Thermally induced oxidative decarboxylation of copper complexes of amino acids and formation of strecker aldehyde

Article abstract of DOI:10.1021/jf502751n

In the Maillard reaction, independent degradations of amino acids play an important role in the generation of amino-acid-specific products, such as Strecker aldehydes or their Schiff bases. Such oxidative decarboxylation reactions are expected to be enhan

Full text of DOI:10.1021/jf502751n

Article
pubs.acs.org/JAFC
Thermally Induced Oxidative Decarboxylation of Copper Complexes
of Amino Acids and Formation of Strecker Aldehyde
Ossanna Nashalian and Varoujan A. Yaylayan*
Department of Food Science and Agricultural Chemistry, McGill University, 21111 Lakeshore, Sainte Anne de Bellevue, Quebec H9X
3V9, Canada
ABSTRACT: In the Maillard reaction, independent degradations of amino acids play an important role in the generation of
amino-acid-specific products, such as Strecker aldehydes or their Schiff bases. Such oxidative decarboxylation reactions are
expected to be enhanced in the presence of metals. Preliminary studies performed through heating of alanine and various metal
salts (Cu, Fe, Zn, and Ca) under pyrolytic conditions indicated that copper(II) and iron(III) because of their high oxidation
potentials were the only metals able to induce oxidative decarboxylation of amino acids and formation of Strecker aldehyde or its
derivatives as detected by gas chromatography/mass spectrometry. Furthermore, studies performed with synthetic alanine and
glycine copper complexes indicated that they constituted the critical intermediates undergoing free-radical oxidative degradation,
followed by the loss of carbon dioxide and the generation of Strecker aldehydes, which were detected either as stable Schiff base
adducts or incorporated in moieties, such as pyrazine or pyridine derivatives.
KEYWORDS: Strecker aldehydes, metal salts, amino acid metal complexes, oxidative decarboxylation of amino acids
such complexes in the Maillard reaction has not been studied,
enhancements in the Strecker aldehyde formation were
reported in the presence of copper ions by Hofmann and
Schieberle.9 Furthermore, copper was found to catalyze the
oxidative decarboxylation of a synthetic amino dioic acid in
aqueous solution, converting it into an amino monocarboxylic
acid.10 Recently, Yablokov et al.11 reported the kinetics of
thermal degradation of copper complexes of several amino
acids at 230 °C, indicating an oxidative degradation mechanism
of the complex initiated with the formation of carboxylamine
free radical, followed by the loss of carbon dioxide and the
generation of alkylamine radicals and its recombination
products. Furthermore, the rate constants for the thermal
degradation reactions of various amino acid copper complexes
were highly dependent upon the nature and bulkiness of their
side chains, and rates of degradation of such amino acid
complexes decreased in the following order: Ser > Phe > Leu >
Ile > Val > Gly > Ala.
The objective of this study was to explore the thermal
degradation of synthetic copper complexes of various amino
acids, particularly alanine and glycine, and their degradations in
the presence of various metal salts {iron(II) or iron(III)
chlorides, zinc chloride, calcium chloride [two generally
recognized as safe (GRAS) metal chloride salts], and copper(I)
and (II) chlorides}. The chloride salts of metals were selected
because of their ability to form more volatile chlorinated
reaction products.12 In this study, we explore the role of metal
salts and amino acid metal complexes in the decomposition
reactions of amino acids.
INTRODUCTION
■
Metal salts, such as copper sulfate, are commonly found or
added to foods for a variety of reasons. The corresponding
metal complexes formed during processing with amino acids
and sugars (Figure 1) or other Maillard reaction products1 may
alter their chemical reactivity during thermal treatment,
generating or enhancing specific reaction products, such as
those of oxidation or decomposition. Understanding how these
independent complexes thermally degrade provides important
insight into their fundamental behavior in the context of the
Maillard reaction. Over the past few decades, the role of metal
salts and their potential to affect the Maillard reaction has been
studied by several researchers.1−4 Considerable efforts have
been expended to study the catalytic role of metal salts in
promoting oxidation pathways and affecting the rates of the
reaction in aqueous systems. Moreover, recently, it has been
demonstrated that metal ions can accelerate the formation of
specific Maillard reaction products, such as advanced
glycosylation end product (AGE)-modified peptides.5 On the
other hand, the use of cations and their salts were explored in
mitigation strategies in lowering or preventing the formation of
certain Maillard reaction products, such as acrylamide, in both
model systems involving asparagine and sugars and food
models.6,7 Although the exact mechanism of how metal ions
can affect the Maillard reaction is still not clear, it has been
proposed that metals increase the rate of the reaction because
of their ability to oxidize Amadori compounds, accelerating the
formation of melanoidins.3 High-molecular-weight reductones
have also been reported to form complexes with metal ions
through polynuclear ligands.1,3 Their ability to complex with
amino acids, change the structural configurations of proteins,
catalyze redox reactions, and act as Lewis acid catalysts by
forming metal coordination complexes with sugars has also
been reported.8 Amino acids could be considered as prime
targets for metal complex formation, and although the role of
Received: June 9, 2014
Revised: July 30, 2014
Accepted: July 31, 2014
Published: July 31, 2014
© 2014 American Chemical Society
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Figure 1. Possible complexes of copper with glucose and alanine.
