of reactions of (2E)-4,4-dimethoxybut-2-enal with
Grignard compounds. Russ. J. Org. Chem. 2010, 46, 406–409.
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asymmetric catalysis. Chem. Rev. 2007, 107, 5713–5743.
Because of their highly versatile nature and reactivity, amino
acids may thus exert several functions in the myriad of reactions
occurring during thermal treatment of food. Their condensation
with carbohydrates in the Maillard reaction is of extreme impor-
tance in heated food products, but the catalysis of 2-alkylfuran
formation described here illustrates their broad catalytic poten-
tial in food.
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