The separation was performed using a fused silica DB-5MS column
(50 m length × 0.2 mm inner diameter × 33 μm film thickness). The
GC method used for the analysis of the volatiles was as follows: The
GC column flow rate was regulated by an electronic flow controller
(EFC) and set at a pressure pulse of 55 psi for the first 3 min, then
decreased to 32 psi at the rate of 300 psi/min, and finally, increased to
70 psi at a rate of 1.23 psi/min for the rest of the run. The GC oven
temperature was set at −5 °C for first 5 min using CO2 as the
cryogenic cooling source and then increased to 50 °C at a rate of 50
°C/min. Then, the oven temperature was again increased to 270 °C at
a rate of 8 °C/min and kept at 270 °C for 5 min. The samples were
detected using an ion-trap mass spectrometer. The MS transfer-line
temperature was set at 250 °C; the manifold temperature was set at 50
°C; and the ion-trap temperature was set at 175 °C. The ionization
voltage of 70 eV was used, and electron multiplier voltage (EMV) was
set at 2000 V. The generated data was analyzed using the AMDIS_32,
version 2.69, computer software, and peak identification will be
performed using the National Institute of Standards and Technology
(NIST), version 2.0, mass spectral research program.
Liquid Chromatography−Mass Spectrometry (LC−MS) Anal-
ysis of the Amino Acid−Copper Synthetic Adducts. The dry
samples were dissolved in deionized water (Millipore, Billerica, MA) to
a concentration of 1 mL/mg. The sample was then diluted 10-fold in
10% methanol prior to analysis by LC−MS. The LC−electrospray
ionization (ESI)−MS analysis comprised of a Dionex Ultimate 3000
RS liquid chromatograph (Dionex, Germering, Germany) coupled to a
Bruker Maxis Impact quadruple-time-of-flight mass spectrometer
(Bruker Daltonics, Bremen, Germany) in positive mode. A total of 1
μL of the sample was injected directly into LC−MS. The electrospray
interphase settings were the following: nebulizer pressure, 0.6 bar;
drying gas, 8 L/min; temperature, 180 °C; and capillary voltage, 4500
V. The scan range was from m/z 100 to 1000. The data were analyzed
using Bruker Compass DataAnalysis software, version 4.1.
MATERIALS AND METHODS
■
Materials and Reagents. L-Alanine (99%), L-methionine (99%),
L-leucine (99%), L-glycine (98%), copper(II) chloride (CuCl2)
(99.9%), copper(I) chloride (CuCl) (97%), calcium chloride
(CaCl2), zinc chloride (ZnCl2) (98%), chloral hydrate (98%), 2-
methylpyridine (98%), potassium hydroxide, D-glucose, and acetalde-
hyde (99.5%) were purchased from Sigma-Aldrich Chemical Co.
(Oakville, Ontario, Canada). [13C-1]Alanine (98%), [13C-2]alanine
(98%), [13C-3]alanine (98%), [15N]alanine, and [13C-1, 13C-2]glycine
(98%) were purchased from Cambridge Isotope Laboratories
(Andover, MI).
Preparation of Model Systems. Model systems comprising of
amino acids alone, amino acids with metal salts (in a 2:1 relative molar
ratio), or synthetic amino acid metal complexes (see Table 1) were
Table 1. Composition of the Reactants in the Model Systems
a
amino acid
metal salt
synthetic metal complex
alanine
CuCl2/CuCl
FeCl2/FeCl3
ZnCl2
(Gly)2Cu
glycine
(Ala)2Cu
leucine
(Gly)Cu(Ala)
methionine
CaCl2
a
All of the listed amino acids were studied in the presence and absence
of metal ions. The amino acids alanine and glycine were studied with
all of the metal salts listed in the table and as synthetic metal
complexes. Leucine and methionine were studied only with CuCl2.
homogenized and mixed using a microscale porcelain mortar and
pestle. After thorough homogenization, approximately 0.5 mg of the
individual reactants or their mixtures were weighed into a quartz tube
(0.3 mm thickness), plugged at both ends with glass wool (Supelco,
Bellefonte, PA), inserted inside the coil probe, and pyrolyzed at 250
°C for 20 s under a helium atmosphere, as described below. All
samples were analyzed in duplicate.
Synthesis of Amino Acid−Metal Adducts. Alanine−copper and
glycine−copper complexes or mixed complexes were prepared by
dissolving alanine (0.89 g), glycine (0.75 g), [13C-1, 13C-2]glycine, or
an equimolar mixture of alanine and glycine at room temperature in
methanol (10 mL) in the presence of KOH (0.05 g) and stirred until
all completely dissolved, followed by the addition of 0.5 mol of CuCl2.
The dark blue (with CuCl2) precipitates were filtered, washed with
methanol, and dried.
Structural Identification. Specific reaction products (listed in
Table 2) were identified by comparison of their retention times and
mass spectra to commercial or in-situ-generated standards in addition
to NIST library matches and stable isotope labeling data (Table 2).
Elemental composition of selected metal complexes was based on their
accurate mass determination by ESI−QqTOF analysis [alanine−
copper complex at m/z 261.9985 [M + Na] (calculated for
C6H12CuN2NaO4 with an error of −1.4 ppm and glycine copper
complex at m/z 233.9665 [M + Na] (calculated for C4H8N2O4CuNa
with an error of 5.443 ppm].
Pyrolysis−Gas Chromatography/Mass Spectrometry (Py−
GC/MS). A Varian CP-3800 gas chromatograph coupled to a Saturn
2000 ion-trap detector interfaced to a CDS Pyroprobe 2000 unit
through a valved interface (CDS 1500) was used for Py−GC/MS
analysis of model systems shown in Table 1. The samples were packed
in the quartz tube inserted inside the coil probe and pyrolyzed at 250
°C for 20 s under a helium atmosphere.
RESULTS AND DISCUSSION
■
Under the Maillard reaction conditions, independent degrada-
tion reactions of amino acids play an important role in the
generation of amino-acid-specific products that result from
thermal decarboxylation or oxidative decarboxylation (Figure
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Table 2. Decomposition Products Observed in the Alanine−Copper Complex and Alanine CuCl2 Mixtures
a
b
Values in parentheses represent those of the commercial or synthesized standards. The in situ preparation of 2-ethylideneamino propanoic acid
comprised 0.5 mg of alanine and 1 μL of acetaldehyde mixed and pyrolyzed under standard pyrolytic conditions.
2), leading to the formation of amines13 and Strecker aldehydes
captured as imines by the amino acid or its corresponding
aldehydes, enhancement of the pathways generating them could
be a critical factor in flavor production and control. Because of
the oxidative capacity of some metal salts, such as copper salts,
it was predicted that, in their presence, the oxidative
decarboxylation reactions could be encouraged. To understand
metal−amino acid interactions and their influence on the
production of specific reaction products, such as Strecker
aldehydes, model systems containing amino acids were studied
in the presence and absence of metal ions as both their free
salts or synthesized amino acid−copper complexes (see Table
1). The purpose of the latter was to eliminate the effect of
counterions, such as chlorides, and assess its importance as an
intermediate.
Unlike other amino acids with longer side chains, such as
leucine and methionine,15 alanine when pyrolyzed in the
absence of metal salts did not generate any significant amount
of degradation products that could be observed under the
experimental conditions. However, when the synthetic copper
salt (Ala2Cu) of alanine was pyrolyzed, a single major peak was
observed that was characterized, resulting from the Schiff base
formation between the intact alanine and its Strecker aldehyde
(see Table 2). The same peak was also observed in the various
copper and iron(III) salt/alanine mixtures along with other
products. On the other hand, leucine and methionine in the
absence of metal salts generated under pyrolytic conditions
Schiff bases of Strecker aldehydes with their corresponding
decarboxylated amino acids15 (see Figure 2). Similarly, the
formation of these Schiff bases was increased in the presence of
copper salts. These observations clearly indicated that, although
some amino acids can undergo thermally induced oxidative
decarboxylation on their own and generate Strecker aldehydes,
however, in the presence of copper or iron(III) salts, this ability
is greatly facilitated or even accelerated by the formation of
copper complexes.
Figure 2. Proposed mechanism of thermally induced oxidative
autodecarboxylation of amino acids.
amine.14,15 Such oxidative decarboxylation reactions are
expected to be enhanced in the presence of oxidizing species,
such as copper,10 and contribute to the pool of aroma-active
aldehydes. On the other hand, oxidative decomposition of
sugars leads to the formation of α-dicarbonyl compounds, such
as glucosone and deoxyglucosone, that can trigger the Strecker
reaction of amino acids, producing even more Strecker
aldehydes along with 2-aminocarbonyl compounds, the
precursors of pyrazines. Currently, the relative contribution of
the two pathways of generating Strecker aldehydes is not
known in food products. Because of the importance of these
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Figure 3. Proposed mechanism of copper-assisted thermal decarboxylation of alanine and acetaldehyde formation.
To confirm the generality of this observation, another amino
acid glycine was chosen that behaves similar to alanine and does
not undergo oxidative decarboxylation on its own. However,
when glycine was pyrolyzed in the presence of copper salts or
as its copper complex, it generated various Strecker aldehyde
derivatives, such as pyrazines,16 that were verified through
NIST library searches and isotope-labeling studies using
synthetic [13C-1, 13C-2]glycine−copper complex, which gen-
erated pyrazine and two of its derivatives methyl- and 2,6-
dimethyl pyrazines, incorporating four, five, and six glycine
atoms, respectively. This label incorporation pattern was
consistent with those reported by Guerra and Yaylayan.16
Pyrazine and alkyl-substituted pyrazines can be formed through
dimerization of azomethine ylides, which, in turn, can be
formed from the Schiff base adducts of glycine and form-
aldehyde, the Strecker aldehyde of glycine. The same complex
also generated 4-hydroxybutanal, the aldol product of two
acetaldehyde molecules. Guerra and Yaylayan16 also demon-
strated that acetaldehyde can be generated from glycine and
formaldehyde through a chain elongation reaction.
Although other metals, such as calcium and zinc, can also
complex with amino acids, however, when copper or iron(III)
chloride was replaced with calcium or zinc chlorides in the
reaction mixtures, they did not promote Strecker aldehyde
formation. This observation underlines the significance of the
nature and reactivity of the metal ions in these complexes,
considering that copper in its oxidation sate +2 and iron in its
oxidation state +3 are known to have higher redox potentials
compared to the other metals studied. The high redox potential
enhances the electron affinity of copper, which, in turn, affects
its reactivity and how easily it can accept and donate
electrons.17 In addition, the Lewis acid character of the metal
ions also influences the stability of the complexes by forming
metal−amino acid bonds with differing bond strengths
depending upon the metal type, which, in turn, would affect
their thermal dissociation capacity. On the other hand, when
alanine was replaced with other amino acids, such as leucine
and methionine, in the CuCl2 reaction mixtures, similar
enhancements in their corresponding Strecker aldehyde
concentrations were observed relative to the model systems
lacking CuCl2 salts. Interestingly, when the mixed glycine/
alanine copper complex (AlaCuGly) was analyzed, a significant
enhancement in the formation of pyrroles, pyridines, and
pyrazines relative to single amino acid complexes was observed.
Furthermore, ethylamine and acetic acid were observed in
iron(III) chloride/alanine model systems, in addition to the
Schiff base adducts of Strecker aldehyde, indicating the ability
of the iron to oxidize acetaldehyde and generate nitrogen-
centered free radicals.
Proposed Mechanism of Copper-Assisted Oxidative
Decarboxylation of Alanine and Acetaldehyde Forma-
tion. The analysis of the metal salt−alanine reaction products
indicated that the influence of the metal ions on the thermal
degradation profile of alanine was highly dependent upon the
type of metal ion. Among the metal chloride salts investigated,
CuCl, CuCl2, and FeCl3 were found to be the only salts that
induced significant changes to the thermal degradation profile
of alanine that could be observed under the experimental
conditions, causing the generation of new products (see Table
2). The results pointed to the role of copper to be
stoichiometric rather than catalytic, as supported by the
generated products and their formation mechanism (see Figure
3). CuCl2 can form stable complexes with 2 mol of alanine with
the release of HCl, as shown in Figure 3, and the resulting
complex can undergo thermal decomposition through an
intramolecular redox reaction to generate a copper-centered
radical cation and a carboxylamine radical.11,18 Both products
can be converted into the ethylamine radical, the former
through the loss of CuO and CO and the latter through the
thermal decarboxylation mechanism. A subsequent dispropor-
tionation reaction between the carboxylamine radical and
ethylamine radical generates free alanine and ethanimine
molecules. The hydrolysis of imine can lead to the formation
of the Strecker aldehyde, which can immediately form a Schiff
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base adduct with the released free alanine, as shown in Figure 3.
This identified adduct, which confirms the formation of both
acetaldehyde and alanine, as predicted in the proposed
mechanism, was the main product observed when the synthetic
copper complex of alanine was pyrolyzed. The identity of this
adduct was verified through comparison of its retention time to
that of pyrolytically generated standard from alanine and
acetaldehyde and through isotope-labeling experiments with
[13C-1]alanine, [13C-2]alanine, [13C-3]alanine, and [15N]-
alanine (see Table 2), in addition to its mass spectral
fragmentation pattern, which was consistent with the proposed
structure (see Figure 4). Further evidence for the release of free
chlorine atom incorporation was confirmed through its natural
isotope abundance pattern. In addition to chloral hydrate,
several chlorinated and non-chlorinated methylpyridines were
also detected in CuCl2−alanine model systems. Isotope-
labeling studies have indicated that 2-methylpyridine incorpo-
rated three 13C-2, three 13C-3 atoms, and one nitrogen atom
from alanine, implying an aldol-type condensation involving
acetaldehyde and ethanimine, followed by cyclization (Figure
5). The mass spectra and retention time of 2-methylpyridine
were further confirmed using a commercially available standard.
Copper(II) and iron(III) were found to be the most reactive
metals that are able to complex with amino acids, promote their
oxidative decarboxylation in a thermally induced free-radical
pathway, and enhance the formation of important Strecker
aldehydes. This ability was demonstrated when the amino acid
was replaced with the alanine copper complex in a Maillard
model system consisting of glucose and alanine. In this reaction
system, the total intensity of ethyl-substituted pyrazines that
incorporated 13C-2 atoms of alanine was increased by 10-fold
compared to the total intensity of corresponding ethyl-
substituted pyrazines in the glucose alanine model system,
indicating enhanced production of the Strecker aldehyde
acetaldehyde and its subsequent reaction with dihydropyrazines
to generate various ethyl-substituted derivatives. With the
absence of any toxicological evidence to prohibit their use as
flavor-potentiating additives, these amino acid−metal com-
plexes might provide a cost-effective way to enhance the flavor
of heated foods. This pathway of generating Strecker aldehydes
through thermal degradation of easily available metal−amino
acid complexes in the absence of sugar-derived α-dicarbonyl
compounds can be considered a convenient strategy to
introduce such aroma-active volatiles in foods lacking
carbohydrates or a sugar source.
Figure 4. Mass spectral fragmentation patterns of 2-ethylidineamino
propionic acid (values in parentheses represent the observed alanine
atom incorporations).
alanine was the detection of 3,6-dimethyl piperazine-2,5-dione,
as shown in Table 2. The same diketopiperazine was also
observed in the alanine copper complexes analyzed at 270 °C
by Yablokov et al.11 On the other hand, the model systems
containing chloride salts, such as CuCl2, that can release HCl
provided additional evidence for the formation of acetaldehyde
because of the ability of HCl to chlorinate acetaldehyde and
form chloral hydrate (Figure 3). The structure of chloral
hydrate was confirmed by comparison of its mass spectrum and
retention time to a commercially available standard (Table 2).
Additionally, the labeling studies confirmed that chloral hydrate
contains 13C-2 and 13C-3 atoms of alanine, while the number of
AUTHOR INFORMATION
■
Corresponding Author
*Telephone: 514-398-7918. Fax: 514-398-7977. E-mail:
varoujan.yaylayan@mcgill.ca.
Funding
The authors acknowledge funding for this research from the
Natural Sciences and Engineering Research Council of Canada
(NSERC).
Notes
The authors declare no competing financial interest.
Figure 5. Proposed mechanism of 2-methylpyridine formation from acetaldehyde.
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